linux-kernel - Re: [PATCH] block: Fix lock unbalance caused by lock disconnect

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Message-ID: <4FC2DFB6.6080701@redhat.com>
Date:	Mon, 28 May 2012 10:15:18 +0800
From:	Asias He <asias@...hat.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Jens Axboe <axboe@...nel.dk>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: Fix lock unbalance caused by lock disconnect

On 05/28/2012 08:07 AM, Tejun Heo wrote:
> On Fri, May 25, 2012 at 10:10:59AM +0800, Asias He wrote:
>> Commit 777eb1bf15b8532c396821774bf6451e563438f5 disconnects externally
>> supplied queue_lock before blk_drain_queue(). This would introduce lock
>> unbalance because theads which have taken the external lock might unlock
>> the internal lock in the during the queue drain.
>>
>> This patch fixes this by disconnecting the lock after the queue drain.
>
> I don't think the patch description is correct.  The lock switcihng is
> inherently broken and the patch doesn't really fix the problem
> although it *might* make the problem less likely.  Trying to switch
> locks while there are other accessors of the lock is simply broken, it
> can never work without outer synchronization.

Since the lock switching is broken, is it a good idea to force all the 
drivers to use the block layer provided lock? i.e. Change the API from
blk_init_queue(rfn, driver_lock) to blk_init_queue(rfn). Any reason not 
to use the block layer provided one.

> Your patch might make
> the problem somewhat less likely simply because queue draining makes a
> lot of request_queue users go away.

Who will use the request_queue after blk_cleanup_queue()?

> So, can you please update the commit description?  It doesn't really
> *fix* anything but I think we're still better off with the change.

Sure. Will send v2.

-- 
Asias
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