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

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Lee Revell wrote:
On Wed, 2006-01-25 at 10:26 -0800, Howard Chu wrote:
The SUSv3 text seems pretty clear. It says "WHEN
pthread_mutex_unlock() is called, ... the scheduling policy SHALL decide ..." It doesn't say MAY, and it doesn't say "some undefined time after the call."

This does NOT require pthread_mutex_unlock() to cause the scheduler to
immediately pick a new runnable process.  It only says it's up the the
scheduling POLICY what to do.  The policy could be "let the unlocking
thread finish its timeslice then reschedule".

This is obviously some very old ground.

http://groups.google.com/groups?threadm=etai7.108188%24B37.2381726%40news1.rdc1.bc.home.com

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.

http://groups.google.com/group/comp.programming.threads/msg/16c01eac398a1139?hl=en&;

But then we have to deal with you folks' bizarre notion that sched_yield() can legitimately be a no-op, which also defies the POSIX spec. Again, in SUSv3 "The /sched_yield/() function shall force the running thread to relinquish the processor until it again becomes the head of its thread list. It takes no arguments." There is no language here saying "sched_yield *may* do nothing at all." There are of course cases where it will have no effect, such as when called in a single-threaded program, but those are the exceptions that define the rule. Otherwise, the expectation is that some other runnable thread will acquire the CPU. Again, note that sched_yield() is a core function of the Threads specification, while scheduling policies are an optional feature. The function's core behavior (give up the CPU and make some other runnable thread run) is invariant; the current thread gives up the CPU regardless of which scheduling policy is in effect or even if scheduling policies are implemented at all. The only behavior that's open to implementors is which *of the other runnable threads* is chosen to take the place of the current thread.

--
 -- Howard Chu
 Chief Architect, Symas Corp.  http://www.symas.com
 Director, Highland Sun        http://highlandsun.com/hyc
 OpenLDAP Core Team            http://www.openldap.org/project/

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