pthread_cond_timedwait() broken in 9-stable? (from JAN 10)

[Jens Axboe]

On 2012-02-16 22:06, Julian Elischer wrote:
> On 2/16/12 9:34 AM, Andriy Gapon wrote:
>> on 15/02/2012 23:41 Julian Elischer said the following:
>>> The program fio (an IO test in ports) uses pthreads
>>>
>>> the following code (from fio-2.0.3, but its in earlier code too)
>>> has suddenly started misbehaving.
>>>
>>>          clock_gettime(CLOCK_REALTIME,&t);
>>>          t.tv_sec += seconds + 10;
>>>
>>>          pthread_mutex_lock(&mutex->lock);
>>>
>>>          while (!mutex->value&&  !ret) {
>>>                  mutex->waiters++;
>>>                  ret = pthread_cond_timedwait(&mutex->cond,&mutex->lock,&t);
>>>                  mutex->waiters--;
>>>          }
>>>
>>>          if (!ret) {
>>>                  mutex->value--;
>>>                  pthread_mutex_unlock(&mutex->lock);
>>>          }
>>>
>>>
>>> It turns out that 'ret' sometimes comes back instantly (on my machine) with a
>>> value of 60 (ETIMEDOUT)
>>> despite the fact that we set the timeout 10 seconds into the future.
>>>
>>> Has anyone else seen anything like this?
>>> (and yes the condition variable attribute have been set to use the REALTIME clock).
>> But why?
>>
>> Just a hypothesis that maybe there is some issue with time keeping on that system.
>> How would that code work out for you with MONOTONIC?
> 
> Jens Axboe, (CC'd) tried both CLOCK_REALTIME and CLOCK_MONOTONIC, and 
> they both had the same problem..
> i.e. random early returns with ETIMEDOUT.

Yep indeed, using either MONOTONIC or REALTIME (and having set both with
pthread_condattr_setclock()), no change in behaviour.

-- 
Jens Axboe