Re: [PATCH V7 5/6] block: support PREEMPT_ONLY

[Ming Lei]

On Mon, Oct 02, 2017 at 04:27:59PM +0000, Bart Van Assche wrote:
> On Sat, 2017-09-30 at 14:12 +0800, Ming Lei wrote:
> > +void blk_set_preempt_only(struct request_queue *q, bool preempt_only)
> > +{
> > +	blk_mq_freeze_queue(q);
> > +	if (preempt_only)
> > +		queue_flag_set_unlocked(QUEUE_FLAG_PREEMPT_ONLY, q);
> > +	else
> > +		queue_flag_clear_unlocked(QUEUE_FLAG_PREEMPT_ONLY, q);
> > +	blk_mq_unfreeze_queue(q);
> > +}
> > +EXPORT_SYMBOL(blk_set_preempt_only);
> > +
> >  /**
> >   * __blk_run_queue_uncond - run a queue whether or not it has been stopped
> >   * @q:	The queue to run
> > @@ -771,9 +782,18 @@ int blk_queue_enter(struct request_queue *q, unsigned flags)
> >  	while (true) {
> >  		int ret;
> >  
> > +		/*
> > +		 * preempt_only flag has to be set after queue is frozen,
> > +		 * so it can be checked here lockless and safely
> > +		 */
> > +		if (blk_queue_preempt_only(q)) {
> > +			if (!(flags & BLK_REQ_PREEMPT))
> > +				goto slow_path;
> > +		}
> > +
> >  		if (percpu_ref_tryget_live(&q->q_usage_counter))
> >  			return 0;
> 
> Sorry but I don't think that it is possible with these changes to prevent
> that a non-preempt request gets allocated after a (SCSI) queue has been
> quiesced. If the CPU that calls blk_queue_enter() observes the set of the
> PREEMPT_ONLY flag after the queue has been unfrozen and after the SCSI
> device state has been changed into QUIESCED then blk_queue_enter() can
> succeed for a non-preempt request. I think this is exactly the scenario

OK, looks one issue, will address it in next version.


-- 
Ming