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

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Date:	Tue, 29 May 2012 09:49:05 +0800
From:	Asias He <asias@...hat.com>
To:	Tejun Heo <tj@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	Christoph Hellwig <hch@...radead.org>
CC:	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 06:20 PM, Tejun Heo wrote:
> Hello, Asias.
>
> On Mon, May 28, 2012 at 10:15:18AM +0800, Asias He wrote:
>>> 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.
>
> I think hch tried to do that a while ago.  Dunno what happened to the
> patches.  IIRC, the whole external lock thing was about sharing a
> single lock across different request_queues.  Not sure whether it's
> actually beneficial enough or just a crazy broken optimization.

Do we have any existing use case of sharing a single lock across 
different request_queues? What's point of this sharing. Christoph?

If nobody has any objections I'd like to make the patches. Jens, any 
comments?

>>> 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()?
>
> Anyone who still holds a ref might try to issue a new request on a
> dead queue.  ie. blkdev with filesystem mounted goes away and the FS
> issues a new read request after blk_cleanup_queue() finishes drainig.

OK. Thanks for explaining.


-- 
Asias
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