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

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Date:	Wed, 8 Apr 2015 10:00:28 -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 09:40:34AM -0400, Jarod Wilson wrote:
> 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.

Are you working on that atomic_t version then, or should I dive into it?

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

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