Resurrecting a dead horse...
*From: *Lee Revell
*Date: * Sat Aug 20 2005 - 15:57:36 EST
------------------------------------------------------------------------
On Sat, 2005-08-20 at 11:38 -0700, Howard Chu wrote:
>/ But I also found that I needed to add a new /
>/ yield(), to work around yet another unexpected issue on this system -/
>/ we have a number of threads waiting on a condition variable, and the/
>/ thread holding the mutex signals the var, unlocks the mutex, and then /
>/ immediately relocks it. The expectation here is that upon unlocking/
>/ the mutex, the calling thread would block while some waiting thread/
>/ (that just got signaled) would get to run. In fact what happened is/
>/ that the calling thread unlocked and relocked the mutex without/
>/ allowing any of the waiting threads to run. In this case the only/
>/ solution was to insert a yield() after the mutex_unlock(). /
That's exactly the behavior I would expect. Why would you expect
unlocking a mutex to cause a reschedule, if the calling thread still has
timeslice left?
Lee
POSIX requires a reschedule to occur, as noted here:
http://blog.firetree.net/2005/06/22/thread-yield-after-mutex-unlock/
The relevant SUSv3 text is here
http://www.opengroup.org/onlinepubs/000095399/functions/pthread_mutex_unlock.html
I suppose if pthread_mutex_unlock() actually behaved correctly we could
remove the other sched_yield() hacks that didn't belong there in the
first place and go on our merry way.
--
-- 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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