On 12/18/24 19:10, Ming Lei wrote: > On Wed, Dec 18, 2024 at 05:03:00PM +0530, Nilay Shroff wrote: >> >> >> On 12/18/24 07:39, Ming Lei wrote: >>> On Tue, Dec 17, 2024 at 08:07:06AM -0800, Damien Le Moal wrote: >>>> On 2024/12/16 23:30, Ming Lei wrote: >>>>> On Tue, Dec 17, 2024 at 08:19:28AM +0100, Christoph Hellwig wrote: >>>>>> On Tue, Dec 17, 2024 at 03:05:48PM +0800, Ming Lei wrote: >>>>>>> On Tue, Dec 17, 2024 at 05:40:56AM +0100, Christoph Hellwig wrote: >>>>>>>> On Tue, Dec 17, 2024 at 09:52:51AM +0800, Ming Lei wrote: >>>>>>>>> The local copy can be updated in any way with any data, so does another >>>>>>>>> concurrent update on q->limits really matter? >>>>>>>> >>>>>>>> Yes, because that means one of the updates get lost even if it is >>>>>>>> for entirely separate fields. >>>>>>> >>>>>>> Right, but the limits are still valid anytime. >>>>>>> >>>>>>> Any suggestion for fixing this deadlock? >>>>>> >>>>>> What is "this deadlock"? >>>>> >>>>> The commit log provides two reports: >>>>> >>>>> - lockdep warning >>>>> >>>>> https://lore.kernel.org/linux-block/Z1A8fai9_fQFhs1s@xxxxxxxxxxxxxxxxxxxx/ >>>>> >>>>> - real deadlock report >>>>> >>>>> https://lore.kernel.org/linux-scsi/ZxG38G9BuFdBpBHZ@fedora/ >>>>> >>>>> It is actually one simple ABBA lock: >>>>> >>>>> 1) queue_attr_store() >>>>> >>>>> blk_mq_freeze_queue(q); //queue freeze lock >>>>> res = entry->store(disk, page, length); >>>>> queue_limits_start_update //->limits_lock >>>>> ... >>>>> queue_limits_commit_update >>>>> blk_mq_unfreeze_queue(q); >>>> >>>> The locking + freeze pattern should be: >>>> >>>> lim = queue_limits_start_update(q); >>>> ... >>>> blk_mq_freeze_queue(q); >>>> ret = queue_limits_commit_update(q, &lim); >>>> blk_mq_unfreeze_queue(q); >>>> >>>> This pattern is used in most places and anything that does not use it is likely >>>> susceptible to a similar ABBA deadlock. We should probably look into trying to >>>> integrate the freeze/unfreeze calls directly into queue_limits_commit_update(). >>>> >>>> Fixing queue_attr_store() to use this pattern seems simpler than trying to fix >>>> sd_revalidate_disk(). >>> >>> This way looks good, just commit af2814149883 ("block: freeze the queue in >>> queue_attr_store") needs to be reverted, and freeze/unfreeze has to be >>> added to each queue attribute .store() handler. >>> >> Wouldn't it be feasible to add blk-mq freeze in queue_limits_start_update() >> and blk-mq unfreeze in queue_limits_commit_update()? If we do this then >> the pattern would be, >> >> queue_limits_start_update(): limit-lock + freeze >> queue_limits_commit_update() : unfreeze + limit-unlock >> >> Then in queue_attr_store() we shall just remove freeze/unfreeze. >> >> We also need to fix few call sites where we've code block, >> >> { >> blk_mq_freeze_queue() >> ... >> queue_limits_start_update() >> ... >> queue_limits_commit_update() >> ... >> blk_mq_unfreeze_queue() >> >> } >> >> In the above code block, we may then replace blk_mq_freeze_queue() with >> queue_limits_commit_update() and similarly replace blk_mq_unfreeze_queue() >> with queue_limits_commit_update(). >> >> { >> queue_limits_start_update() >> ... >> ... >> ... >> queue_limits_commit_update() > > In sd_revalidate_disk(), blk-mq request is allocated under queue_limits_start_update(), > then ABBA deadlock is triggered since blk_queue_enter() implies same lock(freeze lock) > from blk_mq_freeze_queue(). > Yeah agreed but I see sd_revalidate_disk() is probably the only exception which allocates the blk-mq request. Can't we fix it? One simple way I could think of would be updating queue_limit_xxx() APIs and then use it, queue_limits_start_update(struct request_queue *q, bool freeze) { ... mutex_lock(&q->limits_lock); if(freeze) blk_mq_freeze_queue(q); ... } queue_limits_commit_update(struct request_queue *q, struct queue_limits *lim, bool unfreeze) { ... ... if(unfreeze) blk_mq_unfreeze_queue(q); mutex_unlock(&q->limits_lock); } sd_revalidate_disk() { ... ... queue_limits_start_update(q, false); ... ... blk_mq_freeze_queue(q); queue_limits_commit_update(q, lim, true); } Thanks, --Nilay
