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

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Message-ID: <20120528102055.GA15202@dhcp-172-17-108-109.mtv.corp.google.com>
Date:	Mon, 28 May 2012 19:20:55 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Asias He <asias@...hat.com>
Cc:	Jens Axboe <axboe@...nel.dk>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH] block: Fix lock unbalance caused by lock disconnect

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.

> >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.

Thanks.

-- 
tejun
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