Date: Wed, 30 Jan 2008 16:12:01 +0100 From: "Attilio Rao" <attilio@freebsd.org> To: "Yar Tikhiy" <yar@comp.chem.msu.su> Cc: Kostik Belousov <kostikbel@gmail.com>, Doug Barton <dougb@freebsd.org>, freebsd-current@freebsd.org Subject: Re: panic: System call lstat returning with 1 locks held Message-ID: <3bbf2fe10801300712j2fa58aao163059ec9ed91882@mail.gmail.com> In-Reply-To: <3bbf2fe10801300707u3fd121c0k199605c2f0be6cbf@mail.gmail.com> References: <3bbf2fe10801250000k5852c2f2j5d1897c900096818@mail.gmail.com> <3bbf2fe10801260918k56c1a73hf28ff154eca0b74c@mail.gmail.com> <479BBDAA.6000008@FreeBSD.org> <3bbf2fe10801261657x7d7c9de4q71adeaf3a2dd8159@mail.gmail.com> <479C0B5B.9030709@FreeBSD.org> <3bbf2fe10801270642m5ec609d8xb29add77ced36d8a@mail.gmail.com> <479FA3E8.10606@FreeBSD.org> <3bbf2fe10801291411v302dd33at54ebe538397e8fac@mail.gmail.com> <20080130130820.GA88429@comp.chem.msu.su> <3bbf2fe10801300707u3fd121c0k199605c2f0be6cbf@mail.gmail.com>
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2008/1/30, Attilio Rao <attilio@freebsd.org>: > 2008/1/30, Yar Tikhiy <yar@comp.chem.msu.su>: > > On Tue, Jan 29, 2008 at 11:11:13PM +0100, Attilio Rao wrote: > > > > > > I'm committing my WITNESS patch now to perforce so that other people > > > can hopefully stress-test it before to be committed. > > > > Do you think that that patch is applicable in my case? I.e., shall > > I use it to get more debug info on my panics? > > > > If so, where is the patched file in the depot? > > Sorry but I had to delay the operation so far. > In the end, a suitable patch is located here: > http://www.freebsd.org/~attilio/witness_lockmgr.diff > > I tried it and it alredy reported 4 LORs just when booting the kernel :) > So I would expect reasonably LOR cascades with this patch. > > If you all 3 (Scot, Yar and Doug) could try and test it I would > appreciate a lot. > > Thanks, > Attilio > > PS: This is the commit log to perforce: > Add WITNESS support to lockmgr. > A couple of notes: > - Two options have been added in order to serve WITNESS: > * LK_NOWITNESS which disables the support for the specified > lock > * LK_NODUP which disallows the usual DUPOK behaviour > assumed as the default with lockmgr > - In the case of lockmgr_disown() the lock is simply dropped. > This means that a printout won't show the lock held even if it > is basically held by LK_KERNPROC > - In the case of upgrade we can have 3 different cases: > * The shared lock is unheld but consequent acquisition > fails; in this case the lock is reported dropped > * We are the first upgrader so there is an effective > WITNESS_UPGRADE > * We are not the first upgrader so after the shared unlocking > we need to acquire the lock in exclusive mode; this will be > reported with 2 different WITNESS steps. > - In the case of LK_DRAIN the lock will be only checked about the > order but it won't be marked as acquired. This happens because a > drained lock is directly destroyed and not really released, so > witness_destroy() would badly panic in this case What I forgot to mention in this log is that the patch also fixes what seems a bug to me in the case a thread holds an exclusive lock and tries to acquire the same lock in a shared way. What happens in current CVS code is that the lock cames downgraded, but it seems to be handled badly so this patch should fix the behaviour. Attilio -- Peace can only be achieved by understanding - A. Einstein
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