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New pthread rwlock that is more scalable.

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This replaces the pthread rwlock with a new implementation that uses a
more scalable algorithm (primarily through not using a critical section
anymore to make state changes).  The fast path for rdlock acquisition and
release is now basically a single atomic read-modify write or CAS and a few
branches.  See nptl/pthread_rwlock_common.c for details.

	* nptl/DESIGN-rwlock.txt: Remove.
	* nptl/lowlevelrwlock.sym: Remove.
	* nptl/Makefile: Add new tests.
	* nptl/pthread_rwlock_common.c: New file.  Contains the new rwlock.
	* nptl/pthreadP.h (PTHREAD_RWLOCK_PREFER_READER_P): Remove.
	(PTHREAD_RWLOCK_WRPHASE, PTHREAD_RWLOCK_WRLOCKED,
	PTHREAD_RWLOCK_RWAITING, PTHREAD_RWLOCK_READER_SHIFT,
	PTHREAD_RWLOCK_READER_OVERFLOW, PTHREAD_RWLOCK_WRHANDOVER,
	PTHREAD_RWLOCK_FUTEX_USED): New.
	* nptl/pthread_rwlock_init.c (__pthread_rwlock_init): Adapt to new
	implementation.
	* nptl/pthread_rwlock_rdlock.c (__pthread_rwlock_rdlock_slow): Remove.
	(__pthread_rwlock_rdlock): Adapt.
	* nptl/pthread_rwlock_timedrdlock.c
	(pthread_rwlock_timedrdlock): Adapt.
	* nptl/pthread_rwlock_timedwrlock.c
	(pthread_rwlock_timedwrlock): Adapt.
	* nptl/pthread_rwlock_trywrlock.c (pthread_rwlock_trywrlock): Adapt.
	* nptl/pthread_rwlock_tryrdlock.c (pthread_rwlock_tryrdlock): Adapt.
	* nptl/pthread_rwlock_unlock.c (pthread_rwlock_unlock): Adapt.
	* nptl/pthread_rwlock_wrlock.c (__pthread_rwlock_wrlock_slow): Remove.
	(__pthread_rwlock_wrlock): Adapt.
	* nptl/tst-rwlock10.c: Adapt.
	* nptl/tst-rwlock11.c: Adapt.
	* nptl/tst-rwlock17.c: New file.
	* nptl/tst-rwlock18.c: New file.
	* nptl/tst-rwlock19.c: New file.
	* nptl/tst-rwlock2b.c: New file.
	* nptl/tst-rwlock8.c: Adapt.
	* nptl/tst-rwlock9.c: Adapt.
	* sysdeps/aarch64/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/arm/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/hppa/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/ia64/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/m68k/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/microblaze/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/mips/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/nios2/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/s390/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/sh/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/sparc/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/tile/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* sysdeps/unix/sysv/linux/alpha/bits/pthreadtypes.h
	(pthread_rwlock_t): Adapt.
	* sysdeps/unix/sysv/linux/powerpc/bits/pthreadtypes.h
	(pthread_rwlock_t): Adapt.
	* sysdeps/x86/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
	* nptl/nptl-printers.py (): Adapt.
	* nptl/nptl_lock_constants.pysym: Adapt.
	* nptl/test-rwlock-printers.py: Adapt.
	* nptl/test-rwlockattr-printers.c: Adapt.
	* nptl/test-rwlockattr-printers.py: Adapt.
This commit is contained in:
Torvald Riegel 2014-05-22 16:00:12 +02:00
parent fbb31e20bc
commit cc25c8b4c1
42 changed files with 1548 additions and 915 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

55
ChangeLog
View File

@ -1,3 +1,58 @@
2016-01-10 Torvald Riegel <triegel@redhat.com>
* nptl/DESIGN-rwlock.txt: Remove.
* nptl/lowlevelrwlock.sym: Remove.
* nptl/Makefile: Add new tests.
* nptl/pthread_rwlock_common.c: New file. Contains the new rwlock.
* nptl/pthreadP.h (PTHREAD_RWLOCK_PREFER_READER_P): Remove.
(PTHREAD_RWLOCK_WRPHASE, PTHREAD_RWLOCK_WRLOCKED,
PTHREAD_RWLOCK_RWAITING, PTHREAD_RWLOCK_READER_SHIFT,
PTHREAD_RWLOCK_READER_OVERFLOW, PTHREAD_RWLOCK_WRHANDOVER,
PTHREAD_RWLOCK_FUTEX_USED): New.
* nptl/pthread_rwlock_init.c (__pthread_rwlock_init): Adapt to new
implementation.
* nptl/pthread_rwlock_rdlock.c (__pthread_rwlock_rdlock_slow): Remove.
(__pthread_rwlock_rdlock): Adapt.
* nptl/pthread_rwlock_timedrdlock.c
(pthread_rwlock_timedrdlock): Adapt.
* nptl/pthread_rwlock_timedwrlock.c
(pthread_rwlock_timedwrlock): Adapt.
* nptl/pthread_rwlock_trywrlock.c (pthread_rwlock_trywrlock): Adapt.
* nptl/pthread_rwlock_tryrdlock.c (pthread_rwlock_tryrdlock): Adapt.
* nptl/pthread_rwlock_unlock.c (pthread_rwlock_unlock): Adapt.
* nptl/pthread_rwlock_wrlock.c (__pthread_rwlock_wrlock_slow): Remove.
(__pthread_rwlock_wrlock): Adapt.
* nptl/tst-rwlock10.c: Adapt.
* nptl/tst-rwlock11.c: Adapt.
* nptl/tst-rwlock17.c: New file.
* nptl/tst-rwlock18.c: New file.
* nptl/tst-rwlock19.c: New file.
* nptl/tst-rwlock2b.c: New file.
* nptl/tst-rwlock8.c: Adapt.
* nptl/tst-rwlock9.c: Adapt.
* sysdeps/aarch64/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
* sysdeps/arm/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
* sysdeps/hppa/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
* sysdeps/ia64/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
* sysdeps/m68k/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
* sysdeps/microblaze/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
* sysdeps/mips/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
* sysdeps/nios2/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
* sysdeps/s390/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
* sysdeps/sh/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
* sysdeps/sparc/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
* sysdeps/tile/nptl/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
* sysdeps/unix/sysv/linux/alpha/bits/pthreadtypes.h
(pthread_rwlock_t): Adapt.
* sysdeps/unix/sysv/linux/powerpc/bits/pthreadtypes.h
(pthread_rwlock_t): Adapt.
* sysdeps/x86/bits/pthreadtypes.h (pthread_rwlock_t): Adapt.
* nptl/nptl-printers.py (): Adapt.
* nptl/nptl_lock_constants.pysym: Adapt.
* nptl/test-rwlock-printers.py: Adapt.
* nptl/test-rwlockattr-printers.c: Adapt.
* nptl/test-rwlockattr-printers.py: Adapt.
2017-01-10 Joseph Myers <joseph@codesourcery.com> 2017-01-10 Joseph Myers <joseph@codesourcery.com>
* math/libm-test.inc (XFAIL_IBM128_LIBGCC): New macro. * math/libm-test.inc (XFAIL_IBM128_LIBGCC): New macro.

113
nptl/DESIGN-rwlock.txt
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@ -1,113 +0,0 @@
Reader Writer Locks pseudocode
==============================
pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
pthread_rwlock_unlock(pthread_rwlock_t *rwlock);
pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);
struct pthread_rwlock_t {
unsigned int lock:
- internal mutex
unsigned int writers_preferred;
- locking mode: 0 recursive, readers preferred
1 nonrecursive, writers preferred
unsigned int readers;
- number of read-only references various threads have
pthread_t writer;
- descriptor of the writer or 0
unsigned int readers_wakeup;
- 'all readers should wake up' futex.
unsigned int writer_wakeup;
- 'one writer should wake up' futex.
unsigned int nr_readers_queued;
- number of readers queued up.
unsigned int nr_writers_queued;
- number of writers queued up.
}
pthread_rwlock_rdlock(pthread_rwlock_t *rwlock)
{
lll_lock(rwlock->lock);
for (;;) {
if (!rwlock->writer && (!rwlock->nr_writers_queued ||
!rwlock->writers_preferred))
break;
rwlock->nr_readers_queued++;
val = rwlock->readers_wakeup;
lll_unlock(rwlock->lock);
futex_wait(&rwlock->readers_wakeup, val)
lll_lock(rwlock->lock);
rwlock->nr_readers_queued--;
}
rwlock->readers++;
lll_unlock(rwlock->lock);
}
pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock)
{
int result = EBUSY;
lll_lock(rwlock->lock);
if (!rwlock->writer && (!rwlock->nr_writers_queued ||
!rwlock->writers_preferred))
rwlock->readers++;
lll_unlock(rwlock->lock);
return result;
}
pthread_rwlock_wrlock(pthread_rwlock_t *rwlock)
{
lll_lock(rwlock->lock);
for (;;) {
if (!rwlock->writer && !rwlock->readers)
break;
rwlock->nr_writers_queued++;
val = rwlock->writer_wakeup;
lll_unlock(rwlock->lock);
futex_wait(&rwlock->writer_wakeup, val);
lll_lock(rwlock->lock);
rwlock->nr_writers_queued--;
}
rwlock->writer = pthread_self();
lll_unlock(rwlock->lock);
}
pthread_rwlock_unlock(pthread_rwlock_t *rwlock)
{
lll_lock(rwlock->lock);
if (rwlock->writer)
rwlock->writer = 0;
else
rwlock->readers--;
if (!rwlock->readers) {
if (rwlock->nr_writers_queued) {
++rwlock->writer_wakeup;
lll_unlock(rwlock->lock);
futex_wake(&rwlock->writer_wakeup, 1);
return;
} else
if (rwlock->nr_readers_queued) {
++rwlock->readers_wakeup;
lll_unlock(rwlock->lock);
futex_wake(&rwlock->readers_wakeup, MAX_INT);
return;
}
}
lll_unlock(rwlock->lock);
}

11
nptl/Makefile
View File

@ -237,10 +237,11 @@ tests = tst-typesizes \
tst-robust6 tst-robust7 tst-robust8 tst-robust9 \ tst-robust6 tst-robust7 tst-robust8 tst-robust9 \
tst-robustpi1 tst-robustpi2 tst-robustpi3 tst-robustpi4 tst-robustpi5 \ tst-robustpi1 tst-robustpi2 tst-robustpi3 tst-robustpi4 tst-robustpi5 \
tst-robustpi6 tst-robustpi7 tst-robustpi8 tst-robustpi9 \ tst-robustpi6 tst-robustpi7 tst-robustpi8 tst-robustpi9 \
tst-rwlock1 tst-rwlock2 tst-rwlock2a tst-rwlock3 tst-rwlock4 \ tst-rwlock1 tst-rwlock2 tst-rwlock2a tst-rwlock2b tst-rwlock3 \
tst-rwlock5 tst-rwlock6 tst-rwlock7 tst-rwlock8 tst-rwlock9 \ tst-rwlock4 tst-rwlock5 tst-rwlock6 tst-rwlock7 tst-rwlock8 \
tst-rwlock10 tst-rwlock11 tst-rwlock12 tst-rwlock13 tst-rwlock14 \ tst-rwlock9 tst-rwlock10 tst-rwlock11 tst-rwlock12 tst-rwlock13 \
tst-rwlock15 tst-rwlock16 \ tst-rwlock14 tst-rwlock15 tst-rwlock16 tst-rwlock17 tst-rwlock18 \
tst-rwlock19 \
tst-once1 tst-once2 tst-once3 tst-once4 tst-once5 \ tst-once1 tst-once2 tst-once3 tst-once4 tst-once5 \
tst-key1 tst-key2 tst-key3 tst-key4 \ tst-key1 tst-key2 tst-key3 tst-key4 \
tst-sem1 tst-sem2 tst-sem3 tst-sem4 tst-sem5 tst-sem6 tst-sem7 \ tst-sem1 tst-sem2 tst-sem3 tst-sem4 tst-sem5 tst-sem6 tst-sem7 \
@ -306,7 +307,7 @@ test-xfail-tst-once5 = yes
# Files which must not be linked with libpthread. # Files which must not be linked with libpthread.
tests-nolibpthread = tst-unload tests-nolibpthread = tst-unload
gen-as-const-headers = pthread-errnos.sym lowlevelrwlock.sym \ gen-as-const-headers = pthread-errnos.sym \
unwindbuf.sym \ unwindbuf.sym \
lowlevelrobustlock.sym pthread-pi-defines.sym lowlevelrobustlock.sym pthread-pi-defines.sym

16
nptl/lowlevelrwlock.sym
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@ -1,16 +0,0 @@
#include <stddef.h>
#include <stdio.h>
#include <bits/pthreadtypes.h>
#include <bits/wordsize.h>
--
MUTEX offsetof (pthread_rwlock_t, __data.__lock)
NR_READERS offsetof (pthread_rwlock_t, __data.__nr_readers)
READERS_WAKEUP offsetof (pthread_rwlock_t, __data.__readers_wakeup)
WRITERS_WAKEUP offsetof (pthread_rwlock_t, __data.__writer_wakeup)
READERS_QUEUED offsetof (pthread_rwlock_t, __data.__nr_readers_queued)
WRITERS_QUEUED offsetof (pthread_rwlock_t, __data.__nr_writers_queued)
FLAGS offsetof (pthread_rwlock_t, __data.__flags)
WRITER offsetof (pthread_rwlock_t, __data.__writer)
PSHARED offsetof (pthread_rwlock_t, __data.__shared)

48
nptl/nptl-printers.py
View File

@ -430,12 +430,10 @@ class RWLockPrinter(object):
""" """
data = rwlock['__data'] data = rwlock['__data']
self.readers = data['__nr_readers'] self.readers = data['__readers']
self.queued_readers = data['__nr_readers_queued'] self.cur_writer = data['__cur_writer']
self.queued_writers = data['__nr_writers_queued']
self.writer_id = data['__writer']
self.shared = data['__shared'] self.shared = data['__shared']
self.prefers_writers = data['__flags'] self.flags = data['__flags']
self.values = [] self.values = []
self.read_values() self.read_values()
@ -468,20 +466,19 @@ class RWLockPrinter(object):
def read_status(self): def read_status(self):
"""Read the status of the rwlock.""" """Read the status of the rwlock."""
# Right now pthread_rwlock_destroy doesn't do anything, so there's no if self.readers & PTHREAD_RWLOCK_WRPHASE:
# way to check if an rwlock is destroyed. if self.readers & PTHREAD_RWLOCK_WRLOCKED:
self.values.append(('Status', 'Acquired (Write)'))
if self.writer_id: self.values.append(('Writer ID', self.cur_writer))
self.values.append(('Status', 'Locked (Write)')) else:
self.values.append(('Writer ID', self.writer_id)) self.values.append(('Status', 'Not acquired'))
elif self.readers:
self.values.append(('Status', 'Locked (Read)'))
self.values.append(('Readers', self.readers))
else: else:
self.values.append(('Status', 'Unlocked')) r = self.readers >> PTHREAD_RWLOCK_READER_SHIFT
if r > 0:
self.values.append(('Queued readers', self.queued_readers)) self.values.append(('Status', 'Acquired (Read)'))
self.values.append(('Queued writers', self.queued_writers)) self.values.append(('Readers', r))
else:
self.values.append(('Status', 'Not acquired'))
def read_attributes(self): def read_attributes(self):
"""Read the attributes of the rwlock.""" """Read the attributes of the rwlock."""
@ -491,10 +488,12 @@ class RWLockPrinter(object):
else: else:
self.values.append(('Shared', 'No')) self.values.append(('Shared', 'No'))
if self.prefers_writers: if self.flags == PTHREAD_RWLOCK_PREFER_READER_NP:
self.values.append(('Prefers', 'Readers'))
elif self.flags == PTHREAD_RWLOCK_PREFER_WRITER_NP:
self.values.append(('Prefers', 'Writers')) self.values.append(('Prefers', 'Writers'))
else: else:
self.values.append(('Prefers', 'Readers')) self.values.append(('Prefers', 'Writers no recursive readers'))
class RWLockAttributesPrinter(object): class RWLockAttributesPrinter(object):
"""Pretty printer for pthread_rwlockattr_t. """Pretty printer for pthread_rwlockattr_t.
@ -555,13 +554,12 @@ class RWLockAttributesPrinter(object):
# PTHREAD_PROCESS_PRIVATE # PTHREAD_PROCESS_PRIVATE
self.values.append(('Shared', 'No')) self.values.append(('Shared', 'No'))
if (rwlock_type == PTHREAD_RWLOCK_PREFER_READER_NP or if rwlock_type == PTHREAD_RWLOCK_PREFER_READER_NP:
rwlock_type == PTHREAD_RWLOCK_PREFER_WRITER_NP):
# This is a known bug. Using PTHREAD_RWLOCK_PREFER_WRITER_NP will
# still make the rwlock prefer readers.
self.values.append(('Prefers', 'Readers')) self.values.append(('Prefers', 'Readers'))
elif rwlock_type == PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP: elif rwlock_type == PTHREAD_RWLOCK_PREFER_WRITER_NP:
self.values.append(('Prefers', 'Writers')) self.values.append(('Prefers', 'Writers'))
else:
self.values.append(('Prefers', 'Writers no recursive readers'))
def register(objfile): def register(objfile):
"""Register the pretty printers within the given objfile.""" """Register the pretty printers within the given objfile."""

5
nptl/nptl_lock_constants.pysym
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@ -57,6 +57,11 @@ PTHREAD_RWLOCK_PREFER_READER_NP
PTHREAD_RWLOCK_PREFER_WRITER_NP PTHREAD_RWLOCK_PREFER_WRITER_NP
PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP
-- Rwlock
PTHREAD_RWLOCK_WRPHASE
PTHREAD_RWLOCK_WRLOCKED
PTHREAD_RWLOCK_READER_SHIFT
-- 'Shared' attribute values -- 'Shared' attribute values
PTHREAD_PROCESS_PRIVATE PTHREAD_PROCESS_PRIVATE
PTHREAD_PROCESS_SHARED PTHREAD_PROCESS_SHARED

13
nptl/pthreadP.h
View File

@ -150,9 +150,16 @@ enum
| PTHREAD_MUTEXATTR_PROTOCOL_MASK | PTHREAD_MUTEXATTR_PRIO_CEILING_MASK) | PTHREAD_MUTEXATTR_PROTOCOL_MASK | PTHREAD_MUTEXATTR_PRIO_CEILING_MASK)
/* Check whether rwlock prefers readers. */ /* For the following, see pthread_rwlock_common.c. */
#define PTHREAD_RWLOCK_PREFER_READER_P(rwlock) \ #define PTHREAD_RWLOCK_WRPHASE 1
((rwlock)->__data.__flags == 0) #define PTHREAD_RWLOCK_WRLOCKED 2
#define PTHREAD_RWLOCK_RWAITING 4
#define PTHREAD_RWLOCK_READER_SHIFT 3
#define PTHREAD_RWLOCK_READER_OVERFLOW ((unsigned int) 1 \
<< (sizeof (unsigned int) * 8 - 1))
#define PTHREAD_RWLOCK_WRHANDOVER ((unsigned int) 1 \
<< (sizeof (unsigned int) * 8 - 1))
#define PTHREAD_RWLOCK_FUTEX_USED 2
/* Bits used in robust mutex implementation. */ /* Bits used in robust mutex implementation. */

924
nptl/pthread_rwlock_common.c Normal file
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@ -0,0 +1,924 @@
/* POSIX reader--writer lock: core parts.
Copyright (C) 2016-2017 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */
#include <errno.h>
#include <sysdep.h>
#include <pthread.h>
#include <pthreadP.h>
#include <sys/time.h>
#include <stap-probe.h>
#include <atomic.h>
#include <futex-internal.h>
/* A reader--writer lock that fulfills the POSIX requirements (but operations
on this lock are not necessarily full barriers, as one may interpret the
POSIX requirement about "synchronizing memory"). All critical sections are
in a total order, writers synchronize with prior writers and readers, and
readers synchronize with prior writers.
A thread is allowed to acquire a read lock recursively (i.e., have rdlock
critical sections that overlap in sequenced-before) unless the kind of the
rwlock is set to PTHREAD_RWLOCK_PREFER_WRITERS_NONRECURSIVE_NP.
This lock is built so that workloads of mostly readers can be executed with
low runtime overheads. This matches that the default kind of the lock is
PTHREAD_RWLOCK_PREFER_READER_NP. Acquiring a read lock requires a single
atomic addition if the lock is or was previously acquired by other
readers; releasing the lock is a single CAS if there are no concurrent
writers.
Workloads consisting of mostly writers are of secondary importance.
An uncontended write lock acquisition is as fast as for a normal
exclusive mutex but writer contention is somewhat more costly due to
keeping track of the exact number of writers. If the rwlock kind requests
writers to be preferred (i.e., PTHREAD_RWLOCK_PREFER_WRITERS_NP or the
no-recursive-readers variant of it), then writer--to--writer lock ownership
hand-over is fairly fast and bypasses lock acquisition attempts by readers.
The costs of lock ownership transfer between readers and writers vary. If
the program asserts that there are no recursive readers and writers are
preferred, then write lock acquisition attempts will block subsequent read
lock acquisition attempts, so that new incoming readers do not prolong a
phase in which readers have acquired the lock.
The main components of the rwlock are a writer-only lock that allows only
one of the concurrent writers to be the primary writer, and a
single-writer-multiple-readers lock that decides between read phases, in
which readers have acquired the rwlock, and write phases in which a primary
writer or a sequence of different primary writers have acquired the rwlock.
The single-writer-multiple-readers lock is the central piece of state
describing the rwlock and is encoded in the __readers field (see below for
a detailed explanation):
State WP WL R RW Notes
---------------------------
#1 0 0 0 0 Lock is idle (and in a read phase).
#2 0 0 >0 0 Readers have acquired the lock.
#3 0 1 0 0 Lock is not acquired; a writer is waiting for a write
phase to start or will try to start one.
#4 0 1 >0 0 Readers have acquired the lock; a writer is waiting
and explicit hand-over to the writer is required.
#4a 0 1 >0 1 Same as #4 except that there are further readers
waiting because the writer is to be preferred.
#5 1 0 0 0 Lock is idle (and in a write phase).
#6 1 0 >0 0 Write phase; readers are waiting for a read phase to
start or will try to start one.
#7 1 1 0 0 Lock is acquired by a writer.
#8 1 1 >0 0 Lock acquired by a writer and readers are waiting;
explicit hand-over to the readers is required.
WP (PTHREAD_RWLOCK_WRPHASE) is true if the lock is in a write phase, so
potentially acquired by a primary writer.
WL (PTHREAD_RWLOCK_WRLOCKED) is true if there is a primary writer (i.e.,
the thread that was able to set this bit from false to true).
R (all bits in __readers except the number of least-significant bits
denoted in PTHREAD_RWLOCK_READER_SHIFT) is the number of readers that have
or are trying to acquired the lock. There may be more readers waiting if
writers are preferred and there will be no recursive readers, in which
case RW (PTHREAD_RWLOCK_RWAITING) is true in state #4a.
We want to block using futexes but using __readers as a futex word directly
is not a good solution. First, we want to wait on different conditions
such as waiting for a phase change vs. waiting for the primary writer to
release the writer-only lock. Second, the number of readers could change
frequently, which would make it likely that a writer's futex_wait fails
frequently too because the expected value does not match the value of
__readers anymore.
Therefore, we split out the futex words into the __wrphase_futex and
__writers_futex fields. The former tracks the value of the WP bit and is
changed after changing WP by the thread that changes WP. However, because
of the POSIX requirements regarding mutex/rwlock destruction (i.e., that
destroying a rwlock is allowed as soon as no thread has acquired or will
acquire the lock), we have to be careful and hand over lock ownership (via
a phase change) carefully to those threads waiting. Specifically, we must
prevent a situation in which we are not quite sure whether we still have
to unblock another thread through a change to memory (executing a
futex_wake on a former futex word that is now used for something else is
fine).
The scheme we use for __wrphase_futex is that waiting threads that may
use the futex word to block now all have to use the futex word to block; it
is not allowed to take the short-cut and spin-wait on __readers because
then the waking thread cannot just make one final change to memory to
unblock all potentially waiting threads. If, for example, a reader
increments R in states #7 or #8, it has to then block until __wrphase_futex
is 0 and it can confirm that the value of 0 was stored by the primary
writer; in turn, the primary writer has to change to a read phase too when
releasing WL (i.e., to state #2), and it must change __wrphase_futex to 0
as the next step. This ensures that the waiting reader will not be able to
acquire, release, and then destroy the lock concurrently with the pending
futex unblock operations by the former primary writer. This scheme is
called explicit hand-over in what follows.
Note that waiting threads can cancel waiting only if explicit hand-over has
not yet started (e.g., if __readers is still in states #7 or #8 in the
example above).
Writers determine the primary writer through WL. Blocking using futexes
is performed using __writers_futex as a futex word; primary writers will
enable waiting on this futex by setting it to 1 after they acquired the WL
bit and will disable waiting by setting it to 0 before they release WL.
This leaves small windows where blocking using futexes is not possible
although a primary writer exists, but in turn decreases complexity of the
writer--writer synchronization and does not affect correctness.
If writers are preferred, writers can hand over WL directly to other
waiting writers that registered by incrementing __writers: If the primary
writer can CAS __writers from a non-zero value to the same value with the
PTHREAD_RWLOCK_WRHANDOVER bit set, it effectively transfers WL ownership
to one of the registered waiting writers and does not reset WL; in turn,
a registered writer that can clear PTHREAD_RWLOCK_WRHANDOVER using a CAS
then takes over WL. Note that registered waiting writers can cancel
waiting by decrementing __writers, but the last writer to unregister must
become the primary writer if PTHREAD_RWLOCK_WRHANDOVER is set.
Also note that adding another state/bit to signal potential writer--writer
contention (e.g., as done in the normal mutex algorithm) would not be
helpful because we would have to conservatively assume that there is in
fact no other writer, and wake up readers too.
To avoid having to call futex_wake when no thread uses __wrphase_futex or
__writers_futex, threads will set the PTHREAD_RWLOCK_FUTEX_USED bit in the
respective futex words before waiting on it (using a CAS so it will only be
set if in a state in which waiting would be possible). In the case of
__writers_futex, we wake only one thread but several threads may share
PTHREAD_RWLOCK_FUTEX_USED, so we must assume that there are still others.
This is similar to what we do in pthread_mutex_lock. We do not need to
do this for __wrphase_futex because there, we always wake all waiting
threads.
Blocking in the state #4a simply uses __readers as futex word. This
simplifies the algorithm but suffers from some of the drawbacks discussed
before, though not to the same extent because R can only decrease in this
state, so the number of potentially failing futex_wait attempts will be
bounded. All threads moving from state #4a to another state must wake
up threads blocked on the __readers futex.
The ordering invariants that we have to take care of in the implementation
are primarily those necessary for a reader--writer lock; this is rather
straightforward and happens during write/read phase switching (potentially
through explicit hand-over), and between writers through synchronization
involving the PTHREAD_RWLOCK_WRLOCKED or PTHREAD_RWLOCK_WRHANDOVER bits.
Additionally, we need to take care that modifications of __writers_futex
and __wrphase_futex (e.g., by otherwise unordered readers) take place in
the writer critical sections or read/write phases, respectively, and that
explicit hand-over observes stores from the previous phase. How this is
done is explained in more detail in comments in the code.
Many of the accesses to the futex words just need relaxed MO. This is
possible because we essentially drive both the core rwlock synchronization
and the futex synchronization in parallel. For example, an unlock will
unlock the rwlock and take part in the futex synchronization (using
PTHREAD_RWLOCK_FUTEX_USED, see above); even if they are not tightly
ordered in some way, the futex synchronization ensures that there are no
lost wake-ups, and woken threads will then eventually see the most recent
state of the rwlock. IOW, waiting threads will always be woken up, while
not being able to wait using futexes (which can happen) is harmless; in
turn, this means that waiting threads don't need special ordering wrt.
waking threads.
The futex synchronization consists of the three-state futex word:
(1) cannot block on it, (2) can block on it, and (3) there might be a
thread blocked on it (i.e., with PTHREAD_RWLOCK_FUTEX_USED set).
Relaxed-MO atomic read-modify-write operations are sufficient to maintain
this (e.g., using a CAS to go from (2) to (3) but not from (1) to (3)),
but we need ordering of the futex word modifications by the waking threads
so that they collectively make correct state changes between (1)-(3).
The futex-internal synchronization (i.e., the conceptual critical sections
around futex operations in the kernel) then ensures that even an
unconstrained load (i.e., relaxed MO) inside of futex_wait will not lead to
lost wake-ups because either the waiting thread will see the change from
(3) to (1) when a futex_wake came first, or this futex_wake will wake this
waiting thread because the waiting thread came first.
POSIX allows but does not require rwlock acquisitions to be a cancellation
point. We do not support cancellation.
TODO We do not try to elide any read or write lock acquisitions currently.
While this would be possible, it is unclear whether HTM performance is
currently predictable enough and our runtime tuning is good enough at
deciding when to use elision so that enabling it would lead to consistently
better performance. */
static int
__pthread_rwlock_get_private (pthread_rwlock_t *rwlock)
{
return rwlock->__data.__shared != 0 ? FUTEX_SHARED : FUTEX_PRIVATE;
}
static __always_inline void
__pthread_rwlock_rdunlock (pthread_rwlock_t *rwlock)
{
int private = __pthread_rwlock_get_private (rwlock);
/* We decrease the number of readers, and if we are the last reader and
there is a primary writer, we start a write phase. We use a CAS to
make this atomic so that it is clear whether we must hand over ownership
explicitly. */
unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
unsigned int rnew;
for (;;)
{
rnew = r - (1 << PTHREAD_RWLOCK_READER_SHIFT);
/* If we are the last reader, we also need to unblock any readers
that are waiting for a writer to go first (PTHREAD_RWLOCK_RWAITING)
so that they can register while the writer is active. */
if ((rnew >> PTHREAD_RWLOCK_READER_SHIFT) == 0)
{
if ((rnew & PTHREAD_RWLOCK_WRLOCKED) != 0)
rnew |= PTHREAD_RWLOCK_WRPHASE;
rnew &= ~(unsigned int) PTHREAD_RWLOCK_RWAITING;
}
/* We need release MO here for three reasons. First, so that we
synchronize with subsequent writers. Second, we might have been the
first reader and set __wrphase_futex to 0, so we need to synchronize
with the last reader that will set it to 1 (note that we will always
change __readers before the last reader, or we are the last reader).
Third, a writer that takes part in explicit hand-over needs to see
the first reader's store to __wrphase_futex (or a later value) if
the writer observes that a write phase has been started. */
if (atomic_compare_exchange_weak_release (&rwlock->__data.__readers,
&r, rnew))
break;
/* TODO Back-off. */
}
if ((rnew & PTHREAD_RWLOCK_WRPHASE) != 0)
{
/* We need to do explicit hand-over. We need the acquire MO fence so
that our modification of _wrphase_futex happens after a store by
another reader that started a read phase. Relaxed MO is sufficient
for the modification of __wrphase_futex because it is just used
to delay acquisition by a writer until all threads are unblocked
irrespective of whether they are looking at __readers or
__wrphase_futex; any other synchronizes-with relations that are
necessary are established through __readers. */
atomic_thread_fence_acquire ();
if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 1)
& PTHREAD_RWLOCK_FUTEX_USED) != 0)
futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private);
}
/* Also wake up waiting readers if we did reset the RWAITING flag. */
if ((r & PTHREAD_RWLOCK_RWAITING) != (rnew & PTHREAD_RWLOCK_RWAITING))
futex_wake (&rwlock->__data.__readers, INT_MAX, private);
}
static __always_inline int
__pthread_rwlock_rdlock_full (pthread_rwlock_t *rwlock,
const struct timespec *abstime)
{
unsigned int r;
/* Make sure we are not holding the rwlock as a writer. This is a deadlock
situation we recognize and report. */
if (__glibc_unlikely (atomic_load_relaxed (&rwlock->__data.__cur_writer)
== THREAD_GETMEM (THREAD_SELF, tid)))
return EDEADLK;
/* If we prefer writers, recursive rdlock is disallowed, we are in a read
phase, and there are other readers present, we try to wait without
extending the read phase. We will be unblocked by either one of the
other active readers, or if the writer gives up WRLOCKED (e.g., on
timeout).
If there are no other readers, we simply race with any existing primary
writer; it would have been a race anyway, and changing the odds slightly
will likely not make a big difference. */
if (rwlock->__data.__flags == PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP)
{
r = atomic_load_relaxed (&rwlock->__data.__readers);
while (((r & PTHREAD_RWLOCK_WRPHASE) == 0)
&& ((r & PTHREAD_RWLOCK_WRLOCKED) != 0)
&& ((r >> PTHREAD_RWLOCK_READER_SHIFT) > 0))
{
/* TODO Spin first. */
/* Try setting the flag signaling that we are waiting without having
incremented the number of readers. Relaxed MO is fine because
this is just about waiting for a state change in __readers. */
if (atomic_compare_exchange_weak_relaxed
(&rwlock->__data.__readers, &r, r | PTHREAD_RWLOCK_RWAITING))
{
/* Wait for as long as the flag is set. An ABA situation is
harmless because the flag is just about the state of
__readers, and all threads set the flag under the same
conditions. */
while ((atomic_load_relaxed (&rwlock->__data.__readers)
& PTHREAD_RWLOCK_RWAITING) != 0)
{
int private = __pthread_rwlock_get_private (rwlock);
int err = futex_abstimed_wait (&rwlock->__data.__readers,
r, abstime, private);
/* We ignore EAGAIN and EINTR. On time-outs, we can just
return because we don't need to clean up anything. */
if (err == ETIMEDOUT)
return err;
}
/* It makes sense to not break out of the outer loop here
because we might be in the same situation again. */
}
else
{
/* TODO Back-off. */
}
}
}
/* Register as a reader, using an add-and-fetch so that R can be used as
expected value for future operations. Acquire MO so we synchronize with
prior writers as well as the last reader of the previous read phase (see
below). */
r = atomic_fetch_add_acquire (&rwlock->__data.__readers,
(1 << PTHREAD_RWLOCK_READER_SHIFT)) + (1 << PTHREAD_RWLOCK_READER_SHIFT);
/* Check whether there is an overflow in the number of readers. We assume
that the total number of threads is less than half the maximum number
of readers that we have bits for in __readers (i.e., with 32-bit int and
PTHREAD_RWLOCK_READER_SHIFT of 3, we assume there are less than
1 << (32-3-1) concurrent threads).
If there is an overflow, we use a CAS to try to decrement the number of
readers if there still is an overflow situation. If so, we return
EAGAIN; if not, we are not a thread causing an overflow situation, and so
we just continue. Using a fetch-add instead of the CAS isn't possible
because other readers might release the lock concurrently, which could
make us the last reader and thus responsible for handing ownership over
to writers (which requires a CAS too to make the decrement and ownership
transfer indivisible). */
while (__glibc_unlikely (r >= PTHREAD_RWLOCK_READER_OVERFLOW))
{
/* Relaxed MO is okay because we just want to undo our registration and
cannot have changed the rwlock state substantially if the CAS
succeeds. */
if (atomic_compare_exchange_weak_relaxed (&rwlock->__data.__readers, &r,
r - (1 << PTHREAD_RWLOCK_READER_SHIFT)))
return EAGAIN;
}
/* We have registered as a reader, so if we are in a read phase, we have
acquired a read lock. This is also the reader--reader fast-path.
Even if there is a primary writer, we just return. If writers are to
be preferred and we are the only active reader, we could try to enter a
write phase to let the writer proceed. This would be okay because we
cannot have acquired the lock previously as a reader (which could result
in deadlock if we would wait for the primary writer to run). However,
this seems to be a corner case and handling it specially not be worth the
complexity. */
if (__glibc_likely ((r & PTHREAD_RWLOCK_WRPHASE) == 0))
return 0;
/* If there is no primary writer but we are in a write phase, we can try
to install a read phase ourself. */
while (((r & PTHREAD_RWLOCK_WRPHASE) != 0)
&& ((r & PTHREAD_RWLOCK_WRLOCKED) == 0))
{
/* Try to enter a read phase: If the CAS below succeeds, we have
ownership; if it fails, we will simply retry and reassess the
situation.
Acquire MO so we synchronize with prior writers. */
if (atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers, &r,
r ^ PTHREAD_RWLOCK_WRPHASE))
{
/* We started the read phase, so we are also responsible for
updating the write-phase futex. Relaxed MO is sufficient.
Note that there can be no other reader that we have to wake
because all other readers will see the read phase started by us
(or they will try to start it themselves); if a writer started
the read phase, we cannot have started it. Furthermore, we
cannot discard a PTHREAD_RWLOCK_FUTEX_USED flag because we will
overwrite the value set by the most recent writer (or the readers
before it in case of explicit hand-over) and we know that there
are no waiting readers. */
atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 0);
return 0;
}
else
{
/* TODO Back off before retrying. Also see above. */
}
}
if ((r & PTHREAD_RWLOCK_WRPHASE) != 0)
{
/* We are in a write phase, and there must be a primary writer because
of the previous loop. Block until the primary writer gives up the
write phase. This case requires explicit hand-over using
__wrphase_futex.
However, __wrphase_futex might not have been set to 1 yet (either
because explicit hand-over to the writer is still ongoing, or because
the writer has started the write phase but does not yet have updated
__wrphase_futex). The least recent value of __wrphase_futex we can
read from here is the modification of the last read phase (because
we synchronize with the last reader in this read phase through
__readers; see the use of acquire MO on the fetch_add above).
Therefore, if we observe a value of 0 for __wrphase_futex, we need
to subsequently check that __readers now indicates a read phase; we
need to use acquire MO for this so that if we observe a read phase,
we will also see the modification of __wrphase_futex by the previous
writer. We then need to load __wrphase_futex again and continue to
wait if it is not 0, so that we do not skip explicit hand-over.
Relaxed MO is sufficient for the load from __wrphase_futex because
we just use it as an indicator for when we can proceed; we use
__readers and the acquire MO accesses to it to eventually read from
the proper stores to __wrphase_futex. */
unsigned int wpf;
bool ready = false;
for (;;)
{
while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex))
| PTHREAD_RWLOCK_FUTEX_USED) == (1 | PTHREAD_RWLOCK_FUTEX_USED))
{
int private = __pthread_rwlock_get_private (rwlock);
if (((wpf & PTHREAD_RWLOCK_FUTEX_USED) == 0)
&& !atomic_compare_exchange_weak_relaxed
(&rwlock->__data.__wrphase_futex,
&wpf, wpf | PTHREAD_RWLOCK_FUTEX_USED))
continue;
int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex,
1 | PTHREAD_RWLOCK_FUTEX_USED, abstime, private);
if (err == ETIMEDOUT)
{
/* If we timed out, we need to unregister. If no read phase
has been installed while we waited, we can just decrement
the number of readers. Otherwise, we just acquire the
lock, which is allowed because we give no precise timing
guarantees, and because the timeout is only required to
be in effect if we would have had to wait for other
threads (e.g., if futex_wait would time-out immediately
because the given absolute time is in the past). */
r = atomic_load_relaxed (&rwlock->__data.__readers);
while ((r & PTHREAD_RWLOCK_WRPHASE) != 0)
{
/* We don't need to make anything else visible to
others besides unregistering, so relaxed MO is
sufficient. */
if (atomic_compare_exchange_weak_relaxed
(&rwlock->__data.__readers, &r,
r - (1 << PTHREAD_RWLOCK_READER_SHIFT)))
return ETIMEDOUT;
/* TODO Back-off. */
}
/* Use the acquire MO fence to mirror the steps taken in the
non-timeout case. Note that the read can happen both
in the atomic_load above as well as in the failure case
of the CAS operation. */
atomic_thread_fence_acquire ();
/* We still need to wait for explicit hand-over, but we must
not use futex_wait anymore because we would just time out
in this case and thus make the spin-waiting we need
unnecessarily expensive. */
while ((atomic_load_relaxed (&rwlock->__data.__wrphase_futex)
| PTHREAD_RWLOCK_FUTEX_USED)
== (1 | PTHREAD_RWLOCK_FUTEX_USED))
{
/* TODO Back-off? */
}
ready = true;
break;
}
/* If we got interrupted (EINTR) or the futex word does not have the
expected value (EAGAIN), retry. */
}
if (ready)
/* See below. */
break;
/* We need acquire MO here so that we synchronize with the lock
release of the writer, and so that we observe a recent value of
__wrphase_futex (see below). */
if ((atomic_load_acquire (&rwlock->__data.__readers)
& PTHREAD_RWLOCK_WRPHASE) == 0)
/* We are in a read phase now, so the least recent modification of
__wrphase_futex we can read from is the store by the writer
with value 1. Thus, only now we can assume that if we observe
a value of 0, explicit hand-over is finished. Retry the loop
above one more time. */
ready = true;
}
}
return 0;
}
static __always_inline void
__pthread_rwlock_wrunlock (pthread_rwlock_t *rwlock)
{
int private = __pthread_rwlock_get_private (rwlock);
atomic_store_relaxed (&rwlock->__data.__cur_writer, 0);
/* Disable waiting by writers. We will wake up after we decided how to
proceed. */
bool wake_writers = ((atomic_exchange_relaxed
(&rwlock->__data.__writers_futex, 0) & PTHREAD_RWLOCK_FUTEX_USED) != 0);
if (rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP)
{
/* First, try to hand over to another writer. */
unsigned int w = atomic_load_relaxed (&rwlock->__data.__writers);
while (w != 0)
{
/* Release MO so that another writer that gets WRLOCKED from us will
synchronize with us and thus can take over our view of
__readers (including, for example, whether we are in a write
phase or not). */
if (atomic_compare_exchange_weak_release (&rwlock->__data.__writers,
&w, w | PTHREAD_RWLOCK_WRHANDOVER))
/* Another writer will take over. */
goto done;
/* TODO Back-off. */
}
}
/* We have done everything we needed to do to prefer writers, so now we
either hand over explicitly to readers if there are any, or we simply
stay in a write phase. See pthread_rwlock_rdunlock for more details. */
unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
/* Release MO so that subsequent readers or writers synchronize with us. */
while (!atomic_compare_exchange_weak_release
(&rwlock->__data.__readers, &r, (r ^ PTHREAD_RWLOCK_WRLOCKED)
^ ((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0 ? 0
: PTHREAD_RWLOCK_WRPHASE)))
{
/* TODO Back-off. */
}
if ((r >> PTHREAD_RWLOCK_READER_SHIFT) != 0)
{
/* We must hand over explicitly through __wrphase_futex. Relaxed MO is
sufficient because it is just used to delay acquisition by a writer;
any other synchronizes-with relations that are necessary are
established through __readers. */
if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 0)
& PTHREAD_RWLOCK_FUTEX_USED) != 0)
futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private);
}
done:
/* We released WRLOCKED in some way, so wake a writer. */
if (wake_writers)
futex_wake (&rwlock->__data.__writers_futex, 1, private);
}
static __always_inline int
__pthread_rwlock_wrlock_full (pthread_rwlock_t *rwlock,
const struct timespec *abstime)
{
/* Make sure we are not holding the rwlock as a writer. This is a deadlock
situation we recognize and report. */
if (__glibc_unlikely (atomic_load_relaxed (&rwlock->__data.__cur_writer)
== THREAD_GETMEM (THREAD_SELF, tid)))
return EDEADLK;
/* First we try to acquire the role of primary writer by setting WRLOCKED;
if it was set before, there already is a primary writer. Acquire MO so
that we synchronize with previous primary writers.
We do not try to change to a write phase right away using a fetch_or
because we would have to reset it again and wake readers if there are
readers present (some readers could try to acquire the lock more than
once, so setting a write phase in the middle of this could cause
deadlock). Changing to a write phase eagerly would only speed up the
transition from a read phase to a write phase in the uncontended case,
but it would slow down the contended case if readers are preferred (which
is the default).
We could try to CAS from a state with no readers to a write phase, but
this could be less scalable if readers arrive and leave frequently. */
bool may_share_futex_used_flag = false;
unsigned int r = atomic_fetch_or_acquire (&rwlock->__data.__readers,
PTHREAD_RWLOCK_WRLOCKED);
if (__glibc_unlikely ((r & PTHREAD_RWLOCK_WRLOCKED) != 0))
{
/* There is another primary writer. */
bool prefer_writer =
(rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP);
if (prefer_writer)
{
/* We register as a waiting writer, so that we can make use of
writer--writer hand-over. Relaxed MO is fine because we just
want to register. We assume that the maximum number of threads
is less than the capacity in __writers. */
atomic_fetch_add_relaxed (&rwlock->__data.__writers, 1);
}
for (;;)
{
/* TODO Spin until WRLOCKED is 0 before trying the CAS below.
But pay attention to not delay trying writer--writer hand-over
for too long (which we must try eventually anyway). */
if ((r & PTHREAD_RWLOCK_WRLOCKED) == 0)
{
/* Try to become the primary writer or retry. Acquire MO as in
the fetch_or above. */
if (atomic_compare_exchange_weak_acquire
(&rwlock->__data.__readers, &r,
r | PTHREAD_RWLOCK_WRLOCKED))
{
if (prefer_writer)
{
/* Unregister as a waiting writer. Note that because we
acquired WRLOCKED, WRHANDOVER will not be set.
Acquire MO on the CAS above ensures that
unregistering happens after the previous writer;
this sorts the accesses to __writers by all
primary writers in a useful way (e.g., any other
primary writer acquiring after us or getting it from
us through WRHANDOVER will see both our changes to
__writers).
??? Perhaps this is not strictly necessary for
reasons we do not yet know of. */
atomic_fetch_add_relaxed (&rwlock->__data.__writers,
-1);
}
break;
}
/* Retry if the CAS fails (r will have been updated). */
continue;
}
/* If writer--writer hand-over is available, try to become the
primary writer this way by grabbing the WRHANDOVER token. If we
succeed, we own WRLOCKED. */
if (prefer_writer)
{
unsigned int w = atomic_load_relaxed
(&rwlock->__data.__writers);
if ((w & PTHREAD_RWLOCK_WRHANDOVER) != 0)
{
/* Acquire MO is required here so that we synchronize with
the writer that handed over WRLOCKED. We also need this
for the reload of __readers below because our view of
__readers must be at least as recent as the view of the
writer that handed over WRLOCKED; we must avoid an ABA
through WRHANDOVER, which could, for example, lead to us
assuming we are still in a write phase when in fact we
are not. */
if (atomic_compare_exchange_weak_acquire
(&rwlock->__data.__writers,
&w, (w - PTHREAD_RWLOCK_WRHANDOVER - 1)))
{
/* Reload so our view is consistent with the view of
the previous owner of WRLOCKED. See above. */
r = atomic_load_relaxed (&rwlock->__data.__readers);
break;
}
/* We do not need to reload __readers here. We should try
to perform writer--writer hand-over if possible; if it
is not possible anymore, we will reload __readers
elsewhere in this loop. */
continue;
}
}
/* We did not acquire WRLOCKED nor were able to use writer--writer
hand-over, so we block on __writers_futex. */
int private = __pthread_rwlock_get_private (rwlock);
unsigned int wf = atomic_load_relaxed
(&rwlock->__data.__writers_futex);
if (((wf & ~(unsigned int) PTHREAD_RWLOCK_FUTEX_USED) != 1)
|| ((wf != (1 | PTHREAD_RWLOCK_FUTEX_USED))
&& !atomic_compare_exchange_weak_relaxed
(&rwlock->__data.__writers_futex, &wf,
1 | PTHREAD_RWLOCK_FUTEX_USED)))
{
/* If we cannot block on __writers_futex because there is no
primary writer, or we cannot set PTHREAD_RWLOCK_FUTEX_USED,
we retry. We must reload __readers here in case we cannot
block on __writers_futex so that we can become the primary
writer and are not stuck in a loop that just continuously
fails to block on __writers_futex. */
r = atomic_load_relaxed (&rwlock->__data.__readers);
continue;
}
/* We set the flag that signals that the futex is used, or we could
have set it if we had been faster than other waiters. As a
result, we may share the flag with an unknown number of other
writers. Therefore, we must keep this flag set when we acquire
the lock. We do not need to do this when we do not reach this
point here because then we are not part of the group that may
share the flag, and another writer will wake one of the writers
in this group. */
may_share_futex_used_flag = true;
int err = futex_abstimed_wait (&rwlock->__data.__writers_futex,
1 | PTHREAD_RWLOCK_FUTEX_USED, abstime, private);
if (err == ETIMEDOUT)
{
if (prefer_writer)
{
/* We need to unregister as a waiting writer. If we are the
last writer and writer--writer hand-over is available,
we must make use of it because nobody else will reset
WRLOCKED otherwise. (If we use it, we simply pretend
that this happened before the timeout; see
pthread_rwlock_rdlock_full for the full reasoning.)
Also see the similar code above. */
unsigned int w = atomic_load_relaxed
(&rwlock->__data.__writers);
while (!atomic_compare_exchange_weak_acquire
(&rwlock->__data.__writers, &w,
(w == PTHREAD_RWLOCK_WRHANDOVER + 1 ? 0 : w - 1)))
{
/* TODO Back-off. */
}
if (w == PTHREAD_RWLOCK_WRHANDOVER + 1)
{
/* We must continue as primary writer. See above. */
r = atomic_load_relaxed (&rwlock->__data.__readers);
break;
}
}
/* We cleaned up and cannot have stolen another waiting writer's
futex wake-up, so just return. */
return ETIMEDOUT;
}
/* If we got interrupted (EINTR) or the futex word does not have the
expected value (EAGAIN), retry after reloading __readers. */
r = atomic_load_relaxed (&rwlock->__data.__readers);
}
/* Our snapshot of __readers is up-to-date at this point because we
either set WRLOCKED using a CAS or were handed over WRLOCKED from
another writer whose snapshot of __readers we inherit. */
}
/* If we are in a read phase and there are no readers, try to start a write
phase. */
while (((r & PTHREAD_RWLOCK_WRPHASE) == 0)
&& ((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0))
{
/* Acquire MO so that we synchronize with prior writers and do
not interfere with their updates to __writers_futex, as well
as regarding prior readers and their updates to __wrphase_futex,
respectively. */
if (atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers,
&r, r | PTHREAD_RWLOCK_WRPHASE))
{
/* We have started a write phase, so need to enable readers to wait.
See the similar case in__pthread_rwlock_rdlock_full. */
atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1);
/* Make sure we fall through to the end of the function. */
r |= PTHREAD_RWLOCK_WRPHASE;
break;
}
/* TODO Back-off. */
}
/* We are the primary writer; enable blocking on __writers_futex. Relaxed
MO is sufficient for futex words; acquire MO on the previous
modifications of __readers ensures that this store happens after the
store of value 0 by the previous primary writer. */
atomic_store_relaxed (&rwlock->__data.__writers_futex,
1 | (may_share_futex_used_flag ? PTHREAD_RWLOCK_FUTEX_USED : 0));
if (__glibc_unlikely ((r & PTHREAD_RWLOCK_WRPHASE) == 0))
{
/* We are not in a read phase and there are readers (because of the
previous loop). Thus, we have to wait for explicit hand-over from
one of these readers.
We basically do the same steps as for the similar case in
__pthread_rwlock_rdlock_full, except that we additionally might try
to directly hand over to another writer and need to wake up
other writers or waiting readers (i.e., PTHREAD_RWLOCK_RWAITING). */
unsigned int wpf;
bool ready = false;
for (;;)
{
while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex))
| PTHREAD_RWLOCK_FUTEX_USED) == PTHREAD_RWLOCK_FUTEX_USED)
{
int private = __pthread_rwlock_get_private (rwlock);
if (((wpf & PTHREAD_RWLOCK_FUTEX_USED) == 0)
&& !atomic_compare_exchange_weak_relaxed
(&rwlock->__data.__wrphase_futex, &wpf,
PTHREAD_RWLOCK_FUTEX_USED))
continue;
int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex,
PTHREAD_RWLOCK_FUTEX_USED, abstime, private);
if (err == ETIMEDOUT)
{
if (rwlock->__data.__flags
!= PTHREAD_RWLOCK_PREFER_READER_NP)
{
/* We try writer--writer hand-over. */
unsigned int w = atomic_load_relaxed
(&rwlock->__data.__writers);
if (w != 0)
{
/* We are about to hand over WRLOCKED, so we must
release __writers_futex too; otherwise, we'd have
a pending store, which could at least prevent
other threads from waiting using the futex
because it could interleave with the stores
by subsequent writers. In turn, this means that
we have to clean up when we do not hand over
WRLOCKED.
Release MO so that another writer that gets
WRLOCKED from us can take over our view of
__readers. */
unsigned int wf = atomic_exchange_relaxed
(&rwlock->__data.__writers_futex, 0);
while (w != 0)
{
if (atomic_compare_exchange_weak_release
(&rwlock->__data.__writers, &w,
w | PTHREAD_RWLOCK_WRHANDOVER))
{
/* Wake other writers. */
if ((wf & PTHREAD_RWLOCK_FUTEX_USED) != 0)
futex_wake
(&rwlock->__data.__writers_futex, 1,
private);
return ETIMEDOUT;
}
/* TODO Back-off. */
}
/* We still own WRLOCKED and someone else might set
a write phase concurrently, so enable waiting
again. Make sure we don't loose the flag that
signals whether there are threads waiting on
this futex. */
atomic_store_relaxed
(&rwlock->__data.__writers_futex, wf);
}
}
/* If we timed out and we are not in a write phase, we can
just stop being a primary writer. Otherwise, we just
acquire the lock. */
r = atomic_load_relaxed (&rwlock->__data.__readers);
if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
{
/* We are about to release WRLOCKED, so we must release
__writers_futex too; see the handling of
writer--writer hand-over above. */
unsigned int wf = atomic_exchange_relaxed
(&rwlock->__data.__writers_futex, 0);
while ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
{
/* While we don't need to make anything from a
caller's critical section visible to other
threads, we need to ensure that our changes to
__writers_futex are properly ordered.
Therefore, use release MO to synchronize with
subsequent primary writers. Also wake up any
waiting readers as they are waiting because of
us. */
if (atomic_compare_exchange_weak_release
(&rwlock->__data.__readers, &r,
(r ^ PTHREAD_RWLOCK_WRLOCKED)
& ~(unsigned int) PTHREAD_RWLOCK_RWAITING))
{
/* Wake other writers. */
if ((wf & PTHREAD_RWLOCK_FUTEX_USED) != 0)
futex_wake (&rwlock->__data.__writers_futex,
1, private);
/* Wake waiting readers. */
if ((r & PTHREAD_RWLOCK_RWAITING) != 0)
futex_wake (&rwlock->__data.__readers,
INT_MAX, private);
return ETIMEDOUT;
}
}
/* We still own WRLOCKED and someone else might set a
write phase concurrently, so enable waiting again.
Make sure we don't loose the flag that signals
whether there are threads waiting on this futex. */
atomic_store_relaxed (&rwlock->__data.__writers_futex,
wf);
}
/* Use the acquire MO fence to mirror the steps taken in the
non-timeout case. Note that the read can happen both
in the atomic_load above as well as in the failure case
of the CAS operation. */
atomic_thread_fence_acquire ();
/* We still need to wait for explicit hand-over, but we must
not use futex_wait anymore. */
while ((atomic_load_relaxed
(&rwlock->__data.__wrphase_futex)
| PTHREAD_RWLOCK_FUTEX_USED)
== PTHREAD_RWLOCK_FUTEX_USED)
{
/* TODO Back-off. */
}
ready = true;
break;
}
/* If we got interrupted (EINTR) or the futex word does not have
the expected value (EAGAIN), retry. */
}
/* See pthread_rwlock_rdlock_full. */
if (ready)
break;
if ((atomic_load_acquire (&rwlock->__data.__readers)
& PTHREAD_RWLOCK_WRPHASE) != 0)
ready = true;
}
}
atomic_store_relaxed (&rwlock->__data.__cur_writer,
THREAD_GETMEM (THREAD_SELF, tid));
return 0;
}

25
nptl/pthread_rwlock_init.c
View File

@ -18,7 +18,6 @@
#include "pthreadP.h" #include "pthreadP.h"
#include <string.h> #include <string.h>
#include <kernel-features.h>
static const struct pthread_rwlockattr default_rwlockattr = static const struct pthread_rwlockattr default_rwlockattr =
@ -28,6 +27,7 @@ static const struct pthread_rwlockattr default_rwlockattr =
}; };
/* See pthread_rwlock_common.c. */
int int
__pthread_rwlock_init (pthread_rwlock_t *rwlock, __pthread_rwlock_init (pthread_rwlock_t *rwlock,
const pthread_rwlockattr_t *attr) const pthread_rwlockattr_t *attr)
@ -38,27 +38,10 @@ __pthread_rwlock_init (pthread_rwlock_t *rwlock,
memset (rwlock, '\0', sizeof (*rwlock)); memset (rwlock, '\0', sizeof (*rwlock));
rwlock->__data.__flags rwlock->__data.__flags = iattr->lockkind;
= iattr->lockkind == PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP;
/* The __SHARED field is computed to minimize the work that needs to /* The value of __SHARED in a private rwlock must be zero. */
be done while handling the futex. There are two inputs: the rwlock->__data.__shared = (iattr->pshared != PTHREAD_PROCESS_PRIVATE);
availability of private futexes and whether the rwlock is shared
or private. Unfortunately the value of a private rwlock is
fixed: it must be zero. The PRIVATE_FUTEX flag has the value
0x80 in case private futexes are available and zero otherwise.
This leads to the following table:
| pshared | result
| shared private | shared private |
------------+-----------------+-----------------+
!avail 0 | 0 0 | 0 0 |
avail 0x80 | 0x80 0 | 0 0x80 |
If the pshared value is in locking functions XORed with avail
we get the expected result. */
rwlock->__data.__shared = (iattr->pshared == PTHREAD_PROCESS_PRIVATE
? 0 : FUTEX_PRIVATE_FLAG);
return 0; return 0;
} }

158
nptl/pthread_rwlock_rdlock.c
View File

@ -16,165 +16,17 @@
License along with the GNU C Library; if not, see License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */ <http://www.gnu.org/licenses/>. */
#include <errno.h> #include "pthread_rwlock_common.c"
#include <sysdep.h>
#include <lowlevellock.h>
#include <futex-internal.h>
#include <pthread.h>
#include <pthreadP.h>
#include <stap-probe.h>
#include <elide.h>
#include <stdbool.h>
/* Acquire read lock for RWLOCK. Slow path. */
static int __attribute__((noinline))
__pthread_rwlock_rdlock_slow (pthread_rwlock_t *rwlock)
{
int result = 0;
bool wake = false;
int futex_shared =
rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED;
/* Lock is taken in caller. */
while (1)
{
/* Make sure we are not holding the rwlock as a writer. This is
a deadlock situation we recognize and report. */
if (__builtin_expect (rwlock->__data.__writer
== THREAD_GETMEM (THREAD_SELF, tid), 0))
{
result = EDEADLK;
break;
}
/* Remember that we are a reader. */
if (__glibc_unlikely (++rwlock->__data.__nr_readers_queued == 0))
{
/* Overflow on number of queued readers. */
--rwlock->__data.__nr_readers_queued;
result = EAGAIN;
break;
}
int waitval = rwlock->__data.__readers_wakeup;
/* Free the lock. */
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
/* Wait for the writer to finish. We do not check the return value
because we decide how to continue based on the state of the rwlock. */
futex_wait_simple (&rwlock->__data.__readers_wakeup, waitval,
futex_shared);
/* Get the lock. */
lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
--rwlock->__data.__nr_readers_queued;
/* Get the rwlock if there is no writer... */
if (rwlock->__data.__writer == 0
/* ...and if either no writer is waiting or we prefer readers. */
&& (!rwlock->__data.__nr_writers_queued
|| PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
{
/* Increment the reader counter. Avoid overflow. */
if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
{
/* Overflow on number of readers. */
--rwlock->__data.__nr_readers;
result = EAGAIN;
}
else
{
LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
/* See pthread_rwlock_rdlock. */
if (rwlock->__data.__nr_readers == 1
&& rwlock->__data.__nr_readers_queued > 0
&& rwlock->__data.__nr_writers_queued > 0)
{
++rwlock->__data.__readers_wakeup;
wake = true;
}
}
break;
}
}
/* We are done, free the lock. */
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
if (wake)
futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX, futex_shared);
return result;
}
/* Fast path of acquiring read lock on RWLOCK. */
/* See pthread_rwlock_common.c. */
int int
__pthread_rwlock_rdlock (pthread_rwlock_t *rwlock) __pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
{ {
int result = 0;
bool wake = false;
int futex_shared =
rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED;
LIBC_PROBE (rdlock_entry, 1, rwlock); LIBC_PROBE (rdlock_entry, 1, rwlock);
if (ELIDE_LOCK (rwlock->__data.__rwelision, int result = __pthread_rwlock_rdlock_full (rwlock, NULL);
rwlock->__data.__lock == 0 LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
&& rwlock->__data.__writer == 0 return result;
&& rwlock->__data.__nr_readers == 0))
return 0;
/* Make sure we are alone. */
lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
/* Get the rwlock if there is no writer... */
if (rwlock->__data.__writer == 0
/* ...and if either no writer is waiting or we prefer readers. */
&& (!rwlock->__data.__nr_writers_queued
|| PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
{
/* Increment the reader counter. Avoid overflow. */
if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
{
/* Overflow on number of readers. */
--rwlock->__data.__nr_readers;
result = EAGAIN;
}
else
{
LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
/* If we are the first reader, and there are blocked readers and
writers (which we don't prefer, see above), then it can be the
case that we stole the lock from a writer that was already woken
to acquire it. That means that we need to take over the writer's
responsibility to wake all readers (see pthread_rwlock_unlock).
Thus, wake all readers in this case. */
if (rwlock->__data.__nr_readers == 1
&& rwlock->__data.__nr_readers_queued > 0
&& rwlock->__data.__nr_writers_queued > 0)
{
++rwlock->__data.__readers_wakeup;
wake = true;
}
}
/* We are done, free the lock. */
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
if (wake)
futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX, futex_shared);
return result;
}
return __pthread_rwlock_rdlock_slow (rwlock);
} }
weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock) weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)

125
nptl/pthread_rwlock_timedrdlock.c
View File

@ -16,121 +16,22 @@
License along with the GNU C Library; if not, see License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */ <http://www.gnu.org/licenses/>. */
#include <errno.h> #include "pthread_rwlock_common.c"
#include <sysdep.h>
#include <lowlevellock.h>
#include <futex-internal.h>
#include <pthread.h>
#include <pthreadP.h>
#include <sys/time.h>
#include <stdbool.h>
/* See pthread_rwlock_common.c. */
/* Try to acquire read lock for RWLOCK or return after specfied time. */
int int
pthread_rwlock_timedrdlock (pthread_rwlock_t *rwlock, pthread_rwlock_timedrdlock (pthread_rwlock_t *rwlock,
const struct timespec *abstime) const struct timespec *abstime)
{ {
int result = 0; /* Make sure the passed in timeout value is valid. Note that the previous
bool wake = false; implementation assumed that this check *must* not be performed if there
int futex_shared = would in fact be no blocking; however, POSIX only requires that "the
rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED; validity of the abstime parameter need not be checked if the lock can be
immediately acquired" (i.e., we need not but may check it). */
/* ??? Just move this to __pthread_rwlock_rdlock_full? */
if (__glibc_unlikely (abstime->tv_nsec >= 1000000000
|| abstime->tv_nsec < 0))
return EINVAL;
/* Make sure we are alone. */ return __pthread_rwlock_rdlock_full (rwlock, abstime);
lll_lock(rwlock->__data.__lock, rwlock->__data.__shared);
while (1)
{
int err;
/* Get the rwlock if there is no writer... */
if (rwlock->__data.__writer == 0
/* ...and if either no writer is waiting or we prefer readers. */
&& (!rwlock->__data.__nr_writers_queued
|| PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
{
/* Increment the reader counter. Avoid overflow. */
if (++rwlock->__data.__nr_readers == 0)
{
/* Overflow on number of readers. */
--rwlock->__data.__nr_readers;
result = EAGAIN;
}
else
{
/* See pthread_rwlock_rdlock. */
if (rwlock->__data.__nr_readers == 1
&& rwlock->__data.__nr_readers_queued > 0
&& rwlock->__data.__nr_writers_queued > 0)
{
++rwlock->__data.__readers_wakeup;
wake = true;
}
}
break;
}
/* Make sure we are not holding the rwlock as a writer. This is
a deadlock situation we recognize and report. */
if (__builtin_expect (rwlock->__data.__writer
== THREAD_GETMEM (THREAD_SELF, tid), 0))
{
result = EDEADLK;
break;
}
/* Make sure the passed in timeout value is valid. Ideally this
test would be executed once. But since it must not be
performed if we would not block at all simply moving the test
to the front is no option. Replicating all the code is
costly while this test is not. */
if (__builtin_expect (abstime->tv_nsec >= 1000000000
|| abstime->tv_nsec < 0, 0))
{
result = EINVAL;
break;
}
/* Remember that we are a reader. */
if (++rwlock->__data.__nr_readers_queued == 0)
{
/* Overflow on number of queued readers. */
--rwlock->__data.__nr_readers_queued;
result = EAGAIN;
break;
}
int waitval = rwlock->__data.__readers_wakeup;
/* Free the lock. */
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
/* Wait for the writer to finish. We handle ETIMEDOUT below; on other
return values, we decide how to continue based on the state of the
rwlock. */
err = futex_abstimed_wait (&rwlock->__data.__readers_wakeup, waitval,
abstime, futex_shared);
/* Get the lock. */
lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
--rwlock->__data.__nr_readers_queued;
/* Did the futex call time out? */
if (err == ETIMEDOUT)
{
/* Yep, report it. */
result = ETIMEDOUT;
break;
}
}
/* We are done, free the lock. */
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
if (wake)
futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX, futex_shared);
return result;
} }

125
nptl/pthread_rwlock_timedwrlock.c
View File

@ -16,121 +16,22 @@
License along with the GNU C Library; if not, see License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */ <http://www.gnu.org/licenses/>. */
#include <errno.h> #include "pthread_rwlock_common.c"
#include <sysdep.h>
#include <lowlevellock.h>
#include <futex-internal.h>
#include <pthread.h>
#include <pthreadP.h>
#include <sys/time.h>
#include <stdbool.h>
/* See pthread_rwlock_common.c. */
/* Try to acquire write lock for RWLOCK or return after specfied time. */
int int
pthread_rwlock_timedwrlock (pthread_rwlock_t *rwlock, pthread_rwlock_timedwrlock (pthread_rwlock_t *rwlock,
const struct timespec *abstime) const struct timespec *abstime)
{ {
int result = 0; /* Make sure the passed in timeout value is valid. Note that the previous
bool wake_readers = false; implementation assumed that this check *must* not be performed if there
int futex_shared = would in fact be no blocking; however, POSIX only requires that "the
rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED; validity of the abstime parameter need not be checked if the lock can be
immediately acquired" (i.e., we need not but may check it). */
/* ??? Just move this to __pthread_rwlock_wrlock_full? */
if (__glibc_unlikely (abstime->tv_nsec >= 1000000000
|| abstime->tv_nsec < 0))
return EINVAL;
/* Make sure we are alone. */ return __pthread_rwlock_wrlock_full (rwlock, abstime);
lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
while (1)
{
int err;
/* Get the rwlock if there is no writer and no reader. */
if (rwlock->__data.__writer == 0 && rwlock->__data.__nr_readers == 0)
{
/* Mark self as writer. */
rwlock->__data.__writer = THREAD_GETMEM (THREAD_SELF, tid);
break;
}
/* Make sure we are not holding the rwlock as a writer. This is
a deadlock situation we recognize and report. */
if (__builtin_expect (rwlock->__data.__writer
== THREAD_GETMEM (THREAD_SELF, tid), 0))
{
result = EDEADLK;
break;
}
/* Make sure the passed in timeout value is valid. Ideally this
test would be executed once. But since it must not be
performed if we would not block at all simply moving the test
to the front is no option. Replicating all the code is
costly while this test is not. */
if (__builtin_expect (abstime->tv_nsec >= 1000000000
|| abstime->tv_nsec < 0, 0))
{
result = EINVAL;
break;
}
/* Remember that we are a writer. */
if (++rwlock->__data.__nr_writers_queued == 0)
{
/* Overflow on number of queued writers. */
--rwlock->__data.__nr_writers_queued;
result = EAGAIN;
break;
}
int waitval = rwlock->__data.__writer_wakeup;
/* Free the lock. */
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
/* Wait for the writer or reader(s) to finish. We handle ETIMEDOUT
below; on other return values, we decide how to continue based on
the state of the rwlock. */
err = futex_abstimed_wait (&rwlock->__data.__writer_wakeup, waitval,
abstime, futex_shared);
/* Get the lock. */
lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
/* To start over again, remove the thread from the writer list. */
--rwlock->__data.__nr_writers_queued;
/* Did the futex call time out? */
if (err == ETIMEDOUT)
{
result = ETIMEDOUT;
/* If we prefer writers, it can have happened that readers blocked
for us to acquire the lock first. If we have timed out, we need
to wake such readers if there are any, and if there is no writer
currently (otherwise, the writer will take care of wake-up).
Likewise, even if we prefer readers, we can be responsible for
wake-up (see pthread_rwlock_unlock) if no reader or writer has
acquired the lock. We have timed out and thus not consumed a
futex wake-up; therefore, if there is no other blocked writer
that would consume the wake-up and thus take over responsibility,
we need to wake blocked readers. */
if ((!PTHREAD_RWLOCK_PREFER_READER_P (rwlock)
|| ((rwlock->__data.__nr_readers == 0)
&& (rwlock->__data.__nr_writers_queued == 0)))
&& (rwlock->__data.__nr_readers_queued > 0)
&& (rwlock->__data.__writer == 0))
{
++rwlock->__data.__readers_wakeup;
wake_readers = true;
}
break;
}
}
/* We are done, free the lock. */
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
/* Might be required after timeouts. */
if (wake_readers)
futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX, futex_shared);
return result;
} }

105
nptl/pthread_rwlock_tryrdlock.c
View File

@ -18,56 +18,95 @@
#include <errno.h> #include <errno.h>
#include "pthreadP.h" #include "pthreadP.h"
#include <lowlevellock.h> #include <atomic.h>
#include <futex-internal.h>
#include <elide.h>
#include <stdbool.h> #include <stdbool.h>
#include "pthread_rwlock_common.c"
/* See pthread_rwlock_common.c for an overview. */
int int
__pthread_rwlock_tryrdlock (pthread_rwlock_t *rwlock) __pthread_rwlock_tryrdlock (pthread_rwlock_t *rwlock)
{ {
int result = EBUSY; /* For tryrdlock, we could speculate that we will succeed and go ahead and
bool wake = false; register as a reader. However, if we misspeculate, we have to do the
int futex_shared = same steps as a timed-out rdlock, which will increase contention.
rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED; Therefore, there is a trade-off between being able to use a combinable
read-modify-write operation and a CAS loop as used below; we pick the
if (ELIDE_TRYLOCK (rwlock->__data.__rwelision, latter because it simplifies the code, and should perform better when
rwlock->__data.__lock == 0 tryrdlock is used in cases where writers are infrequent.
&& rwlock->__data.__nr_readers == 0 Because POSIX does not require a failed trylock to "synchronize memory",
&& rwlock->__data.__writer, 0)) relaxed MO is sufficient here and on the failure path of the CAS
return 0; below. */
unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
lll_lock (rwlock->__data.__lock, rwlock->__data.__shared); unsigned int rnew;
do
if (rwlock->__data.__writer == 0
&& (rwlock->__data.__nr_writers_queued == 0
|| PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
{ {
if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0)) if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
{ {
--rwlock->__data.__nr_readers; /* If we are in a read phase, try to acquire unless there is a
result = EAGAIN; primary writer and we prefer writers and there will be no
recursive read locks. */
if (((r & PTHREAD_RWLOCK_WRLOCKED) != 0)
&& (rwlock->__data.__flags
== PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP))
return EBUSY;
rnew = r + (1 << PTHREAD_RWLOCK_READER_SHIFT);
} }
else else
{ {
result = 0; /* If there is a writer that has acquired the lock and we are in
/* See pthread_rwlock_rdlock. */ a write phase, fail. */
if (rwlock->__data.__nr_readers == 1 if ((r & PTHREAD_RWLOCK_WRLOCKED) != 0)
&& rwlock->__data.__nr_readers_queued > 0 return EBUSY;
&& rwlock->__data.__nr_writers_queued > 0) else
{ {
++rwlock->__data.__readers_wakeup; /* If we do not care about potentially waiting writers, just
wake = true; try to acquire. */
rnew = (r + (1 << PTHREAD_RWLOCK_READER_SHIFT))
^ PTHREAD_RWLOCK_WRPHASE;
} }
} }
/* If we could have caused an overflow or take effect during an
overflow, we just can / need to return EAGAIN. There is no need to
have actually modified the number of readers because we could have
done that and cleaned up immediately. */
if (rnew >= PTHREAD_RWLOCK_READER_OVERFLOW)
return EAGAIN;
}
/* If the CAS fails, we retry; this prevents that tryrdlock fails spuriously
(i.e., fails to acquire the lock although there is no writer), which is
fine for C++14 but not currently allowed by POSIX.
However, because tryrdlock must not appear to block, we should avoid
starving this CAS loop due to constant changes to __readers:
While normal rdlock readers that won't be able to acquire will just block
(and we expect timeouts on timedrdlock to be longer than one retry of the
CAS loop), we can have concurrently failing tryrdlock calls due to
readers or writers that acquire and release in the meantime. Using
randomized exponential back-off to make a live-lock unlikely should be
sufficient.
TODO Back-off.
Acquire MO so we synchronize with prior writers. */
while (!atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers,
&r, rnew));
if ((r & PTHREAD_RWLOCK_WRPHASE) != 0)
{
/* Same as in __pthread_rwlock_rdlock_full:
We started the read phase, so we are also responsible for
updating the write-phase futex. Relaxed MO is sufficient.
Note that there can be no other reader that we have to wake
because all other readers will see the read phase started by us
(or they will try to start it themselves); if a writer started
the read phase, we cannot have started it. Furthermore, we
cannot discard a PTHREAD_RWLOCK_FUTEX_USED flag because we will
overwrite the value set by the most recent writer (or the readers
before it in case of explicit hand-over) and we know that there
are no waiting readers. */
atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 0);
} }
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared); return 0;
if (wake)
futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX, futex_shared);
return result;
} }
strong_alias (__pthread_rwlock_tryrdlock, pthread_rwlock_tryrdlock) strong_alias (__pthread_rwlock_tryrdlock, pthread_rwlock_tryrdlock)

53
nptl/pthread_rwlock_trywrlock.c
View File

@ -18,31 +18,44 @@
#include <errno.h> #include <errno.h>
#include "pthreadP.h" #include "pthreadP.h"
#include <lowlevellock.h> #include <atomic.h>
#include <elide.h>
/* See pthread_rwlock_common.c for an overview. */
int int
__pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock) __pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock)
{ {
int result = EBUSY; /* When in a trywrlock, we can acquire the write lock if it is in states
#1 (idle and read phase) and #5 (idle and write phase), and also in #6
if (ELIDE_TRYLOCK (rwlock->__data.__rwelision, (readers waiting, write phase) if we prefer writers.
rwlock->__data.__lock == 0 If we observe any other state, we are allowed to fail and do not need to
&& rwlock->__data.__nr_readers == 0 "synchronize memory" as specified by POSIX (hence relaxed MO is
&& rwlock->__data.__writer, 1)) sufficient for the first load and the CAS failure path).
return 0; We face a similar issue as in tryrdlock in that we need to both avoid
live-locks / starvation and must not fail spuriously (see there for
lll_lock (rwlock->__data.__lock, rwlock->__data.__shared); further comments) -- and thus must loop until we get a definitive
observation or state change. */
if (rwlock->__data.__writer == 0 && rwlock->__data.__nr_readers == 0) unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
bool prefer_writer =
(rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP);
while (((r & PTHREAD_RWLOCK_WRLOCKED) == 0)
&& (((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0)
|| (prefer_writer && ((r & PTHREAD_RWLOCK_WRPHASE) != 0))))
{ {
rwlock->__data.__writer = THREAD_GETMEM (THREAD_SELF, tid); /* Try to transition to states #7 or #8 (i.e., acquire the lock). */
result = 0; if (atomic_compare_exchange_weak_acquire (
&rwlock->__data.__readers, &r,
r | PTHREAD_RWLOCK_WRPHASE | PTHREAD_RWLOCK_WRLOCKED))
{
atomic_store_relaxed (&rwlock->__data.__writers_futex, 1);
atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1);
atomic_store_relaxed (&rwlock->__data.__cur_writer,
THREAD_GETMEM (THREAD_SELF, tid));
return 0;
}
/* TODO Back-off. */
/* See above. */
} }
return EBUSY;
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
return result;
} }
strong_alias (__pthread_rwlock_trywrlock, pthread_rwlock_trywrlock) strong_alias (__pthread_rwlock_trywrlock, pthread_rwlock_trywrlock)

53
nptl/pthread_rwlock_unlock.c
View File

@ -18,60 +18,29 @@
#include <errno.h> #include <errno.h>
#include <sysdep.h> #include <sysdep.h>
#include <lowlevellock.h>
#include <futex-internal.h> #include <futex-internal.h>
#include <pthread.h> #include <pthread.h>
#include <pthreadP.h> #include <pthreadP.h>
#include <stap-probe.h> #include <stap-probe.h>
#include <elide.h>
#include "pthread_rwlock_common.c"
/* Unlock RWLOCK. */ /* See pthread_rwlock_common.c for an overview. */
int int
__pthread_rwlock_unlock (pthread_rwlock_t *rwlock) __pthread_rwlock_unlock (pthread_rwlock_t *rwlock)
{ {
int futex_shared =
rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED;
LIBC_PROBE (rwlock_unlock, 1, rwlock); LIBC_PROBE (rwlock_unlock, 1, rwlock);
/* Trying to elide an unlocked lock may crash the process. This /* We distinguish between having acquired a read vs. a write lock by looking
is expected and is compatible with POSIX.1-2008: "results are at the writer TID. If it's equal to our TID, we must be the writer
undefined if the read-write lock rwlock is not held by the because nobody else can have stored this value. Also, if we are a
calling thread". */ reader, we will read from the wrunlock store with value 0 by the most
if (ELIDE_UNLOCK (rwlock->__data.__writer == 0 recent writer because that writer happens-before us. */
&& rwlock->__data.__nr_readers == 0)) if (atomic_load_relaxed (&rwlock->__data.__cur_writer)
return 0; == THREAD_GETMEM (THREAD_SELF, tid))
__pthread_rwlock_wrunlock (rwlock);
lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
if (rwlock->__data.__writer)
rwlock->__data.__writer = 0;
else else
--rwlock->__data.__nr_readers; __pthread_rwlock_rdunlock (rwlock);
/* If there are still readers present, we do not yet need to wake writers
nor are responsible to wake any readers. */
if (rwlock->__data.__nr_readers == 0)
{
/* Note that if there is a blocked writer, we effectively make it
responsible for waking any readers because we don't wake readers in
this case. */
if (rwlock->__data.__nr_writers_queued)
{
++rwlock->__data.__writer_wakeup;
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
futex_wake (&rwlock->__data.__writer_wakeup, 1, futex_shared);
return 0;
}
else if (rwlock->__data.__nr_readers_queued)
{
++rwlock->__data.__readers_wakeup;
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX,
futex_shared);
return 0;
}
}
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
return 0; return 0;
} }

106
nptl/pthread_rwlock_wrlock.c
View File

@ -16,114 +16,18 @@
License along with the GNU C Library; if not, see License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */ <http://www.gnu.org/licenses/>. */
#include <errno.h> #include "pthread_rwlock_common.c"
#include <sysdep.h>
#include <lowlevellock.h>
#include <futex-internal.h>
#include <pthread.h>
#include <pthreadP.h>
#include <stap-probe.h>
#include <elide.h>
/* Acquire write lock for RWLOCK. */
static int __attribute__((noinline))
__pthread_rwlock_wrlock_slow (pthread_rwlock_t *rwlock)
{
int result = 0;
int futex_shared =
rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED;
/* Caller has taken the lock. */
while (1)
{
/* Make sure we are not holding the rwlock as a writer. This is
a deadlock situation we recognize and report. */
if (__builtin_expect (rwlock->__data.__writer
== THREAD_GETMEM (THREAD_SELF, tid), 0))
{
result = EDEADLK;
break;
}
/* Remember that we are a writer. */
if (++rwlock->__data.__nr_writers_queued == 0)
{
/* Overflow on number of queued writers. */
--rwlock->__data.__nr_writers_queued;
result = EAGAIN;
break;
}
int waitval = rwlock->__data.__writer_wakeup;
/* Free the lock. */
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
/* Wait for the writer or reader(s) to finish. We do not check the
return value because we decide how to continue based on the state of
the rwlock. */
futex_wait_simple (&rwlock->__data.__writer_wakeup, waitval,
futex_shared);
/* Get the lock. */
lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
/* To start over again, remove the thread from the writer list. */
--rwlock->__data.__nr_writers_queued;
/* Get the rwlock if there is no writer and no reader. */
if (rwlock->__data.__writer == 0 && rwlock->__data.__nr_readers == 0)
{
/* Mark self as writer. */
rwlock->__data.__writer = THREAD_GETMEM (THREAD_SELF, tid);
LIBC_PROBE (wrlock_acquire_write, 1, rwlock);
break;
}
}
/* We are done, free the lock. */
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
return result;
}
/* Fast path of acquiring write lock for RWLOCK. */
/* See pthread_rwlock_common.c. */
int int
__pthread_rwlock_wrlock (pthread_rwlock_t *rwlock) __pthread_rwlock_wrlock (pthread_rwlock_t *rwlock)
{ {
LIBC_PROBE (wrlock_entry, 1, rwlock); LIBC_PROBE (wrlock_entry, 1, rwlock);
if (ELIDE_LOCK (rwlock->__data.__rwelision, int result = __pthread_rwlock_wrlock_full (rwlock, NULL);
rwlock->__data.__lock == 0 LIBC_PROBE (wrlock_acquire_write, 1, rwlock);
&& rwlock->__data.__writer == 0 return result;
&& rwlock->__data.__nr_readers == 0))
return 0;
/* Make sure we are alone. */
lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
/* Get the rwlock if there is no writer and no reader. */
if (__glibc_likely((rwlock->__data.__writer |
rwlock->__data.__nr_readers) == 0))
{
/* Mark self as writer. */
rwlock->__data.__writer = THREAD_GETMEM (THREAD_SELF, tid);
LIBC_PROBE (wrlock_acquire_write, 1, rwlock);
/* We are done, free the lock. */
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
return 0;
}
return __pthread_rwlock_wrlock_slow (rwlock);
} }
weak_alias (__pthread_rwlock_wrlock, pthread_rwlock_wrlock) weak_alias (__pthread_rwlock_wrlock, pthread_rwlock_wrlock)
hidden_def (__pthread_rwlock_wrlock) hidden_def (__pthread_rwlock_wrlock)

8
nptl/test-rwlock-printers.py
View File

@ -35,9 +35,9 @@ try:
break_at(test_source, 'Test locking (reader)') break_at(test_source, 'Test locking (reader)')
continue_cmd() # Go to test_locking_reader continue_cmd() # Go to test_locking_reader
test_printer(var, to_string, {'Status': 'Unlocked'}) test_printer(var, to_string, {'Status': 'Not acquired'})
next_cmd() next_cmd()
test_printer(var, to_string, {'Status': r'Locked \(Read\)', 'Readers': '1'}) test_printer(var, to_string, {'Status': r'Acquired \(Read\)', 'Readers': '1'})
next_cmd() next_cmd()
test_printer(var, to_string, {'Readers': '2'}) test_printer(var, to_string, {'Readers': '2'})
next_cmd() next_cmd()
@ -45,10 +45,10 @@ try:
break_at(test_source, 'Test locking (writer)') break_at(test_source, 'Test locking (writer)')
continue_cmd() # Go to test_locking_writer continue_cmd() # Go to test_locking_writer
test_printer(var, to_string, {'Status': 'Unlocked'}) test_printer(var, to_string, {'Status': 'Not acquired'})
next_cmd() next_cmd()
thread_id = get_current_thread_lwpid() thread_id = get_current_thread_lwpid()
test_printer(var, to_string, {'Status': r'Locked \(Write\)', test_printer(var, to_string, {'Status': r'Acquired \(Write\)',
'Writer ID': thread_id}) 'Writer ID': thread_id})
continue_cmd() # Exit continue_cmd() # Exit

2
nptl/test-rwlockattr-printers.c
View File

@ -74,6 +74,8 @@ test_setkind_np (pthread_rwlock_t *rwlock, pthread_rwlockattr_t *attr)
int result = FAIL; int result = FAIL;
if (SET_KIND (attr, PTHREAD_RWLOCK_PREFER_READER_NP) == 0 /* Set kind. */ if (SET_KIND (attr, PTHREAD_RWLOCK_PREFER_READER_NP) == 0 /* Set kind. */
&& rwlock_reinit (rwlock, attr) == PASS
&& SET_KIND (attr, PTHREAD_RWLOCK_PREFER_WRITER_NP) == 0
&& rwlock_reinit (rwlock, attr) == PASS && rwlock_reinit (rwlock, attr) == PASS
&& SET_KIND (attr, PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP) == 0 && SET_KIND (attr, PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP) == 0
&& rwlock_reinit (rwlock, attr) == PASS) && rwlock_reinit (rwlock, attr) == PASS)

3
nptl/test-rwlockattr-printers.py
View File

@ -46,6 +46,9 @@ try:
next_cmd(2) next_cmd(2)
test_printer(rwlock_var, rwlock_to_string, {'Prefers': 'Writers'}) test_printer(rwlock_var, rwlock_to_string, {'Prefers': 'Writers'})
test_printer(attr_var, attr_to_string, {'Prefers': 'Writers'}) test_printer(attr_var, attr_to_string, {'Prefers': 'Writers'})
next_cmd(2)
test_printer(rwlock_var, rwlock_to_string, {'Prefers': 'Writers no recursive readers'})
test_printer(attr_var, attr_to_string, {'Prefers': 'Writers no recursive readers'})
break_at(test_source, 'Set shared') break_at(test_source, 'Set shared')
continue_cmd() # Go to test_setpshared continue_cmd() # Go to test_setpshared

2
nptl/tst-rwlock10.c
View File

@ -16,5 +16,5 @@
License along with the GNU C Library; see the file COPYING.LIB. If License along with the GNU C Library; see the file COPYING.LIB. If
not, see <http://www.gnu.org/licenses/>. */ not, see <http://www.gnu.org/licenses/>. */
#define INIT PTHREAD_RWLOCK_INITIALIZER #define KIND PTHREAD_RWLOCK_PREFER_READER_NP
#include "tst-rwlock8.c" #include "tst-rwlock8.c"

2
nptl/tst-rwlock11.c
View File

@ -16,5 +16,5 @@
License along with the GNU C Library; see the file COPYING.LIB. If License along with the GNU C Library; see the file COPYING.LIB. If
not, see <http://www.gnu.org/licenses/>. */ not, see <http://www.gnu.org/licenses/>. */
#define INIT PTHREAD_RWLOCK_INITIALIZER #define KIND PTHREAD_RWLOCK_PREFER_READER_NP
#include "tst-rwlock9.c" #include "tst-rwlock9.c"

19
nptl/tst-rwlock17.c Normal file
View File

@ -0,0 +1,19 @@
/* Test program for timedout read/write lock functions.
Copyright (C) 2016-2017 Free Software Foundation, Inc.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public License as
published by the Free Software Foundation; either version 2.1 of the
License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; see the file COPYING.LIB. If
not, see <http://www.gnu.org/licenses/>. */
#define KIND PTHREAD_RWLOCK_PREFER_WRITER_NP
#include "tst-rwlock8.c"

19
nptl/tst-rwlock18.c Normal file
View File

@ -0,0 +1,19 @@
/* Test program for timedout read/write lock functions.
Copyright (C) 2016-2017 Free Software Foundation, Inc.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public License as
published by the Free Software Foundation; either version 2.1 of the
License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; see the file COPYING.LIB. If
not, see <http://www.gnu.org/licenses/>. */
#define KIND PTHREAD_RWLOCK_PREFER_WRITER_NP
#include "tst-rwlock9.c"

127
nptl/tst-rwlock19.c Normal file
View File

@ -0,0 +1,127 @@
/* Test rdlock overflow.
Copyright (C) 2000-2017 Free Software Foundation, Inc.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public License as
published by the Free Software Foundation; either version 2.1 of the
License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; see the file COPYING.LIB. If
not, see <http://www.gnu.org/licenses/>. */
#include <errno.h>
#include <error.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
#include <pthreadP.h>
#define NREADERS 15
#define READTRIES 5000
#define DELAY 1000000
static pthread_rwlock_t lock = PTHREAD_RWLOCK_INITIALIZER;
static int eagain_returned = 0;
static int success_returned = 0;
static void *
reader_thread (void *nr)
{
struct timespec delay;
int n;
delay.tv_sec = 0;
delay.tv_nsec = DELAY;
for (n = 0; n < READTRIES; ++n)
{
int err = pthread_rwlock_rdlock (&lock);
if (err == EAGAIN)
{
atomic_store_relaxed (&eagain_returned, 1);
continue;
}
else if (err == 0)
atomic_store_relaxed (&success_returned, 1);
else
{
puts ("rdlock failed");
exit (1);
}
nanosleep (&delay, NULL);
if (pthread_rwlock_unlock (&lock) != 0)
{
puts ("unlock for reader failed");
exit (1);
}
}
return NULL;
}
static int
do_test (void)
{
pthread_t thrd[NREADERS];
int n;
void *res;
/* Set the rwlock so that it's close to a reader overflow.
PTHREAD_RWLOCK_WRPHASE and PTHREAD_RWLOCK_WRLOCK are zero initially. */
unsigned int readers = PTHREAD_RWLOCK_READER_OVERFLOW
- ((NREADERS / 3) << PTHREAD_RWLOCK_READER_SHIFT);
lock.__data.__readers = readers;
for (n = 0; n < NREADERS; ++n)
if (pthread_create (&thrd[n], NULL, reader_thread,
(void *) (long int) n) != 0)
{
puts ("reader create failed");
exit (1);
}
/* Wait for all the threads. */
for (n = 0; n < NREADERS; ++n)
if (pthread_join (thrd[n], &res) != 0)
{
puts ("reader join failed");
exit (1);
}
if (atomic_load_relaxed (&eagain_returned) == 0)
{
puts ("EAGAIN has never been returned");
exit (1);
}
if (atomic_load_relaxed (&success_returned) == 0)
{
puts ("rdlock was never successfully acquired");
exit (1);
}
if (lock.__data.__readers != readers)
{
puts ("__readers in rwlock differs from initial value");
exit (1);
}
return 0;
}
#define TIMEOUT 30
#define TEST_FUNCTION do_test ()
#include "../test-skeleton.c"

2
nptl/tst-rwlock2b.c Normal file
View File

@ -0,0 +1,2 @@
#define TYPE PTHREAD_RWLOCK_PREFER_WRITER_NP
#include "tst-rwlock2.c"

25
nptl/tst-rwlock8.c
View File

@ -32,11 +32,11 @@
#define DELAY 1000000 #define DELAY 1000000
#ifndef INIT #ifndef KIND
# define INIT PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP # define KIND PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP
#endif #endif
static pthread_rwlock_t lock = INIT; static pthread_rwlock_t lock;
static void * static void *
@ -118,6 +118,25 @@ do_test (void)
pthread_t thrd[NREADERS]; pthread_t thrd[NREADERS];
int n; int n;
void *res; void *res;
pthread_rwlockattr_t a;
if (pthread_rwlockattr_init (&a) != 0)
{
puts ("rwlockattr_t failed");
exit (1);
}
if (pthread_rwlockattr_setkind_np (&a, KIND) != 0)
{
puts ("rwlockattr_setkind failed");
exit (1);
}
if (pthread_rwlock_init (&lock, &a) != 0)
{
puts ("rwlock_init failed");
exit (1);
}
/* Make standard error the same as standard output. */ /* Make standard error the same as standard output. */
dup2 (1, 2); dup2 (1, 2);

25
nptl/tst-rwlock9.c
View File

@ -34,11 +34,11 @@
#define TIMEOUT 1000000 #define TIMEOUT 1000000
#define DELAY 1000000 #define DELAY 1000000
#ifndef INIT #ifndef KIND
# define INIT PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP # define KIND PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP
#endif #endif
static pthread_rwlock_t lock = INIT; static pthread_rwlock_t lock;
static void * static void *
@ -156,6 +156,25 @@ do_test (void)
pthread_t thrd[NREADERS]; pthread_t thrd[NREADERS];
int n; int n;
void *res; void *res;
pthread_rwlockattr_t a;
if (pthread_rwlockattr_init (&a) != 0)
{
puts ("rwlockattr_t failed");
exit (1);
}
if (pthread_rwlockattr_setkind_np (&a, KIND) != 0)
{
puts ("rwlockattr_setkind failed");
exit (1);
}
if (pthread_rwlock_init (&lock, &a) != 0)
{
puts ("rwlock_init failed");
exit (1);
}
/* Make standard error the same as standard output. */ /* Make standard error the same as standard output. */
dup2 (1, 2); dup2 (1, 2);

14
sysdeps/aarch64/nptl/bits/pthreadtypes.h
View File

@ -154,13 +154,13 @@ typedef union
{ {
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
int __writer; int __cur_writer;
int __shared; int __shared;
unsigned long int __pad1; unsigned long int __pad1;
unsigned long int __pad2; unsigned long int __pad2;

14
sysdeps/arm/nptl/bits/pthreadtypes.h
View File

@ -141,12 +141,12 @@ typedef union
{ {
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
#if __BYTE_ORDER == __BIG_ENDIAN #if __BYTE_ORDER == __BIG_ENDIAN
unsigned char __pad1; unsigned char __pad1;
unsigned char __pad2; unsigned char __pad2;
@ -162,7 +162,7 @@ typedef union
unsigned char __pad1; unsigned char __pad1;
unsigned char __pad2; unsigned char __pad2;
#endif #endif
int __writer; int __cur_writer;
} __data; } __data;
char __size[__SIZEOF_PTHREAD_RWLOCK_T]; char __size[__SIZEOF_PTHREAD_RWLOCK_T];
long int __align; long int __align;

14
sysdeps/hppa/nptl/bits/pthreadtypes.h
View File

@ -168,13 +168,13 @@ typedef union
next four words are all set to 1 by the Linuxthreads next four words are all set to 1 by the Linuxthreads
PTHREAD_RWLOCK_INITIALIZER. We ignore them in NPTL. */ PTHREAD_RWLOCK_INITIALIZER. We ignore them in NPTL. */
int __compat_padding[4] __attribute__ ((__aligned__(16))); int __compat_padding[4] __attribute__ ((__aligned__(16)));
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
int __writer; int __cur_writer;
/* An unused word, reserved for future use. It was added /* An unused word, reserved for future use. It was added
to maintain the location of the flags from the Linuxthreads to maintain the location of the flags from the Linuxthreads
layout of this structure. */ layout of this structure. */

14
sysdeps/ia64/nptl/bits/pthreadtypes.h
View File

@ -138,13 +138,13 @@ typedef union
{ {
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
int __writer; int __cur_writer;
int __shared; int __shared;
unsigned long int __pad1; unsigned long int __pad1;
unsigned long int __pad2; unsigned long int __pad2;

14
sysdeps/m68k/nptl/bits/pthreadtypes.h
View File

@ -142,19 +142,19 @@ typedef union
{ {
struct struct
{ {
int __lock __attribute__ ((__aligned__ (4))); unsigned int __readers __attribute__ ((__aligned__ (4)));
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
unsigned char __pad1; unsigned char __pad1;
unsigned char __pad2; unsigned char __pad2;
unsigned char __shared; unsigned char __shared;
/* FLAGS must stay at this position in the structure to maintain /* FLAGS must stay at this position in the structure to maintain
binary compatibility. */ binary compatibility. */
unsigned char __flags; unsigned char __flags;
int __writer; int __cur_writer;
} __data; } __data;
char __size[__SIZEOF_PTHREAD_RWLOCK_T]; char __size[__SIZEOF_PTHREAD_RWLOCK_T];
long int __align; long int __align;

14
sysdeps/microblaze/nptl/bits/pthreadtypes.h
View File

@ -136,12 +136,12 @@ typedef union
{ {
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
# if __BYTE_ORDER == __BIG_ENDIAN # if __BYTE_ORDER == __BIG_ENDIAN
unsigned char __pad1; unsigned char __pad1;
unsigned char __pad2; unsigned char __pad2;
@ -157,7 +157,7 @@ typedef union
unsigned char __pad1; unsigned char __pad1;
unsigned char __pad2; unsigned char __pad2;
# endif # endif
int __writer; int __cur_writer;
} __data; } __data;
char __size[__SIZEOF_PTHREAD_RWLOCK_T]; char __size[__SIZEOF_PTHREAD_RWLOCK_T];
long int __align; long int __align;

28
sysdeps/mips/nptl/bits/pthreadtypes.h
View File

@ -171,13 +171,13 @@ typedef union
# if _MIPS_SIM == _ABI64 # if _MIPS_SIM == _ABI64
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
int __writer; int __cur_writer;
int __shared; int __shared;
unsigned long int __pad1; unsigned long int __pad1;
unsigned long int __pad2; unsigned long int __pad2;
@ -188,12 +188,12 @@ typedef union
# else # else
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
#if __BYTE_ORDER == __BIG_ENDIAN #if __BYTE_ORDER == __BIG_ENDIAN
unsigned char __pad1; unsigned char __pad1;
unsigned char __pad2; unsigned char __pad2;
@ -209,7 +209,7 @@ typedef union
unsigned char __pad1; unsigned char __pad1;
unsigned char __pad2; unsigned char __pad2;
#endif #endif
int __writer; int __cur_writer;
} __data; } __data;
# endif # endif
char __size[__SIZEOF_PTHREAD_RWLOCK_T]; char __size[__SIZEOF_PTHREAD_RWLOCK_T];

14
sysdeps/nios2/nptl/bits/pthreadtypes.h
View File

@ -141,12 +141,12 @@ typedef union
{ {
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
#if __BYTE_ORDER == __BIG_ENDIAN #if __BYTE_ORDER == __BIG_ENDIAN
unsigned char __pad1; unsigned char __pad1;
unsigned char __pad2; unsigned char __pad2;
@ -162,7 +162,7 @@ typedef union
unsigned char __pad1; unsigned char __pad1;
unsigned char __pad2; unsigned char __pad2;
#endif #endif
int __writer; int __cur_writer;
} __data; } __data;
char __size[__SIZEOF_PTHREAD_RWLOCK_T]; char __size[__SIZEOF_PTHREAD_RWLOCK_T];
long int __align; long int __align;

28
sysdeps/s390/nptl/bits/pthreadtypes.h
View File

@ -191,13 +191,13 @@ typedef union
# if __WORDSIZE == 64 # if __WORDSIZE == 64
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
int __writer; int __cur_writer;
int __shared; int __shared;
unsigned long int __pad1; unsigned long int __pad1;
unsigned long int __pad2; unsigned long int __pad2;
@ -208,19 +208,19 @@ typedef union
# else # else
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
unsigned char __pad1; unsigned char __pad1;
unsigned char __pad2; unsigned char __pad2;
unsigned char __shared; unsigned char __shared;
/* FLAGS must stay at this position in the structure to maintain /* FLAGS must stay at this position in the structure to maintain
binary compatibility. */ binary compatibility. */
unsigned char __flags; unsigned char __flags;
int __writer; int __cur_writer;
} __data; } __data;
# endif # endif
char __size[__SIZEOF_PTHREAD_RWLOCK_T]; char __size[__SIZEOF_PTHREAD_RWLOCK_T];

14
sysdeps/sh/nptl/bits/pthreadtypes.h
View File

@ -141,12 +141,12 @@ typedef union
{ {
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
#if __BYTE_ORDER == __BIG_ENDIAN #if __BYTE_ORDER == __BIG_ENDIAN
unsigned char __pad1; unsigned char __pad1;
unsigned char __pad2; unsigned char __pad2;
@ -162,7 +162,7 @@ typedef union
unsigned char __pad1; unsigned char __pad1;
unsigned char __pad2; unsigned char __pad2;
#endif #endif
pthread_t __writer; pthread_t __cur_writer;
} __data; } __data;
char __size[__SIZEOF_PTHREAD_RWLOCK_T]; char __size[__SIZEOF_PTHREAD_RWLOCK_T];
long int __align; long int __align;

28
sysdeps/sparc/nptl/bits/pthreadtypes.h
View File

@ -171,13 +171,13 @@ typedef union
# if __WORDSIZE == 64 # if __WORDSIZE == 64
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
int __writer; int __cur_writer;
int __shared; int __shared;
unsigned long int __pad1; unsigned long int __pad1;
unsigned long int __pad2; unsigned long int __pad2;
@ -188,19 +188,19 @@ typedef union
# else # else
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
unsigned char __pad1; unsigned char __pad1;
unsigned char __pad2; unsigned char __pad2;
unsigned char __shared; unsigned char __shared;
/* FLAGS must stay at this position in the structure to maintain /* FLAGS must stay at this position in the structure to maintain
binary compatibility. */ binary compatibility. */
unsigned char __flags; unsigned char __flags;
int __writer; int __cur_writer;
} __data; } __data;
# endif # endif
char __size[__SIZEOF_PTHREAD_RWLOCK_T]; char __size[__SIZEOF_PTHREAD_RWLOCK_T];

28
sysdeps/tile/nptl/bits/pthreadtypes.h
View File

@ -171,13 +171,13 @@ typedef union
# if __WORDSIZE == 64 # if __WORDSIZE == 64
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
int __writer; int __cur_writer;
int __shared; int __shared;
unsigned long int __pad1; unsigned long int __pad1;
unsigned long int __pad2; unsigned long int __pad2;
@ -188,19 +188,19 @@ typedef union
# else # else
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
/* FLAGS must stay at this position in the structure to maintain /* FLAGS must stay at this position in the structure to maintain
binary compatibility. */ binary compatibility. */
unsigned char __flags; unsigned char __flags;
unsigned char __shared; unsigned char __shared;
unsigned char __pad1; unsigned char __pad1;
unsigned char __pad2; unsigned char __pad2;
int __writer; int __cur_writer;
} __data; } __data;
# endif # endif
char __size[__SIZEOF_PTHREAD_RWLOCK_T]; char __size[__SIZEOF_PTHREAD_RWLOCK_T];

14
sysdeps/unix/sysv/linux/alpha/bits/pthreadtypes.h
View File

@ -137,13 +137,13 @@ typedef union
{ {
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
int __writer; int __cur_writer;
int __shared; int __shared;
unsigned long int __pad1; unsigned long int __pad1;
unsigned long int __pad2; unsigned long int __pad2;

28
sysdeps/unix/sysv/linux/powerpc/bits/pthreadtypes.h
View File

@ -177,13 +177,13 @@ typedef union
# if __WORDSIZE == 64 # if __WORDSIZE == 64
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
int __writer; int __cur_writer;
int __shared; int __shared;
unsigned char __rwelision; unsigned char __rwelision;
unsigned char __pad1[7]; unsigned char __pad1[7];
@ -196,19 +196,19 @@ typedef union
# else # else
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
unsigned char __rwelision; unsigned char __rwelision;
unsigned char __pad2; unsigned char __pad2;
unsigned char __shared; unsigned char __shared;
/* FLAGS must stay at this position in the structure to maintain /* FLAGS must stay at this position in the structure to maintain
binary compatibility. */ binary compatibility. */
unsigned char __flags; unsigned char __flags;
int __writer; int __cur_writer;
#define __PTHREAD_RWLOCK_ELISION_EXTRA 0 #define __PTHREAD_RWLOCK_ELISION_EXTRA 0
} __data; } __data;
# endif # endif

28
sysdeps/x86/bits/pthreadtypes.h
View File

@ -189,13 +189,13 @@ typedef union
# ifdef __x86_64__ # ifdef __x86_64__
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
int __writer; int __cur_writer;
int __shared; int __shared;
signed char __rwelision; signed char __rwelision;
# ifdef __ILP32__ # ifdef __ILP32__
@ -214,12 +214,12 @@ typedef union
# else # else
struct struct
{ {
int __lock; unsigned int __readers;
unsigned int __nr_readers; unsigned int __writers;
unsigned int __readers_wakeup; unsigned int __wrphase_futex;
unsigned int __writer_wakeup; unsigned int __writers_futex;
unsigned int __nr_readers_queued; unsigned int __pad3;
unsigned int __nr_writers_queued; unsigned int __pad4;
/* FLAGS must stay at this position in the structure to maintain /* FLAGS must stay at this position in the structure to maintain
binary compatibility. */ binary compatibility. */
unsigned char __flags; unsigned char __flags;
@ -227,7 +227,7 @@ typedef union
signed char __rwelision; signed char __rwelision;
# define __PTHREAD_RWLOCK_ELISION_EXTRA 0 # define __PTHREAD_RWLOCK_ELISION_EXTRA 0
unsigned char __pad2; unsigned char __pad2;
int __writer; int __cur_writer;
} __data; } __data;
# endif # endif
char __size[__SIZEOF_PTHREAD_RWLOCK_T]; char __size[__SIZEOF_PTHREAD_RWLOCK_T];