On Wed, Apr 29, 2020 at 10:46:36AM -0700, Paul E. McKenney wrote: > On Wed, Apr 29, 2020 at 09:43:27PM +0800, Ming Lei wrote: > > 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 > > If the process is migrated from one CPU to another, each CPU will execute > full barriers (smp_mb() or equivalent) as part of the migration. Do those > barriers help prevent the undesired outcome? No, WRITE on rq->tag will be run on the new CPU which becomes different with another CPU on which READ rq->tag is done. Thanks, Ming
