3154492dcf
The futex and kutex implementations used the same linked list for waiting, however the futex implementation used kutexs and the same thread could be in the same list twice in the case of contention. This case corrupted the wait lists and led to deadlocks and lost wakeups. This change fixes the problem by having separate data structures for futexes and kutexes. Mutexes contended by multiple threads could lead to lost wakeups since only one contended thread was awoken and subsequent unlocks are unaware of the unawakened contended threads. This change fixes the problem with a temporary solution of waking all the contended threads until a better design is implemented. Additional details are tweaked to be more reliable and simpler.
47 lines
1.7 KiB
C
47 lines
1.7 KiB
C
/*
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* Copyright (c) 2013, 2014, 2021, 2022 Jonas 'Sortie' Termansen.
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*
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* Permission to use, copy, modify, and distribute this software for any
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* purpose with or without fee is hereby granted, provided that the above
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* copyright notice and this permission notice appear in all copies.
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*
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* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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*
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* pthread/pthread_mutex_unlock.c
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* Unlocks a mutex.
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*/
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#include <sys/futex.h>
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#include <limits.h>
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#include <pthread.h>
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#include <stdbool.h>
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static const int UNLOCKED = 0;
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static const int CONTENDED = 2;
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int pthread_mutex_unlock(pthread_mutex_t* mutex)
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{
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if ( mutex->type == PTHREAD_MUTEX_RECURSIVE && mutex->recursion )
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{
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mutex->recursion--;
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return 0;
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}
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mutex->owner = 0;
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// TODO: Multiple threads could have caused the contention and this wakes
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// only one such thread, but then the mutex returns to normal and the
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// subsequent unlocks doesn't wake the other contended threads. Work
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// around this bug by waking all of them instead, but it's better to
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// instead count the number of waiting threads.
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if ( __atomic_exchange_n(&mutex->lock, UNLOCKED,
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__ATOMIC_SEQ_CST) == CONTENDED )
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futex(&mutex->lock, FUTEX_WAKE, INT_MAX, NULL);
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return 0;
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}
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