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. Thanks, Ming
