linux-kernel - Re: [PATCH 2/7] block: loop: don't hold lo_ctl_mutex in lo

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Message-ID: <20150408134034.GH14217@redhat.com>
Date:	Wed, 8 Apr 2015 09:40:34 -0400
From:	Jarod Wilson <jarod@...hat.com>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Markus Pargmann <mpa@...gutronix.de>,
	Stefan Weinhuber <wein@...ibm.com>,
	Stefan Haberland <stefan.haberland@...ibm.com>,
	Sebastian Ott <sebott@...ux.vnet.ibm.com>,
	Fabian Frederick <fabf@...net.be>,
	David Herrmann <dh.herrmann@...il.com>,
	Mike Galbraith <bitbucket@...ine.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"nbd-general@...ts.sourceforge.net" 
	<nbd-general@...ts.sourceforge.net>, linux-s390@...r.kernel.org
Subject: Re: [PATCH 2/7] block: loop: don't hold lo_ctl_mutex in lo_open

On Wed, Apr 08, 2015 at 02:50:59PM +0800, Ming Lei wrote:
> Hi Jarod,
> 
> On Wed, Apr 8, 2015 at 2:23 PM, Jarod Wilson <jarod@...hat.com> wrote:
> > From: Ming Lei <ming.lei@...onical.com>
> >
> > The lo_ctl_mutex is held for running all ioctl handlers, and
> > in some ioctl handlers, ioctl_by_bdev(BLKRRPART) is called for
> > rereading partitions, which requires bd_mutex.
> >
> > So it is easy to cause failure because trylock(bd_mutex) may
> > fail inside blkdev_reread_part(), and follows the lock context:
> >
> > blkid or other application:
> >         ->open()
> >                 ->mutex_lock(bd_mutex)
> >                 ->lo_open()
> >                         ->mutex_lock(lo_ctl_mutex)
> >
> > losetup(set fd ioctl):
> >         ->mutex_lock(lo_ctl_mutex)
> >         ->ioctl_by_bdev(BLKRRPART)
> >                 ->trylock(bd_mutex)
> >
> > This patch trys to eliminate the ABBA lock dependency by removing
> > lo_ctl_mutext in lo_open() with the following approach:
> >
> > 1) introduce lo_open_mutex to protect lo_refcnt and avoid acquiring
> > lo_ctl_mutex in lo_open():
> 
> It is a bit quick since I said the lo_open_mutex can be removed,
> and Christoph agreed that too.
> 
> So looks we still need to post another version, :-)

Ah. I missed that bit. Just trying to keep up momentum.

-- 
Jarod Wilson
jarod@...hat.com

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