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

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

On Tue, May 22, 2018 at 10:49:11AM +0800, Ming Lei wrote:
> On Mon, May 21, 2018 at 05:11:31PM -0600, Keith Busch wrote:
> > -static void blk_mq_terminate_expired(struct blk_mq_hw_ctx *hctx,
> > +static void blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
> >  		struct request *rq, void *priv, bool reserved)
> >  {
> > +	unsigned long *next = priv;
> > +
> >  	/*
> > -	 * 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;
> 
> If this request is just completed in normal path and its state isn't
> updated yet, timeout will hold the request, and may complete this
> request again, then this req can be completed two times.

Hi Ming,

In the event the driver requests a normal completion, the timeout work
releasing the last reference doesn't do a second completion: it only
releases the request's tag back for re-allocation.

Thanks,
Keith