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<  Erlang bugs mailing list  ~  Problem with function ethr_rwmutex_tryrlock
Guest
Posted: Wed May 26, 2010 8:37 pm Reply with quote
Guest
Hi list,

I think the fallback version of function ethr_rwmutex_tryrlock in
erts/lib_src/common/ethread.c is not correct. This function should be
similar with pthread_rwlock_tryrdlock. For pthread_rwlock_tryrdlock, the
calling thread acquires the read lock if a writer does not hold the lock and
there are no writers blocked on the lock. But as following code shows,
ethr_rwmutex_tryrlock doesn't get the lock when there is no waiting writer,
and acquires the lock when there are waiting writers. Am I right?

ethr_rwmutex_tryrlock(ethr_rwmutex *rwmtx)
{
int res;
#if ETHR_XCHK
if (!rwmtx || rwmtx->initialized != ETHR_RWMUTEX_INITIALIZED) {
ASSERT(0);
return EINVAL;
}
#endif
res = ethr_mutex_trylock__(&rwmtx->mtx);
if (res != 0)
return res;
if (!rwmtx->waiting_writers) {
res = ethr_mutex_unlock__(&rwmtx->mtx);
if (res == 0)
return EBUSY;
return res;
}
rwmtx->readers++;
return ethr_mutex_unlock__(&rwmtx->mtx);
}

Best Regards,
Jianrong Zhang


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Guest
Posted: Thu May 27, 2010 1:39 pm Reply with quote
Guest
> Hi list,
>
> I think the fallback version of function ethr_rwmutex_tryrlock in
> erts/lib_src/common/ethread.c is not correct. This function should be
> similar with pthread_rwlock_tryrdlock. For pthread_rwlock_tryrdlock, the
> calling thread acquires the read lock if a writer does not hold the lock and
> there are no writers blocked on the lock. But as following code shows,
> ethr_rwmutex_tryrlock doesn't get the lock when there is no waiting writer,
> and acquires the lock when there are waiting writers. Am I right?
>
> ethr_rwmutex_tryrlock(ethr_rwmutex *rwmtx)
> {
> int res;
> #if ETHR_XCHK
> if (!rwmtx || rwmtx->initialized != ETHR_RWMUTEX_INITIALIZED) {
> ASSERT(0);
> return EINVAL;
> }
> #endif
> res = ethr_mutex_trylock__(&rwmtx->mtx);
> if (res != 0)
> return res;
> if (!rwmtx->waiting_writers) {
> res = ethr_mutex_unlock__(&rwmtx->mtx);
> if (res == 0)
> return EBUSY;
> return res;
> }
> rwmtx->readers++;
> return ethr_mutex_unlock__(&rwmtx->mtx);
> }
>
> Best Regards,
> Jianrong Zhang
>

Yes, you are right.

if (!rwmtx->waiting_writers) {

should be

if (rwmtx->waiting_writers) {

Thanks! It will be fixed in the upcomming release.

Regards,
Rickard

--
Rickard Green, Erlang/OTP, Ericsson AB.

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