Date: Thu, 16 Sep 2004 16:15:58 -0600 From: Scott Long <scottl@samsco.org> To: Nate Lawson <nate@root.org> Cc: cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/dev/md md.c Message-ID: <414A109E.4080601@samsco.org> 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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Nate Lawson wrote: > Pawel Jakub Dawidek wrote: > >> On Thu, Sep 16, 2004 at 12:40:23PM -0700, Nate Lawson wrote: >> +> >@@ -379,9 +379,8 @@ >> +> > bp->bio_bcount = bp->bio_length; >> +> > mtx_lock(&sc->queue_mtx); >> +> > bioq_disksort(&sc->bio_queue, bp); >> +> >- mtx_unlock(&sc->queue_mtx); >> +> >- >> +> > wakeup(sc); >> +> >+ mtx_unlock(&sc->queue_mtx); >> +> > } >> +> +> I think the original order is correct since you can occur 2 >> switches if +> you wakeup first and then unlock. >> >> Nope, this order was wrong: >> >> thread1 thread2 >> ----------------------- >> mtx_lock(mtx) >> ... >> mtx_unlock(mtx) >> mtx_lock(mtx) >> wakeup(ptr) >> msleep(ptr, mtx) <- Race, it will be never woken up. > > > You still have a race, like this: > > thread1 thread2 > ----------------------------- > mtx_lock(mtx) > wakeup(ptr) > mtx_unlock(mtx) > mtx_lock(mtx) > msleep(ptr, mtx) > > 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: > > 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 > > Since the work item is added in thread1 with the mutex held, the check > for it in thread2 is safe and race-free. A wakeup is only there to > kickstart thread2 if it's asleep. If it's running, it needs to check > atomically that there is no work before sleeping. If it doesn't do > this, it's a bug. > Or just use a semaphore. Scott
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