Re: [PATCH V8 07/11] blk-mq: stop to handle IO and drain IO before hctx becomes inactive

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

On Wed, Apr 29, 2020 at 01:27:57PM +0100, Will Deacon wrote:
> On Wed, Apr 29, 2020 at 05:46:16PM +0800, Ming Lei wrote:
> > On Wed, Apr 29, 2020 at 09:07:29AM +0100, Will Deacon wrote:
> > > On Wed, Apr 29, 2020 at 10:16:12AM +0800, Ming Lei wrote:
> > > > On Tue, Apr 28, 2020 at 05:58:37PM +0200, Peter Zijlstra wrote:
> > > > > On Sat, Apr 25, 2020 at 05:48:32PM +0200, Christoph Hellwig wrote:
> > > > > >  		atomic_inc(&data.hctx->nr_active);
> > > > > >  	}
> > > > > >  	data.hctx->tags->rqs[rq->tag] = rq;
> > > > > >  
> > > > > >  	/*
> > > > > > +	 * Ensure updates to rq->tag and tags->rqs[] are seen by
> > > > > > +	 * blk_mq_tags_inflight_rqs.  This pairs with the smp_mb__after_atomic
> > > > > > +	 * in blk_mq_hctx_notify_offline.  This only matters in case a process
> > > > > > +	 * gets migrated to another CPU that is not mapped to this hctx.
> > > > > >  	 */
> > > > > > +	if (rq->mq_ctx->cpu != get_cpu())
> > > > > >  		smp_mb();
> > > > > > +	put_cpu();
> > > > > 
> > > > > This looks exceedingly weird; how do you think you can get to another
> > > > > CPU and not have an smp_mb() implied in the migration itself? Also, what
> > > > 
> > > > What we need is one smp_mb() between the following two OPs:
> > > > 
> > > > 1) 
> > > >    rq->tag = rq->internal_tag;
> > > >    data.hctx->tags->rqs[rq->tag] = rq;
> > > > 
> > > > 2) 
> > > > 	if (unlikely(test_bit(BLK_MQ_S_INACTIVE, &rq->mq_hctx->state)))
> > > > 
> > > > And the pair of the above barrier is in blk_mq_hctx_notify_offline().
> > > 
> > > I'm struggling with this, so let me explain why. My understanding of the
> > > original patch [1] and your explanation above is that you want *either* of
> > > the following behaviours
> > > 
> > >   - __blk_mq_get_driver_tag() (i.e. (1) above) and test_bit(BLK_MQ_S_INACTIVE, ...)
> > >     run on the same CPU with barrier() between them, or
> > > 
> > >   - There is a migration and therefore an implied smp_mb() between them
> > > 
> > > However, given that most CPUs can speculate loads (and therefore the
> > > test_bit() operation), I don't understand how the "everything runs on the
> > > same CPU" is safe if a barrier() is required.  In other words, if the
> > > barrier() is needed to prevent the compiler hoisting the load, then the CPU
> > > can still cause problems.
> > 
> > Do you think the speculate loads may return wrong value of
> > BLK_MQ_S_INACTIVE bit in case of single CPU? BTW, writing the bit is
> > done on the same CPU. If yes, this machine may not obey cache consistency,
> > IMO.
> 
> If the write is on the same CPU, then the read will of course return the
> value written by that write, otherwise we'd have much bigger problems!

OK, then it is nothing to with speculate loads.

> 
> But then I'm confused, because you're saying that the write is done on the
> same CPU, but previously you were saying that migration occuring before (1)
> was problematic. Can you explain a bit more about that case, please? What
> is running before (1) that is relevant here?

Please see the following two code paths:

[1] code path1:
blk_mq_hctx_notify_offline():
	set_bit(BLK_MQ_S_INACTIVE, &hctx->state);

	smp_mb() or smp_mb_after_atomic()

	blk_mq_hctx_drain_inflight_rqs():
		blk_mq_tags_inflight_rqs()
			rq = hctx->tags->rqs[index]
			and
			READ rq->tag

[2] code path2:
	blk_mq_get_driver_tag():

		process might be migrated to other CPU here and chance is small,
		then the follow code will be run on CPU different with code path1

		rq->tag = rq->internal_tag;
		hctx->tags->rqs[rq->tag] = rq;

		barrier() in case that code path2 is run on same CPU with code path1
		OR
		smp_mb() in case that code path2 is run on different CPU with code path1 because
		of process migration
		
		test_bit(BLK_MQ_S_INACTIVE, &data.hctx->state)



Thanks,
Ming