Re: [PATCH 2/2] blk-mq: Fix request handover from timeout path to normal execution

[tj@xxxxxxxxxx]

Hello,

On Mon, Apr 02, 2018 at 09:31:34PM +0000, Bart Van Assche wrote:
> > > > +	 * As nothing prevents from completion happening while
> > > > +	 * ->aborted_gstate is set, this may lead to ignored completions
> > > > +	 * and further spurious timeouts.
> > > > +	 */
> > > > +	if (rq->rq_flags & RQF_MQ_TIMEOUT_RESET)
> > > > +		blk_mq_rq_update_aborted_gstate(rq, 0);
...
> I think it can happen that the above code sees that (rq->rq_flags &
> RQF_MQ_TIMEOUT_RESET) != 0, that blk_mq_start_request() executes the
> following code:
> 
> 	blk_mq_rq_update_state(rq, MQ_RQ_IN_FLIGHT);
> 	blk_add_timer(rq);
> 
> and that subsequently blk_mq_rq_update_aborted_gstate(rq, 0) is called,
> which will cause the next completion to be lost. Is fixing one occurrence
> of a race and reintroducing it in another code path really an improvement?

I'm not following at all.  How would blk_mq_start_request() get called
on the request while it's still owned by the timeout handler?  That
gstate clearing is what transfers the ownership back to the
non-timeout path.

Thanks.

-- 
tejun