Re: [PATCH] blk-mq: don't grab rq's refcount in blk_mq_check_expired()

[Ming Lei <ming.lei@redhat.com>]

On Wed, Aug 11, 2021 at 08:55:25AM -0600, Keith Busch wrote:
> On Wed, Aug 11, 2021 at 10:38:38PM +0800, Ming Lei wrote:
> > Inside blk_mq_queue_tag_busy_iter() we already grabbed request's
> > refcount before calling ->fn(), so needn't to grab it one more time
> > in blk_mq_check_expired().
> > 
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >  block/blk-mq.c | 25 +++++++------------------
> >  1 file changed, 7 insertions(+), 18 deletions(-)
> > 
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index d2725f94491d..4d3457d2957f 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -917,6 +917,10 @@ void blk_mq_put_rq_ref(struct request *rq)
> >  		__blk_mq_free_request(rq);
> >  }
> >  
> > +/*
> > + * This request won't be re-allocated because its refcount is held when
> > + * calling this callback.
> > + */
> >  static bool blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
> >  		struct request *rq, void *priv, bool reserved)
> >  {
> > @@ -930,27 +934,12 @@ static bool blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
> >  		return true;
> >  
> >  	/*
> > -	 * 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 (!refcount_inc_not_zero(&rq->ref))
> > -		return true;
> > -
> > -	/*
> > -	 * 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.
> > +	 * Re-verify this exact request is truly expired; if it is not expired,
> > +	 * then the request was completed and reallocated as a new request
> > +	 * after returning from blk_mq_check_expired().
> >  	 */
> >  	if (blk_mq_req_expired(rq, next))
> >  		blk_mq_rq_timed_out(rq, reserved);
> 
> There's no need to check expired twice if the iterator is already taking a
> reference. I had this double check because I didn't want to penalize normal IO
> by preventing it from completing while the timeout handler is running, but it
> looks like the current timeout iterator is going to do that anyway.

Indeed, will clean that in V2.


Thanks,
Ming