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264 lines
12 KiB
C
264 lines
12 KiB
C
/* Low-level lock implementation. Generic futex-based version.
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Copyright (C) 2005-2015 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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<http://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 <lowlevellock-futex.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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atomic_compare_and_exchange_bool_acq (&(lock), 1, 0)
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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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atomic_compare_and_exchange_bool_acq (&(lock), 2, 0)
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extern void __lll_lock_wait_private (int *futex) attribute_hidden;
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extern void __lll_lock_wait (int *futex, int private) attribute_hidden;
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extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden;
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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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/* If FUTEX is 0 (not acquired), set to ID (acquired with no waiters) and
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return 0. Otherwise, ensure that it is set to FUTEX | FUTEX_WAITERS
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(acquired, possibly with waiters) and block until we acquire the lock.
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FUTEX will now be ID | FUTEX_WAITERS and we return 0.
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If the previous owner of the lock dies before we acquire the lock then FUTEX
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will be the value of id as set by the previous owner, with FUTEX_OWNER_DIED
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set (FUTEX_WAITERS may or may not be set). We return this value to indicate
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that the lock is not acquired. */
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#define __lll_robust_lock(futex, id, private) \
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({ \
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int *__futex = (futex); \
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int __val = 0; \
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\
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if (__glibc_unlikely \
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(atomic_compare_and_exchange_bool_acq (__futex, id, 0))) \
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__val = __lll_robust_lock_wait (__futex, private); \
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__val; \
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})
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#define lll_robust_lock(futex, id, private) \
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__lll_robust_lock (&(futex), id, 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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/* As __lll_robust_lock, but set to ID | FUTEX_WAITERS (acquired, possibly with
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waiters) if FUTEX is 0. */
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#define lll_robust_cond_lock(futex, id, private) \
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__lll_robust_lock (&(futex), (id) | FUTEX_WAITERS, private)
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extern int __lll_timedlock_wait (int *futex, const struct timespec *,
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int private) attribute_hidden;
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extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *,
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int private) attribute_hidden;
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/* As __lll_lock, but with a timeout. If the timeout occurs then return
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ETIMEDOUT. If ABSTIME is invalid, return EINVAL. */
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#define __lll_timedlock(futex, abstime, private) \
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({ \
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int *__futex = (futex); \
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int __val = 0; \
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\
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if (__glibc_unlikely \
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(atomic_compare_and_exchange_bool_acq (__futex, 1, 0))) \
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__val = __lll_timedlock_wait (__futex, abstime, private); \
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__val; \
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})
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#define lll_timedlock(futex, abstime, private) \
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__lll_timedlock (&(futex), abstime, private)
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/* As __lll_robust_lock, but with a timeout. If the timeout occurs then return
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ETIMEDOUT. If ABSTIME is invalid, return EINVAL. */
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#define __lll_robust_timedlock(futex, abstime, id, private) \
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({ \
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int *__futex = (futex); \
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int __val = 0; \
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\
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if (__glibc_unlikely \
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(atomic_compare_and_exchange_bool_acq (__futex, id, 0))) \
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__val = __lll_robust_timedlock_wait (__futex, abstime, private); \
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__val; \
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})
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#define lll_robust_timedlock(futex, abstime, id, private) \
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__lll_robust_timedlock (&(futex), abstime, id, 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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#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 __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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/* 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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had FUTEX_WAITERS set then wake any waiters. The waiter that acquires the
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lock will set FUTEX_WAITERS. */
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#define __lll_robust_unlock(futex, private) \
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((void) \
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({ \
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int *__futex = (futex); \
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int __oldval = atomic_exchange_rel (__futex, 0); \
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if (__glibc_unlikely (__oldval & FUTEX_WAITERS)) \
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lll_futex_wake (__futex, 1, private); \
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}))
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#define lll_robust_unlock(futex, private) \
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__lll_robust_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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/* The kernel notifies a process which uses CLONE_CHILD_CLEARTID via futex
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wake-up when the clone terminates. The memory location contains the
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thread ID while the clone is running and is reset to zero by the kernel
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afterwards. The kernel up to version 3.16.3 does not use the private futex
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operations for futex wake-up when the clone terminates. */
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#define lll_wait_tid(tid) \
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do { \
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__typeof (tid) __tid; \
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while ((__tid = (tid)) != 0) \
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lll_futex_wait (&(tid), __tid, LLL_SHARED);\
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} while (0)
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extern int __lll_timedwait_tid (int *, const struct timespec *)
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attribute_hidden;
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/* As lll_wait_tid, but with a timeout. If the timeout occurs then return
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ETIMEDOUT. If ABSTIME is invalid, return EINVAL. */
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#define lll_timedwait_tid(tid, abstime) \
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({ \
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int __res = 0; \
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if ((tid) != 0) \
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__res = __lll_timedwait_tid (&(tid), (abstime)); \
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__res; \
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})
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#endif /* lowlevellock.h */
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