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276 lines
13 KiB
C
276 lines
13 KiB
C
/* Low-level lock implementation. Generic futex-based version.
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Copyright (C) 2005-2025 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_acquire (__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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extern void __lll_lock_wake_private (int *futex);
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libc_hidden_proto (__lll_lock_wake_private)
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extern void __lll_lock_wake (int *futex, int private);
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libc_hidden_proto (__lll_lock_wake)
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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_release (__futex, 0); \
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if (__glibc_unlikely (__oldval > 1)) \
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{ \
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if (__builtin_constant_p (private) && (private) == LLL_PRIVATE) \
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__lll_lock_wake_private (__futex); \
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else \
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__lll_lock_wake (__futex, __private); \
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} \
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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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