mirror of
https://sourceware.org/git/glibc.git
synced 2024-11-22 13:00:06 +00:00
276 lines
13 KiB
C
276 lines
13 KiB
C
/* Low-level lock implementation. Generic futex-based version.
|
|
Copyright (C) 2005-2023 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
|
|
<https://www.gnu.org/licenses/>. */
|
|
|
|
#ifndef _LOWLEVELLOCK_H
|
|
#define _LOWLEVELLOCK_H 1
|
|
|
|
#include <atomic.h>
|
|
#include <elision-conf.h>
|
|
#include <lowlevellock-futex.h>
|
|
#include <time.h>
|
|
|
|
/* Low-level locks use a combination of atomic operations (to acquire and
|
|
release lock ownership) and futex operations (to block until the state
|
|
of a lock changes). A lock can be in one of three states:
|
|
0: not acquired,
|
|
1: acquired with no waiters; no other threads are blocked or about to block
|
|
for changes to the lock state,
|
|
>1: acquired, possibly with waiters; there may be other threads blocked or
|
|
about to block for changes to the lock state.
|
|
|
|
We expect that the common case is an uncontended lock, so we just need
|
|
to transition the lock between states 0 and 1; releasing the lock does
|
|
not need to wake any other blocked threads. If the lock is contended
|
|
and a thread decides to block using a futex operation, then this thread
|
|
needs to first change the state to >1; if this state is observed during
|
|
lock release, the releasing thread will wake one of the potentially
|
|
blocked threads.
|
|
|
|
Much of this code takes a 'private' parameter. This may be:
|
|
LLL_PRIVATE: lock only shared within a process
|
|
LLL_SHARED: lock may be shared across processes.
|
|
|
|
Condition variables contain an optimization for broadcasts that requeues
|
|
waiting threads on a lock's futex. Therefore, there is a special
|
|
variant of the locks (whose name contains "cond") that makes sure to
|
|
always set the lock state to >1 and not just 1.
|
|
|
|
Robust locks set the lock to the id of the owner. This allows detection
|
|
of the case where the owner exits without releasing the lock. Flags are
|
|
OR'd with the owner id to record additional information about lock state.
|
|
Therefore the states of robust locks are:
|
|
0: not acquired
|
|
id: acquired (by user identified by id & FUTEX_TID_MASK)
|
|
|
|
The following flags may be set in the robust lock value:
|
|
FUTEX_WAITERS - possibly has waiters
|
|
FUTEX_OWNER_DIED - owning user has exited without releasing the futex. */
|
|
|
|
|
|
/* If LOCK is 0 (not acquired), set to 1 (acquired with no waiters) and return
|
|
0. Otherwise leave lock unchanged and return non-zero to indicate that the
|
|
lock was not acquired. */
|
|
#define __lll_trylock(lock) \
|
|
__glibc_unlikely (atomic_compare_and_exchange_bool_acq ((lock), 1, 0))
|
|
#define lll_trylock(lock) \
|
|
__lll_trylock (&(lock))
|
|
|
|
/* If LOCK is 0 (not acquired), set to 2 (acquired, possibly with waiters) and
|
|
return 0. Otherwise leave lock unchanged and return non-zero to indicate
|
|
that the lock was not acquired. */
|
|
#define lll_cond_trylock(lock) \
|
|
__glibc_unlikely (atomic_compare_and_exchange_bool_acq (&(lock), 2, 0))
|
|
|
|
extern void __lll_lock_wait_private (int *futex);
|
|
libc_hidden_proto (__lll_lock_wait_private)
|
|
extern void __lll_lock_wait (int *futex, int private);
|
|
libc_hidden_proto (__lll_lock_wait)
|
|
|
|
/* This is an expression rather than a statement even though its value is
|
|
void, so that it can be used in a comma expression or as an expression
|
|
that's cast to void. */
|
|
/* The inner conditional compiles to a call to __lll_lock_wait_private if
|
|
private is known at compile time to be LLL_PRIVATE, and to a call to
|
|
__lll_lock_wait otherwise. */
|
|
/* If FUTEX is 0 (not acquired), set to 1 (acquired with no waiters) and
|
|
return. Otherwise, ensure that it is >1 (acquired, possibly with waiters)
|
|
and then block until we acquire the lock, at which point FUTEX will still be
|
|
>1. The lock is always acquired on return. */
|
|
#define __lll_lock(futex, private) \
|
|
((void) \
|
|
({ \
|
|
int *__futex = (futex); \
|
|
if (__glibc_unlikely \
|
|
(atomic_compare_and_exchange_bool_acq (__futex, 1, 0))) \
|
|
{ \
|
|
if (__builtin_constant_p (private) && (private) == LLL_PRIVATE) \
|
|
__lll_lock_wait_private (__futex); \
|
|
else \
|
|
__lll_lock_wait (__futex, private); \
|
|
} \
|
|
}))
|
|
#define lll_lock(futex, private) \
|
|
__lll_lock (&(futex), private)
|
|
|
|
|
|
/* This is an expression rather than a statement even though its value is
|
|
void, so that it can be used in a comma expression or as an expression
|
|
that's cast to void. */
|
|
/* Unconditionally set FUTEX to 2 (acquired, possibly with waiters). If FUTEX
|
|
was 0 (not acquired) then return. Otherwise, block until the lock is
|
|
acquired, at which point FUTEX is 2 (acquired, possibly with waiters). The
|
|
lock is always acquired on return. */
|
|
#define __lll_cond_lock(futex, private) \
|
|
((void) \
|
|
({ \
|
|
int *__futex = (futex); \
|
|
if (__glibc_unlikely (atomic_exchange_acquire (__futex, 2) != 0)) \
|
|
__lll_lock_wait (__futex, private); \
|
|
}))
|
|
#define lll_cond_lock(futex, private) __lll_cond_lock (&(futex), private)
|
|
|
|
|
|
extern void __lll_lock_wake_private (int *futex);
|
|
libc_hidden_proto (__lll_lock_wake_private)
|
|
extern void __lll_lock_wake (int *futex, int private);
|
|
libc_hidden_proto (__lll_lock_wake)
|
|
|
|
/* This is an expression rather than a statement even though its value is
|
|
void, so that it can be used in a comma expression or as an expression
|
|
that's cast to void. */
|
|
/* Unconditionally set FUTEX to 0 (not acquired), releasing the lock. If FUTEX
|
|
was >1 (acquired, possibly with waiters), then wake any waiters. The waiter
|
|
that acquires the lock will set FUTEX to >1.
|
|
Evaluate PRIVATE before releasing the lock so that we do not violate the
|
|
mutex destruction requirements. Specifically, we need to ensure that
|
|
another thread can destroy the mutex (and reuse its memory) once it
|
|
acquires the lock and when there will be no further lock acquisitions;
|
|
thus, we must not access the lock after releasing it, or those accesses
|
|
could be concurrent with mutex destruction or reuse of the memory. */
|
|
#define __lll_unlock(futex, private) \
|
|
((void) \
|
|
({ \
|
|
int *__futex = (futex); \
|
|
int __private = (private); \
|
|
int __oldval = atomic_exchange_release (__futex, 0); \
|
|
if (__glibc_unlikely (__oldval > 1)) \
|
|
{ \
|
|
if (__builtin_constant_p (private) && (private) == LLL_PRIVATE) \
|
|
__lll_lock_wake_private (__futex); \
|
|
else \
|
|
__lll_lock_wake (__futex, __private); \
|
|
} \
|
|
}))
|
|
#define lll_unlock(futex, private) \
|
|
__lll_unlock (&(futex), private)
|
|
|
|
|
|
#define lll_islocked(futex) \
|
|
((futex) != LLL_LOCK_INITIALIZER)
|
|
|
|
|
|
/* Our internal lock implementation is identical to the binary-compatible
|
|
mutex implementation. */
|
|
|
|
/* Initializers for lock. */
|
|
#define LLL_LOCK_INITIALIZER (0)
|
|
#define LLL_LOCK_INITIALIZER_LOCKED (1)
|
|
|
|
/* Elision support. */
|
|
|
|
#if ENABLE_ELISION_SUPPORT
|
|
/* Force elision for all new locks. This is used to decide whether
|
|
existing DEFAULT locks should be automatically upgraded to elision
|
|
in pthread_mutex_lock. Disabled for suid programs. Only used when
|
|
elision is available. */
|
|
extern int __pthread_force_elision;
|
|
libc_hidden_proto (__pthread_force_elision)
|
|
|
|
extern void __lll_elision_init (void) attribute_hidden;
|
|
extern int __lll_clocklock_elision (int *futex, short *adapt_count,
|
|
clockid_t clockid,
|
|
const struct __timespec64 *timeout,
|
|
int private);
|
|
libc_hidden_proto (__lll_clocklock_elision)
|
|
|
|
extern int __lll_lock_elision (int *futex, short *adapt_count, int private);
|
|
libc_hidden_proto (__lll_lock_elision)
|
|
|
|
# if ELISION_UNLOCK_NEEDS_ADAPT_COUNT
|
|
extern int __lll_unlock_elision (int *lock, short *adapt_count, int private);
|
|
# else
|
|
extern int __lll_unlock_elision (int *lock, int private);
|
|
# endif
|
|
libc_hidden_proto (__lll_unlock_elision)
|
|
|
|
extern int __lll_trylock_elision (int *lock, short *adapt_count);
|
|
libc_hidden_proto (__lll_trylock_elision)
|
|
|
|
# define lll_clocklock_elision(futex, adapt_count, clockid, timeout, private) \
|
|
__lll_clocklock_elision (&(futex), &(adapt_count), clockid, timeout, private)
|
|
# define lll_lock_elision(futex, adapt_count, private) \
|
|
__lll_lock_elision (&(futex), &(adapt_count), private)
|
|
# define lll_trylock_elision(futex, adapt_count) \
|
|
__lll_trylock_elision (&(futex), &(adapt_count))
|
|
# if ELISION_UNLOCK_NEEDS_ADAPT_COUNT
|
|
# define lll_unlock_elision(futex, adapt_count, private) \
|
|
__lll_unlock_elision (&(futex), &(adapt_count), private)
|
|
# else
|
|
# define lll_unlock_elision(futex, adapt_count, private) \
|
|
__lll_unlock_elision (&(futex), private)
|
|
# endif
|
|
|
|
/* Automatically enable elision for existing user lock kinds. */
|
|
# define FORCE_ELISION(m, s) \
|
|
if (__pthread_force_elision) \
|
|
{ \
|
|
/* See concurrency notes regarding __kind in \
|
|
struct __pthread_mutex_s in \
|
|
sysdeps/nptl/bits/thread-shared-types.h. \
|
|
\
|
|
There are the following cases for the kind of a mutex \
|
|
(The mask PTHREAD_MUTEX_ELISION_FLAGS_NP covers the flags \
|
|
PTHREAD_MUTEX_ELISION_NP and PTHREAD_MUTEX_NO_ELISION_NP where \
|
|
only one of both flags can be set): \
|
|
- both flags are not set: \
|
|
This is the first lock operation for this mutex. Enable \
|
|
elision as it is not enabled so far. \
|
|
Note: It can happen that multiple threads are calling e.g. \
|
|
pthread_mutex_lock at the same time as the first lock \
|
|
operation for this mutex. Then elision is enabled for this \
|
|
mutex by multiple threads. Storing with relaxed MO is enough \
|
|
as all threads will store the same new value for the kind of \
|
|
the mutex. But we have to ensure that we always use the \
|
|
elision path regardless if this thread has enabled elision or \
|
|
another one. \
|
|
\
|
|
- PTHREAD_MUTEX_ELISION_NP flag is set: \
|
|
Elision was already enabled for this mutex by a previous lock \
|
|
operation. See case above. Just use the elision path. \
|
|
\
|
|
- PTHREAD_MUTEX_NO_ELISION_NP flag is set: \
|
|
Elision was explicitly disabled by pthread_mutexattr_settype. \
|
|
Do not use the elision path. \
|
|
Note: The flag PTHREAD_MUTEX_NO_ELISION_NP will never be \
|
|
changed after mutex initialization. */ \
|
|
int mutex_kind = atomic_load_relaxed (&((m)->__data.__kind)); \
|
|
if ((mutex_kind & PTHREAD_MUTEX_ELISION_FLAGS_NP) == 0) \
|
|
{ \
|
|
mutex_kind |= PTHREAD_MUTEX_ELISION_NP; \
|
|
atomic_store_relaxed (&((m)->__data.__kind), mutex_kind); \
|
|
} \
|
|
if ((mutex_kind & PTHREAD_MUTEX_ELISION_NP) != 0) \
|
|
{ \
|
|
s; \
|
|
} \
|
|
}
|
|
|
|
#else /* !ENABLE_ELISION_SUPPORT */
|
|
|
|
# define lll_clocklock_elision(futex, adapt_count, clockid, abstime, private) \
|
|
__futex_clocklock64 (&(futex), clockid, abstime, private)
|
|
# define lll_lock_elision(lock, try_lock, private) \
|
|
({ lll_lock (lock, private); 0; })
|
|
# define lll_trylock_elision(a,t) lll_trylock(a)
|
|
# define lll_unlock_elision(a,b,c) ({ lll_unlock (a,c); 0; })
|
|
# define FORCE_ELISION(m, s)
|
|
|
|
#endif /* !ENABLE_ELISION_SUPPORT */
|
|
|
|
#endif /* lowlevellock.h */
|