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

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Date:	Fri, 18 Feb 2011 12:57:39 +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 Thu, 17 Feb 2011 15:00:11 -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.
> 
> Hi Neil,
> 
> I am having a look at queue allocation in md and had few queries.
> 
> I was looking at md_alloc(), where we do
> 
> mddev->queue = blk_alloc_queue(GFP_KERNEL);
> blk_queue_make_request(mddev->queue, md_make_request);
> 
> call to blk_queue_make_request() will make sure queue_lock is initiliazed
> to internal __queue_lock.

That is a relatively recent change. commit a4e7d46407d in July 2009 by the
look of it.

> 
> Then I looked at raid0_run(), which is again setting the queue lock.
> 
> mddev->queue->queue_lock = &mddev->queue->__queue_lock;
> 
> I think this is redundant now as during md_alloc() we already did it.

Yep, it is now redundant.

> 
> Similar seems to be the case for linear.c and multipath.c
> 
> Following seem to be the cases where we overide the default lock.
> 
> raid1.c, raid5.c, raid10.c
> 
> I was going through the raid1.c, and I see that q->queue_lock has
> been initialized to &conf->deivce_lock. Can we do the reverse. Introduce
> spinlock pointer in conf and point it at queue->queue_lock? Anyway you
> mentioned that personality's data structure are freed before request
> queue is cleaned up, so there should not be any lifetime issues.

The reason it is this way is largely historical.  I don't recall the details
exactly but md data structures were always quite independent of request_queue.

The only reason I set ->queue_lock at all is because some blk functions
started checking that it was held ... something to do with plugging and
stacking limits I think.  See commit e7e72bf641b1 (may 2008) for more details.


> 
> Also  I was wondering how does it help sharing the lock between request
> queue and some other data structures in driver.

All it does is silence some warnings.

NeilBrown

> 
> Thanks
> Vivek
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