linux-kernel - Re: [PATCH 2/2] blk-mq: remove unnecessary blk_clear_rq

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Date:	Thu, 18 Sep 2014 18:26:38 +0200
From:	Christoph Hellwig <hch@....de>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ming Lei <ming.lei@...oical.com>
Subject: Re: [PATCH 2/2] blk-mq: remove unnecessary blk_clear_rq_complete()

On Thu, Sep 18, 2014 at 09:33:35AM +0800, Ming Lei wrote:
> > Why yo you think it's not nessecary?  The request is not in the drivers
> 
> The COMPLETED flag will be cleared in blk_mq_start_request(), so
> it needn't to be cleared here, and it is a bit early and might cause
> a tiny race window.
> 
> > hand at this point, so it should not be marked started.  Maybe I'm missing
> > something, but this sounds like it could very likely cause regressions.
> 
> The STARTED flag has been cleared in __blk_mq_requeue_request(),
> I mean the COMPLETED flag, consider the following situation:
> 
> - one req is queued to driver, and the req is completed from
> the device just before its timeout expired
> - the driver checks the result and finds it need to requeue for some reason
> - blk_mq_requeue_request() is called to requeue the req
> - COMPLETED is cleared before clearing STARTED because of
> writes reorder
> - timeout just comes between the two writes
> - then the request may be completed another time because of timeout

Ok, I guess you are right on this one.  Objection cleared.
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