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(). Thanks, Ming
