Commit 2d82f3bd authored by Ming Lei's avatar Ming Lei Committed by Jens Axboe
Browse files

blk-mq: fix lockdep warning in __blk_mq_update_nr_hw_queues 



Commit 5989bfe6 ("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 <nilay@linux.ibm.com>
Cc: Yu Kuai <yukuai3@huawei.com>
Fixes: 5989bfe6 ("block: restore two stage elevator switch while running nr_hw_queue update")
Signed-off-by: default avatarMing Lei <ming.lei@redhat.com>
Reviewed-by: default avatarYu Kuai <yukuai3@huawei.com>
Reviewed-by: default avatarNilay Shroff <nilay@linux.ibm.com>
Link: https://lore.kernel.org/r/20250815131737.331692-1-ming.lei@redhat.com


Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent d0a2b527

block/blk-mq.c

+9 −4
Original line number Diff line number Diff line
@@ -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 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, 
	}

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);