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blk-mq: fix lockdep warning in __blk_mq_update_nr_hw_queues

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Commit 5989bfe6ac ("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: 5989bfe6ac ("block: restore two stage elevator switch while running nr_hw_queue update")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Link: https://lore.kernel.org/r/20250815131737.331692-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Ming Lei 2025-08-15 21:17:37 +08:00 committed by Jens Axboe
parent d0a2b527d8
commit 2d82f3bd89
1 changed files with 9 additions and 4 deletions
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13
block/blk-mq.c
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@ -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);