Re: pthread_mutex_unlock (was Re: sched_yield() makes OpenLDAP slow)

[Mark Lord]

David Schwartz wrote:
> > Kaz's post clearly interprets the POSIX spec differently from you. The
> > policy can decide *which of the waiting threads* gets the mutex, but the
> > releasing thread is totally out of the picture. For good or bad, the
> > current pthread_mutex_unlock() is not POSIX-compliant. Now then, if
> > we're forced to live with that, for efficiency's sake, that's OK,
> > assuming that valid workarounds exist, such as inserting a sched_yield()
> > after the unlock.
>
> 	My thanks to David Hopwood for providing me with the definitive refutation
> of this position. The response is that the as-if rules allows the
> implementation to violate the specification internally provided no compliant
> application could tell the difference.
>
> 	When you call 'pthread_mutex_lock', there is no guarantee regarding how
> long it will or might take until you are actually waiting for the mutex. So
> no conforming application can ever tell whether or not it is waiting for the
> mutex or about to wait for the mutex.
>
> 	So you cannot write an application that can tell the difference.

Not true.  The code for the relinquishing thread could indeed tell the difference.

-ml
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