> On Aug 22, 2024, at 11:54, Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote: > > Hi, > > 在 2024/08/19 11:49, Muchun Song 写道: >> On Mon, Aug 19, 2024 at 10:28 AM Ming Lei <ming.lei@xxxxxxxxxx> wrote: >>> >>> Hi Muchun, >>> >>> On Sun, Aug 11, 2024 at 06:19:19PM +0800, Muchun Song wrote: >>>> Supposing the following scenario with a virtio_blk driver. >>>> >>>> CPU0 CPU1 >>>> >>>> blk_mq_try_issue_directly() >>>> __blk_mq_issue_directly() >>>> q->mq_ops->queue_rq() >>>> virtio_queue_rq() >>>> blk_mq_stop_hw_queue() >>>> virtblk_done() >>>> blk_mq_request_bypass_insert() blk_mq_start_stopped_hw_queues() >>>> /* Add IO request to dispatch list */ 1) store blk_mq_start_stopped_hw_queue() >>>> clear_bit(BLK_MQ_S_STOPPED) 3) store >>>> blk_mq_run_hw_queue() blk_mq_run_hw_queue() >>>> if (!blk_mq_hctx_has_pending()) if (!blk_mq_hctx_has_pending()) 4) load >>>> return return >>>> blk_mq_sched_dispatch_requests() blk_mq_sched_dispatch_requests() >>>> if (blk_mq_hctx_stopped()) 2) load if (blk_mq_hctx_stopped()) >>>> return return >>>> __blk_mq_sched_dispatch_requests() __blk_mq_sched_dispatch_requests() >>>> >>>> The full memory barrier should be inserted between 1) and 2), as well as between >>>> 3) and 4) to make sure that either CPU0 sees BLK_MQ_S_STOPPED is cleared or CPU1 >>>> sees dispatch list or setting of bitmap of software queue. Otherwise, either CPU >>>> will not re-run the hardware queue causing starvation. >>> >>> Yeah, it is one kind of race which is triggered when adding request into >>> ->dispatch list after returning STS_RESOURCE. We were troubled by lots of >>> such kind of race. >> Yes. I saw the similar fix for BLK_MQ_S_SCHED_RESTART. >>> >>> stopping queue is used in very less drivers, and its only purpose should >>> be for throttling hw queue in case that low level queue is busy. There seems >>> more uses of blk_mq_stop_hw_queues(), but most of them should be replaced >>> with blk_mq_quiesce_queue(). >>> >>> IMO, fixing this kind of issue via memory barrier is too tricky to >>> maintain cause WRITE/READ dependency is very hard to follow. I'd suggest to >>> make memory barrier solution as the last resort, and we can try to figure >>> out other easier & more reliable way first. >> I do agree it is hard to maintain the dependencies in the future. We should >> propose an easy-maintainable solution. But I thought it is a long-term issue >> throughout different stable linux distros. Adding a mb is the easy way to fix >> the problem (the code footprint is really small), so it will be very >> easy for others >> to backport those bug fixes to different stable linux distros. Therefore, mb >> should be an interim solution. Then, we could improve it based on the solution >> you've proposed below. What do you think? > > I'll agree with Ming, let's figure out a better fix first. Easy to backport to stables is not first consideration. Hi Kuai, All right. I usually focus on MM, it seems there is a gap between MM and BLock. Anyway, let's figure out if there is any good solution. >> Thanks, >> Muchun. >>> >>> One idea I thought of is to call blk_mq_request_bypass_insert()(or rename >>> & export it) before calling blk_mq_stop_hw_queue() in driver, then >>> return new status code STS_STOP_DISPATCH for notifying blk-mq to stop >>> dispatch simply. > > New status code look good to me, however, I wonder can we just remove > the problematic blk_mq_stop_hw_queue(), and replace it by handling the > new status from block layer? > > - Passing the new status to blk_mq_run_dispatch_ops, and quiesce with I didn't fully understand your suggestion. Let me ask some questions. blk_mq_stop_hw_queue() is usually called in blk_mq_ops->queue_rq path, it'll be easy for this case to pass the new status to blk_mq_run_dispatch_ops. Should we remove blk_mq_stop_hw_queues() as well? How to pass the new status to blk_mq_run_dispatch_ops in this case? > the new status, if no request is inflight, unquiesce immediately; Actually, I didn't understand how to avoid the above race. May you elaborate the scenario? Muhcun, Thanks. > - unquiesce is any IO is done afterwards;
