Date: Tue, 17 Jun 2003 20:08:50 -0700 (PDT) From: Don Lewis <truckman@FreeBSD.org> To: alfred@FreeBSD.org Cc: current@FreeBSD.org Subject: Re: Giant pushdown in kern_descrip.c rev 1.128 Message-ID: <200306180308.h5I38oM7053457@gw.catspoiler.org> In-Reply-To: <20030617220231.GV62025@elvis.mu.org>
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On 17 Jun, Alfred Perlstein wrote: > * Don Lewis <truckman@FreeBSD.org> [030617 13:06] wrote: >> On 17 Jun, Alfred Perlstein wrote: >> > * Don Lewis <truckman@FreeBSD.org> [030617 12:00] wrote: >> >> It's not legal to attempt to aquire Giant in fdrop_locked(), while >> >> FILE_LOCK() is held. The problem is that FILE_LOCK uses the mutex pool, >> >> which should only be used for leaf mutexes. >> >> >> >> It also looks like there is a potential for a lock order reversal if >> >> some callers aquire Giant before FILE_LOCK() and fdrop_locked() does the >> >> opposite. >> >> >> >> It also appears that witness ignores the mutex pool ... >> > >> > Yes, but I think the fix is as simple as just dropping the FILE_LOCK >> > after the decrement as we're the last holders of it, can you try >> > this: >> >> I like simple fixes, especially when the code shrinks ;-) >> >> Unfortunately, I think your point about this only happening because this >> process is the last holder of the file means that this doesn't explain >> Peter's deadlock. > > You can still deadlock because another file's mutex may hash to the same > location. ... or some other user of the mutex pool that happens to hold Giant. I'm in favor of committing your patch, though I think it should be commented to indicate why it is safe to play with fp after the mutex has been unlocked.
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