linux-kernel - Re: blk_throtl_exit taking q->queue

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Message-ID: <20110217113536.2bbf308e@notabene.brown>
Date:	Thu, 17 Feb 2011 11:35:36 +1100
From:	NeilBrown <neilb@...e.de>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Jens Axboe <jaxboe@...ionio.com>, linux-kernel@...r.kernel.org
Subject: Re: blk_throtl_exit  taking q->queue_lock is problematic

On Wed, 16 Feb 2011 10:53:05 -0500 Vivek Goyal <vgoyal@...hat.com> wrote:

> On Wed, Feb 16, 2011 at 06:31:14PM +1100, NeilBrown wrote:
> > 
> > 
> > Hi,
> > 
> >  I recently discovered that blk_throtl_exit takes ->queue_lock when a blockdev
> > is finally released.
> > 
> > This is a problem for because by that time the queue_lock doesn't exist any
> > more.  It is in a separate data structure controlled by the RAID personality
> > and by the time that the block device is being destroyed the raid personality
> > has shutdown and the data structure containing the lock has been freed.
> > 
> > This has not been a problem before.  Nothing else takes queue_lock after
> > blk_cleanup_queue.
> 
> I agree that this is a problem. blk_throtl_exit() needs queue lock to
> avoid other races with cgroup code and for avoiding races for its
> lists etc.
> 
> > 
> > I could of course set queue_lock to point to __queue_lock and initialise that,
> > but it seems untidy and probably violates some locking requirements.
> > 
> > Is there some way you could use some other lock - maybe a global lock, or
> > maybe used __queue_lock directly ???
> 
> Initially I had put blk_throtl_exit() in blk_cleanup_queue() where it is
> known that ->queue_lock is still around. Due to a bug, Jens moved it
> to blk_release_queue(). I still think that blk_cleanup_queue() is a better
> place to call blk_throtl_exit().

Why do you say that it is known that ->queue_lock is still around in
blk_cleanup_queue?  In md it isn't. :-(
Is there some (other) reason that it needs to be?

Thanks,
NeilBrown

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