Message-ID: <43D8386B.6000204@rtr.ca>
Date: Wed, 25 Jan 2006 21:48:11 -0500
From: Mark Lord <lkml@rtr.ca>
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To: davids@webmaster.com
Cc: Lee Revell <rlrevell@joe-job.com>,
       Christopher Friesen <cfriesen@nortel.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       hancockr@shaw.ca
Subject: Re: pthread_mutex_unlock (was Re: sched_yield() makes OpenLDAP slow)
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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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