On Mon, Jan 08, 2018 at 09:06:55PM +0000, Bart Van Assche wrote:
> On Mon, 2018-01-08 at 11:15 -0800, Tejun Heo wrote:
> > +static void blk_mq_rq_update_aborted_gstate(struct request *rq, u64 gstate)
> > +{
> > + unsigned long flags;
> > +
> > + local_irq_save(flags);
> > + u64_stats_update_begin(&rq->aborted_gstate_sync);
> > + rq->aborted_gstate = gstate;
> > + u64_stats_update_end(&rq->aborted_gstate_sync);
> > + local_irq_restore(flags);
> > +}
>
> Please add a comment that explains the purpose of local_irq_save() and
> local_irq_restore(). Please also explain why you chose to disable interrupts
Will do.
> instead of disabling preemption. I think that disabling preemption should be
> sufficient since this is the only code that updates rq->aborted_gstate and
> since this function is always called from thread context.
blk_mq_complete_request() can read it from the irq context. If that
happens between update_begin and end, it'll end up looping infinitely.
> > @@ -801,6 +840,12 @@ void blk_mq_rq_timed_out(struct request *req, bool reserved)
> > __blk_mq_complete_request(req);
> > break;
> > case BLK_EH_RESET_TIMER:
> > + /*
> > + * As nothing prevents from completion happening while
> > + * ->aborted_gstate is set, this may lead to ignored
> > + * completions and further spurious timeouts.
> > + */
> > + blk_mq_rq_update_aborted_gstate(req, 0);
> > blk_add_timer(req);
> > blk_clear_rq_complete(req);
> > break;
>
> Is the race that the comment refers to addressed by one of the later patches?
No, but it's not a new race. It has always been there and I suppose
doesn't lead to practical problems - the race window is pretty small
and the effect isn't critical. I'm just documenting the existing race
condition. Will note that in the description.
Thanks.
--
tejun
