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The elision interfaces are closely aligned between the targets that implement them, so declare them in the generic <lowlevellock.h> file. Empty .c stubs are provided, so that fewer makefile updates under sysdeps are needed. Also simplify initialization via __libc_early_init. The symbols __lll_clocklock_elision, __lll_lock_elision, __lll_trylock_elision, __lll_unlock_elision, __pthread_force_elision move into libc. For the time being, non-hidden references are used from libpthread to access them, but once that part of libpthread is moved into libc, hidden symbols will be used again. (Hidden references seem desirable to reduce the likelihood of transactions aborts.)
266 lines
13 KiB
C
266 lines
13 KiB
C
/* Low-level lock implementation. Generic futex-based version.
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Copyright (C) 2005-2021 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library. If not, see
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<https://www.gnu.org/licenses/>. */
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#ifndef _LOWLEVELLOCK_H
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#define _LOWLEVELLOCK_H 1
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#include <atomic.h>
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#include <elision-conf.h>
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#include <lowlevellock-futex.h>
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#include <time.h>
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/* Low-level locks use a combination of atomic operations (to acquire and
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release lock ownership) and futex operations (to block until the state
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of a lock changes). A lock can be in one of three states:
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0: not acquired,
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1: acquired with no waiters; no other threads are blocked or about to block
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for changes to the lock state,
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>1: acquired, possibly with waiters; there may be other threads blocked or
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about to block for changes to the lock state.
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We expect that the common case is an uncontended lock, so we just need
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to transition the lock between states 0 and 1; releasing the lock does
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not need to wake any other blocked threads. If the lock is contended
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and a thread decides to block using a futex operation, then this thread
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needs to first change the state to >1; if this state is observed during
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lock release, the releasing thread will wake one of the potentially
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blocked threads.
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Much of this code takes a 'private' parameter. This may be:
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LLL_PRIVATE: lock only shared within a process
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LLL_SHARED: lock may be shared across processes.
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Condition variables contain an optimization for broadcasts that requeues
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waiting threads on a lock's futex. Therefore, there is a special
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variant of the locks (whose name contains "cond") that makes sure to
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always set the lock state to >1 and not just 1.
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Robust locks set the lock to the id of the owner. This allows detection
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of the case where the owner exits without releasing the lock. Flags are
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OR'd with the owner id to record additional information about lock state.
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Therefore the states of robust locks are:
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0: not acquired
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id: acquired (by user identified by id & FUTEX_TID_MASK)
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The following flags may be set in the robust lock value:
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FUTEX_WAITERS - possibly has waiters
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FUTEX_OWNER_DIED - owning user has exited without releasing the futex. */
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/* If LOCK is 0 (not acquired), set to 1 (acquired with no waiters) and return
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0. Otherwise leave lock unchanged and return non-zero to indicate that the
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lock was not acquired. */
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#define __lll_trylock(lock) \
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__glibc_unlikely (atomic_compare_and_exchange_bool_acq ((lock), 1, 0))
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#define lll_trylock(lock) \
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__lll_trylock (&(lock))
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/* If LOCK is 0 (not acquired), set to 2 (acquired, possibly with waiters) and
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return 0. Otherwise leave lock unchanged and return non-zero to indicate
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that the lock was not acquired. */
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#define lll_cond_trylock(lock) \
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__glibc_unlikely (atomic_compare_and_exchange_bool_acq (&(lock), 2, 0))
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extern void __lll_lock_wait_private (int *futex);
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libc_hidden_proto (__lll_lock_wait_private)
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extern void __lll_lock_wait (int *futex, int private);
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libc_hidden_proto (__lll_lock_wait)
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/* This is an expression rather than a statement even though its value is
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void, so that it can be used in a comma expression or as an expression
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that's cast to void. */
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/* The inner conditional compiles to a call to __lll_lock_wait_private if
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private is known at compile time to be LLL_PRIVATE, and to a call to
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__lll_lock_wait otherwise. */
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/* If FUTEX is 0 (not acquired), set to 1 (acquired with no waiters) and
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return. Otherwise, ensure that it is >1 (acquired, possibly with waiters)
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and then block until we acquire the lock, at which point FUTEX will still be
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>1. The lock is always acquired on return. */
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#define __lll_lock(futex, private) \
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((void) \
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({ \
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int *__futex = (futex); \
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if (__glibc_unlikely \
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(atomic_compare_and_exchange_bool_acq (__futex, 1, 0))) \
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{ \
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if (__builtin_constant_p (private) && (private) == LLL_PRIVATE) \
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__lll_lock_wait_private (__futex); \
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else \
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__lll_lock_wait (__futex, private); \
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} \
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}))
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#define lll_lock(futex, private) \
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__lll_lock (&(futex), private)
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/* This is an expression rather than a statement even though its value is
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void, so that it can be used in a comma expression or as an expression
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that's cast to void. */
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/* Unconditionally set FUTEX to 2 (acquired, possibly with waiters). If FUTEX
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was 0 (not acquired) then return. Otherwise, block until the lock is
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acquired, at which point FUTEX is 2 (acquired, possibly with waiters). The
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lock is always acquired on return. */
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#define __lll_cond_lock(futex, private) \
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((void) \
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({ \
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int *__futex = (futex); \
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if (__glibc_unlikely (atomic_exchange_acq (__futex, 2) != 0)) \
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__lll_lock_wait (__futex, private); \
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}))
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#define lll_cond_lock(futex, private) __lll_cond_lock (&(futex), private)
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/* This is an expression rather than a statement even though its value is
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void, so that it can be used in a comma expression or as an expression
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that's cast to void. */
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/* Unconditionally set FUTEX to 0 (not acquired), releasing the lock. If FUTEX
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was >1 (acquired, possibly with waiters), then wake any waiters. The waiter
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that acquires the lock will set FUTEX to >1.
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Evaluate PRIVATE before releasing the lock so that we do not violate the
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mutex destruction requirements. Specifically, we need to ensure that
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another thread can destroy the mutex (and reuse its memory) once it
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acquires the lock and when there will be no further lock acquisitions;
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thus, we must not access the lock after releasing it, or those accesses
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could be concurrent with mutex destruction or reuse of the memory. */
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#define __lll_unlock(futex, private) \
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((void) \
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({ \
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int *__futex = (futex); \
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int __private = (private); \
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int __oldval = atomic_exchange_rel (__futex, 0); \
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if (__glibc_unlikely (__oldval > 1)) \
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lll_futex_wake (__futex, 1, __private); \
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}))
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#define lll_unlock(futex, private) \
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__lll_unlock (&(futex), private)
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#define lll_islocked(futex) \
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((futex) != LLL_LOCK_INITIALIZER)
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/* Our internal lock implementation is identical to the binary-compatible
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mutex implementation. */
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/* Initializers for lock. */
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#define LLL_LOCK_INITIALIZER (0)
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#define LLL_LOCK_INITIALIZER_LOCKED (1)
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/* Elision support. */
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#if ENABLE_ELISION_SUPPORT
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/* Force elision for all new locks. This is used to decide whether
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existing DEFAULT locks should be automatically upgraded to elision
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in pthread_mutex_lock. Disabled for suid programs. Only used when
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elision is available. */
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extern int __pthread_force_elision;
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libc_hidden_proto (__pthread_force_elision)
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extern void __lll_elision_init (void) attribute_hidden;
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extern int __lll_clocklock_elision (int *futex, short *adapt_count,
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clockid_t clockid,
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const struct __timespec64 *timeout,
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int private);
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libc_hidden_proto (__lll_clocklock_elision)
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extern int __lll_lock_elision (int *futex, short *adapt_count, int private);
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libc_hidden_proto (__lll_lock_elision)
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# if ELISION_UNLOCK_NEEDS_ADAPT_COUNT
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extern int __lll_unlock_elision (int *lock, short *adapt_count, int private);
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# else
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extern int __lll_unlock_elision (int *lock, int private);
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# endif
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libc_hidden_proto (__lll_unlock_elision)
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extern int __lll_trylock_elision (int *lock, short *adapt_count);
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libc_hidden_proto (__lll_trylock_elision)
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# define lll_clocklock_elision(futex, adapt_count, clockid, timeout, private) \
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__lll_clocklock_elision (&(futex), &(adapt_count), clockid, timeout, private)
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# define lll_lock_elision(futex, adapt_count, private) \
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__lll_lock_elision (&(futex), &(adapt_count), private)
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# define lll_trylock_elision(futex, adapt_count) \
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__lll_trylock_elision (&(futex), &(adapt_count))
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# if ELISION_UNLOCK_NEEDS_ADAPT_COUNT
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# define lll_unlock_elision(futex, adapt_count, private) \
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__lll_unlock_elision (&(futex), &(adapt_count), private)
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# else
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# define lll_unlock_elision(futex, adapt_count, private) \
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__lll_unlock_elision (&(futex), private)
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# endif
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/* Automatically enable elision for existing user lock kinds. */
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# define FORCE_ELISION(m, s) \
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if (__pthread_force_elision) \
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{ \
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/* See concurrency notes regarding __kind in \
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struct __pthread_mutex_s in \
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sysdeps/nptl/bits/thread-shared-types.h. \
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\
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There are the following cases for the kind of a mutex \
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(The mask PTHREAD_MUTEX_ELISION_FLAGS_NP covers the flags \
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PTHREAD_MUTEX_ELISION_NP and PTHREAD_MUTEX_NO_ELISION_NP where \
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only one of both flags can be set): \
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- both flags are not set: \
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This is the first lock operation for this mutex. Enable \
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elision as it is not enabled so far. \
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Note: It can happen that multiple threads are calling e.g. \
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pthread_mutex_lock at the same time as the first lock \
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operation for this mutex. Then elision is enabled for this \
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mutex by multiple threads. Storing with relaxed MO is enough \
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as all threads will store the same new value for the kind of \
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the mutex. But we have to ensure that we always use the \
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elision path regardless if this thread has enabled elision or \
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another one. \
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\
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- PTHREAD_MUTEX_ELISION_NP flag is set: \
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Elision was already enabled for this mutex by a previous lock \
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operation. See case above. Just use the elision path. \
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\
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- PTHREAD_MUTEX_NO_ELISION_NP flag is set: \
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Elision was explicitly disabled by pthread_mutexattr_settype. \
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Do not use the elision path. \
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Note: The flag PTHREAD_MUTEX_NO_ELISION_NP will never be \
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changed after mutex initialization. */ \
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int mutex_kind = atomic_load_relaxed (&((m)->__data.__kind)); \
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if ((mutex_kind & PTHREAD_MUTEX_ELISION_FLAGS_NP) == 0) \
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{ \
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mutex_kind |= PTHREAD_MUTEX_ELISION_NP; \
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atomic_store_relaxed (&((m)->__data.__kind), mutex_kind); \
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} \
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if ((mutex_kind & PTHREAD_MUTEX_ELISION_NP) != 0) \
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{ \
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s; \
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} \
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}
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#else /* !ENABLE_ELISION_SUPPORT */
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# define lll_clocklock_elision(futex, adapt_count, clockid, abstime, private) \
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__futex_clocklock64 (&(futex), clockid, abstime, private)
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# define lll_lock_elision(lock, try_lock, private) \
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({ lll_lock (lock, private); 0; })
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# define lll_trylock_elision(a,t) lll_trylock(a)
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# define lll_unlock_elision(a,b,c) ({ lll_unlock (a,c); 0; })
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# define FORCE_ELISION(m, s)
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#endif /* !ENABLE_ELISION_SUPPORT */
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#endif /* lowlevellock.h */
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