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

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Message-ID: <20150408152857.GK14217@redhat.com>
Date:	Wed, 8 Apr 2015 11:28:57 -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 10:20:22PM +0800, Ming Lei wrote:
> On Wed, Apr 8, 2015 at 10:00 PM, Jarod Wilson <jarod@...hat.com> wrote:
> > 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?
> 
> It has been ready, and I will post them out once it passes my tests.

Okay, I'll hold tight then. Sorry for the noise. :p

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

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