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SubjectRe: [PATCH 2/2] blk-mq: Fix request handover from timeout path to normal execution
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On Mon, 2018-04-02 at 14:10 -0700, Tejun Heo wrote:
> On Mon, Apr 02, 2018 at 02:08:37PM -0700, Bart Van Assche wrote:
> > On 04/02/18 12:01, Tejun Heo 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);
> >
> > Hello Tejun,
> >
> > Since this patch fixes one race but introduces another race, is this
> > patch really an improvement?
>
> Oh, that's not a new race. That's the same non-critical race which
> always existed. It's just being documented.

Hello Tejun,

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?

Thanks,

Bart.



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