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Avoid pthread_rwlock_wrlock() starvation

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Justine Tunney 2024-12-24 10:30:11 -08:00
parent 55b7aa1632
commit ec2db4e40e
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GPG key ID: BE714B4575D6E328
3 changed files with 6 additions and 0 deletions
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3
libc/thread/pthread_rwlock_rdlock.c
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@ -42,6 +42,9 @@ errno_t pthread_rwlock_rdlock(pthread_rwlock_t *lk) {
for (;;)
if (~(w = atomic_load_explicit(&lk->_word, memory_order_relaxed)) & 1)
break;
// xxx: avoid writer starvation in pthread_rwlock_rdlock_test
while (atomic_load(&lk->_waiters))
pthread_yield_np();
if (atomic_compare_exchange_weak_explicit(
&lk->_word, &w, w + 2, memory_order_acquire, memory_order_relaxed))
return 0;

2
libc/thread/pthread_rwlock_wrlock.c
View file

@ -42,8 +42,10 @@ errno_t pthread_rwlock_wrlock(pthread_rwlock_t *rwlock) {
if (atomic_compare_exchange_weak_explicit(
&rwlock->_word, &w, 1, memory_order_acquire, memory_order_relaxed))
return 0;
atomic_fetch_add(&rwlock->_waiters, 1);
for (;;)
if (!(w = atomic_load_explicit(&rwlock->_word, memory_order_relaxed)))
break;
atomic_fetch_sub(&rwlock->_waiters, 1);
}
}

1
libc/thread/thread.h
View file

@ -111,6 +111,7 @@ typedef struct pthread_rwlock_s {
char _pshared;
char _iswrite;
_PTHREAD_ATOMIC(uint32_t) _word;
_PTHREAD_ATOMIC(uint32_t) _waiters;
};
};
} pthread_rwlock_t;