linux-kernel - Re: 2.6.17-rc6-mm1/pktcdvd - BUG: possible circular locking

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Date:	Fri, 14 Jul 2006 15:46:10 +0200
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Peter Osterlund <petero2@...ia.com>
Cc:	Laurent Riffard <laurent.riffard@...e.fr>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	axboe@...e.de
Subject: Re: 2.6.17-rc6-mm1/pktcdvd - BUG: possible circular locking

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....

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