Re: [PATCH 5/5] blk-mq: Rework blk-mq timeout handling again

[Keith Busch <keith.busch@intel.com>]

On Fri, Jul 27, 2018 at 04:51:05PM +0000, Bart Van Assche wrote:
> On Fri, 2018-07-27 at 10:46 -0600, Keith Busch wrote:
> > On Fri, Jul 27, 2018 at 09:20:42AM -0700, Bart Van Assche wrote:
> > > +	ret = req->q->mq_ops->timeout(req, reserved);
> > > +	/*
> > > +	 * BLK_EH_DONT_RESET_TIMER means that the block driver either
> > > +	 * completed the request or still owns the request and will
> > > +	 * continue processing the timeout asynchronously. In the
> > > +	 * latter case, if blk_mq_complete_request() was called while
> > > +	 * the timeout handler was in progress, ignore that call.
> > > +	 */
> > > +	if (ret == BLK_EH_DONT_RESET_TIMER)
> > > +		return;
> > 
> > This is how completions get lost.
> 
> The new approach for handling completions that occur while the .timeout()
> callback in progress is as follows:
> * blk_mq_complete_request() executes the following code:
>        if (blk_mq_change_rq_state(rq, MQ_RQ_TIMED_OUT, MQ_RQ_COMPLETE))
>                return;
> * blk_mq_rq_timed_out() executes the following code:
>        if (blk_mq_rq_state(req) == MQ_RQ_COMPLETE) {
>                __blk_mq_complete_request(req);
>                return;
>        }
> 
> As one can see __blk_mq_complete_request() gets called if this race occurs.

You skip that code if the driver returns BLK_EH_DONT_RESET_TIMER.