Re: [RFC PATCH 3/3] blk-mq: Remove generation seqeunce

Keith Busch
2018-07-12T19:24:37Z

On Thu, Jul 12, 2018 at 06:16:12PM +0000, Bart Van Assche wrote:
> On Mon, 2018-05-21 at 17:11 -0600, Keith Busch wrote:
> > /*
> > - * We marked @rq->aborted_gstate and waited for RCU. If there were
> > - * completions that we lost to, they would have finished and
> > - * updated @rq->gstate by now; otherwise, the completion path is
> > - * now guaranteed to see @rq->aborted_gstate and yield. If
> > - * @rq->aborted_gstate still matches @rq->gstate, @rq is ours.
> > + * Just do a quick check if it is expired before locking the request in
> > + * so we're not unnecessarilly synchronizing across CPUs.
> > */
> > - if (!(rq->rq_flags & RQF_MQ_TIMEOUT_EXPIRED) &&
> > - READ_ONCE(rq->gstate) == rq->aborted_gstate)
> > + if (!blk_mq_req_expired(rq, next))
> > + return;
> > +
> > + /*
> > + * We have reason to believe the request may be expired. Take a
> > + * reference on the request to lock this request lifetime into its
> > + * currently allocated context to prevent it from being reallocated in
> > + * the event the completion by-passes this timeout handler.
> > + *
> > + * If the reference was already released, then the driver beat the
> > + * timeout handler to posting a natural completion.
> > + */
> > + if (!kref_get_unless_zero(&rq->ref))
> > + return;
> > +
> > + /*
> > + * The request is now locked and cannot be reallocated underneath the
> > + * timeout handler's processing. Re-verify this exact request is truly
> > + * expired; if it is not expired, then the request was completed and
> > + * reallocated as a new request.
> > + */
> > + if (blk_mq_req_expired(rq, next))
> > blk_mq_rq_timed_out(rq, reserved);
> > + blk_mq_put_request(rq);
> > }
>
> Hello Keith and Christoph,
>
> What prevents that a request finishes and gets reused after the
> blk_mq_req_expired() call has finished and before kref_get_unless_zero() is
> called? Is this perhaps a race condition that has not yet been triggered by
> any existing block layer test? Please note that there is no such race
> condition in the patch I had posted ("blk-mq: Rework blk-mq timeout handling
> again" - https://www.spinics.net/lists/linux-block/msg26489.html).

I don't think there's any such race in the merged implementation
either. If the request is reallocated, then the kref check may succeed,
but the blk_mq_req_expired() check would surely fail, allowing the
request to proceed as normal. The code comments at least say as much.