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Message-ID: <20150301175009.GE16260@port70.net>
Date: Sun, 1 Mar 2015 18:50:11 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: semaphore redesign
* Szabolcs Nagy <nsz@...t70.net> [2015-03-01 18:30:49 +0100]:
> * Alexander Monakov <amonakov@...ras.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
>
sorry
the code is ok (applications cannot rely on the barrier in case of
failure), it can lead to surprising results if the application
uses relaxed atomics, but it's not a conformance issue
> > }
> > } 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;
> > }
> >
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