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Date: Wed, 27 Aug 2014 17:32:12 +0400 (MSK)
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
Subject: Re: sem_getvalue conformance considerations
On Wed, 27 Aug 2014, Alexander Monakov wrote:
> Why wouldn't the following design work?
>
> val[0] is sem value if >= 0, negated waiters count otherwise
> val[1] is wakeup count, incremented before futex wake, tested and decremented
> by waiters returning from futex wait
Unless I'm missing something, the above can simplify sem ops
(sorry, eliding some details in the following pseudocode)
trywait:
val = sem->val[0]
while (val > 0) {
oldval = val;
if ((val = a_cas(sem->val, val, val-1)) == oldval)
return 0;
}
errno = EAGAIN;
return -1;
wait:
if (atomic_fetch_and_decrement(sem->val) > 0)
return 0;
while (!(futex_wait(sem->val+1, 0) && errno == EINTR)) {
wakecnt = sem->val[1];
while (wakecnt > 0) {
oldwcnt = wakecnt;
if ((wakecnt = a_cas(sem->val+1, wakecnt, wakecnt-1)) == oldwcnt)
return 0;
}
}
return -1;
post:
val = sem->val[0];
do {
if (val == SEM_VALUE_MAX) {
errno = EOVERFLOW;
return -1;
}
oldval = val;
} while ((val = a_cas(sem->val, val, val+1)) != oldval);
if (val < 0) {
a_inc(sem->val+1);
futex_wake(sem->val+1, 1);
}
return 0;
Alexander
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