linux-kernel - Re: [PATCH] loop: queue_lock NULL pointer derefence in blk_throtl

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Message-ID: <20110120151738.GA4665@swordfish.minsk.epam.com>
Date:	Thu, 20 Jan 2011 17:18:22 +0200
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Jens Axboe <jaxboe@...ionio.com>,
	Philipp Reisner <philipp.reisner@...bit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lars Ellenberg <lars.ellenberg@...bit.com>,
	"Stephen M. Cameron" <scameron@...rdog.cce.hp.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] loop: queue_lock NULL pointer derefence in
 blk_throtl_exit (v2)

On (01/20/11 09:35), Vivek Goyal wrote:
> Hi Sergey,
>

Hi Vivek,

> Can we expand a little bit on comment that why do we need to have
> q->queue_lock initialized here now. Basically in the past nobody tried
> to take q->queue_lock in blk_cleanup_queue() path hence things just
> worked. Now blk throttling code is new and it takes q->queue_lock hence we
> run into issues. This could be true for some other future code too.
> 
> Secondly currently blk throttle code seems to be the only user dependent
> on this lock initialization. So it might make sense to move this code
> closer to the actual call and blk_release_queue() might be even better
> place to do it atleast for now.
> 

I'm afraid it's not safe to move NULL-check-and-fix out from blk_cleanup_queue, 
since we're performing elevator_exit(q->elevator) call, which may depend on 
q->queue_lock. For example, (./cfq-iosched.c: .elevator_exit_fn = cfq_exit_queue)
cfq_exit_queue uses q->queue_lock.


	Sergey

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