From: Peter Osterlund <petero2@telia.com>
To: Arjan van de Ven <arjan@linux.intel.com>
Cc: Laurent Riffard <laurent.riffard@free.fr>,
Kernel development list <linux-kernel@vger.kernel.org>,
axboe@suse.de
Subject: Re: 2.6.17-rc6-mm1/pktcdvd - BUG: possible circular locking
Date: 14 Jul 2006 23:06:51 +0200 [thread overview]
Message-ID: <m3odvrc2vo.fsf@telia.com> (raw)
In-Reply-To: <1152884770.3159.37.camel@laptopd505.fenrus.org>
Arjan van de Ven <arjan@linux.intel.com> writes:
> On Fri, 2006-07-14 at 13:22 +0200, Peter Osterlund wrote:
> > > and what locking prevents this? And via multiple opens?
> >
> > You are right that my reasoning was incorrect. If someone is doing
> > "pktsetup ; pktsetup -d" quickly in a loop while someone else is
> > trying to open the device, one thread could be at the start of
> > pkt_open() at the same time as another thread is in pkt_new_dev().
> >
> > However, I added a 5s delay in pkt_open() to enlarge the race window.
> > I still couldn't make the driver lock up though. The explanation is
> > that pkt_new_dev() calls blkdev_get() with the CD device (eg /dev/hdc)
> > as bdev parameter, while do_open() locks the bd_mutex for the pktcdvd
> > device (eg /dev/pktcdvd/0).
> >
> > Do you still think this could deadlock? If not, how should the code be
> > annotated to make this warning go away?
>
> unless we KNOW it won't deadlock (eg we have a "this cannot deadlock
> BECAUSE of X, Y and Z") I don't think annotations are the right idea. In
> addition, the "how to annotate" really depends on what X, Y and Z
> are....
In the first call chain, do_open -> pkt_open, the bd_mutex object that
is being locked corresponds to a pktcdvd block device, because those
are the only devices that have their open method set to pkt_open.
In the second call chain, pkt_ctl_ioctl -> pkt_new_dev -> do_open, the
bd_mutex object that is being locked *does not* correspond to a
pktcdvd block device, because pkt_new_dev will bail out with a "Can't
chain pktcdvd devices" error if you call it with "dev" set to a
pktcdvd device.
Therefore, there is no AB-BA deadlock possibility. The locking
dependencies are A -> B and B -> A', where it is known that A, B and
A' are all different.
So the claim from the lockdep code, "BUG: possible circular locking
deadlock detected!", is a false alarm.
--
Peter Osterlund - petero2@telia.com
http://web.telia.com/~u89404340
next prev parent reply other threads:[~2006-07-14 21:07 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-06-08 19:09 2.6.17-rc6-mm1/pktcdvd - BUG: possible circular locking Laurent Riffard
2006-06-12 15:14 ` Arjan van de Ven
2006-06-12 16:41 ` Jens Axboe
2006-06-21 10:21 ` Arjan van de Ven
2006-06-22 14:50 ` Peter Osterlund
2006-06-22 18:20 ` Arjan van de Ven
2006-07-14 11:22 ` Peter Osterlund
2006-07-14 13:46 ` Arjan van de Ven
2006-07-14 21:06 ` Peter Osterlund [this message]
2006-07-14 22:29 ` Arjan van de Ven
2006-07-15 7:04 ` [patch] lockdep: annotate pktcdvd natural device hierarchy Arjan van de Ven
2006-07-15 10:35 ` Laurent Riffard
2006-07-15 10:57 ` Peter Osterlund
2006-07-16 10:33 ` Laurent Riffard
2006-07-25 2:27 ` Andrew Morton
2006-07-25 5:34 ` Arjan van de Ven
2006-07-25 6:31 ` Andrew Morton
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