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From: Szabolcs Nagy <nsz@port70.net>
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Subject: Re: semaphore redesign
Date: Sun, 1 Mar 2015 18:30:49 +0100
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* Alexander Monakov <amonakov@ispras.ru> [2015-02-28 02:21:22 +0300]:
> int sem_post(sem_t *sem)
> {
> 	int val;
> 	do {
> 		val = sem->__val[0];
> 		if (val == SEM_VALUE_MAX) {
> 			errno = EOVERFLOW;
> 			return -1;

as discussed on irc early return here without a barrier is not ok
(it is a hard to observe corner case, i add the comment here so
it does not get forgotten)

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_11

> 		}
> 	} while (val != a_cas(sem->__val, val, val+1));
> 	if (val < 0) {
> 		int priv = sem->__val[2];
> 		a_inc(sem->__val+1);
> 		__wake(sem->__val+1, 1, priv);
> 	}
> 	return 0;
> }
> 
> int sem_trywait(sem_t *sem)
> {
> 	int val;
> 	do {
> 		val = sem->__val[0];
> 		if (val <= 0) {
> 			errno = EAGAIN;
> 			return -1;

likewise

> 		}
> 	} while (val != a_cas(sem->__val, val, val-1));
> 	return 0;
> }
>