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retry on cas failures in sem_trywait

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this seems counter-intuitive since sem_trywait is supposed to just try
once, not wait for the semaphore. however, the retry loop is not a
wait. instead, it's to handle the case where the value changes due to
a simultaneous post or wait from another thread while the semaphore
value remains positive. in such a case, it's absolutely wrong for
sem_trywait to fail with EAGAIN because the semaphore is not busy.
This commit is contained in:
Rich Felker
2012-07-24 00:51:36 -04:00
parent 5d26d5d15b
commit 07e62953c7
1 changed files with 2 additions and 2 deletions
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4
src/thread/sem_trywait.c
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@ -3,8 +3,8 @@
int sem_trywait(sem_t *sem)
{
int val = sem->__val[0];
if (val>0) {
int val;
while ((val=sem->__val[0]) > 0) {
int new = val-1-(val==1 && sem->__val[1]);
if (a_cas(sem->__val, val, new)==val) return 0;
}