From 2d82f3bd8910eb65e30bb2a3c9b945bfb3b6d661 Mon Sep 17 00:00:00 2001 From: Ming Lei Date: Fri, 15 Aug 2025 21:17:37 +0800 Subject: [PATCH] blk-mq: fix lockdep warning in __blk_mq_update_nr_hw_queues Commit 5989bfe6ac6b ("block: restore two stage elevator switch while running nr_hw_queue update") reintroduced a lockdep warning by calling blk_mq_freeze_queue_nomemsave() before switching the I/O scheduler. The function blk_mq_elv_switch_none() calls elevator_change_done(). Running this while the queue is frozen causes a lockdep warning. Fix this by reordering the operations: first, switch the I/O scheduler to 'none', and then freeze the queue. This ensures that elevator_change_done() is not called on an already frozen queue. And this way is safe because elevator_set_none() does freeze queue before switching to none. Also we still have to rely on blk_mq_elv_switch_back() for switching back, and it has to cover unfrozen queue case. Cc: Nilay Shroff Cc: Yu Kuai Fixes: 5989bfe6ac6b ("block: restore two stage elevator switch while running nr_hw_queue update") Signed-off-by: Ming Lei Reviewed-by: Yu Kuai Reviewed-by: Nilay Shroff Link: https://lore.kernel.org/r/20250815131737.331692-1-ming.lei@redhat.com Signed-off-by: Jens Axboe --- block/blk-mq.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/block/blk-mq.c b/block/blk-mq.c index b67d6c02eceb..ba3a4b77f578 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -5033,6 +5033,7 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, unsigned int memflags; int i; struct xarray elv_tbl, et_tbl; + bool queues_frozen = false; lockdep_assert_held(&set->tag_list_lock); @@ -5056,9 +5057,6 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, blk_mq_sysfs_unregister_hctxs(q); } - list_for_each_entry(q, &set->tag_list, tag_set_list) - blk_mq_freeze_queue_nomemsave(q); - /* * Switch IO scheduler to 'none', cleaning up the data associated * with the previous scheduler. We will switch back once we are done @@ -5068,6 +5066,9 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, if (blk_mq_elv_switch_none(q, &elv_tbl)) goto switch_back; + list_for_each_entry(q, &set->tag_list, tag_set_list) + blk_mq_freeze_queue_nomemsave(q); + queues_frozen = true; if (blk_mq_realloc_tag_set_tags(set, nr_hw_queues) < 0) goto switch_back; @@ -5091,8 +5092,12 @@ fallback: } switch_back: /* The blk_mq_elv_switch_back unfreezes queue for us. */ - list_for_each_entry(q, &set->tag_list, tag_set_list) + list_for_each_entry(q, &set->tag_list, tag_set_list) { + /* switch_back expects queue to be frozen */ + if (!queues_frozen) + blk_mq_freeze_queue_nomemsave(q); blk_mq_elv_switch_back(q, &elv_tbl, &et_tbl); + } list_for_each_entry(q, &set->tag_list, tag_set_list) { blk_mq_sysfs_register_hctxs(q); -- 2.25.1