Date: Thu, 16 Sep 2004 18:25:15 -0400 (EDT) From: Garrett Wollman <wollman@khavrinen.lcs.mit.edu> To: Nate Lawson <nate@root.org> Cc: cvs-all@FreeBSD.ORG Subject: Re: cvs commit: src/sys/dev/md md.c Message-ID: <200409162225.i8GMPFFs010481@khavrinen.lcs.mit.edu> In-Reply-To: <414A1073.8010404@root.org> References: <20040916185923.2F92316A552@hub.freebsd.org> <4149EC27.9080200@root.org> <20040916204321.GE30151@darkness.comp.waw.pl> <414A1073.8010404@root.org>
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<<On Thu, 16 Sep 2004 15:15:15 -0700, Nate Lawson <nate@root.org> said:
> You should be checking the work condition in thread 2 while holding the
> mutex but before going to sleep. Adding work to the queue happens in
> thread 1 where you write "..." and that is done with the mutex held so
> there is no race. The full diagram with this detail included is:
Of course, getting this right is complicated enough that we have an
entire abstraction to assist.
> thread1 thread2
> -----------------------------
> mtx_lock(mtx)
> add work to queue
> mtx_unlock(mtx)
> mtx_lock(mtx)
> wakeup(ptr)
> check queue for work item
> if (!work item)
> msleep(ptr, mtx)
> else
> dequeue work item and loop
mtx_lock(mtx)
add work to queue
cv_signal(worktodo)
mtx_unlock(mtx)
mtx_lock(mtx)
for (;;) {
check queue for work item
if (!work item)
cv_wait(cv, mtx)
else {
dequeue work item
do work
}
}
mtx_unlock(mtx)
-GAWollman
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