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block: fix potential deadlock while running nr_hw_queue update

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Move scheduler tags (sched_tags) allocation and deallocation outside
both the ->elevator_lock and ->freeze_lock when updating nr_hw_queues.
This change breaks the dependency chain from the percpu allocator lock
to the elevator lock, helping to prevent potential deadlocks, as
observed in the reported lockdep splat[1].

This commit introduces batch allocation and deallocation helpers for
sched_tags, which are now used from within __blk_mq_update_nr_hw_queues
routine while iterating through the tagset.

With this change, all sched_tags memory management is handled entirely
outside the ->elevator_lock and the ->freeze_lock context, thereby
eliminating the lock dependency that could otherwise manifest during
nr_hw_queues updates.

[1] https://lore.kernel.org/all/0659ea8d-a463-47c8-9180-43c719e106eb@linux.ibm.com/

Reported-by: Stefan Haberland <sth@linux.ibm.com>
Closes: https://lore.kernel.org/all/0659ea8d-a463-47c8-9180-43c719e106eb@linux.ibm.com/
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Link: https://lore.kernel.org/r/20250730074614.2537382-4-nilay@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Nilay Shroff
2025-07-30 13:16:09 +05:30
committed by Jens Axboe
parent f5a6604f7a
commit 04225d13ae
5 changed files with 89 additions and 15 deletions
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4
block/blk.h
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@@ -12,6 +12,7 @@
#include "blk-crypto-internal.h"
struct elevator_type;
struct elevator_tags;
/*
* Default upper limit for the software max_sectors limit used for regular I/Os.
@@ -330,7 +331,8 @@ bool blk_bio_list_merge(struct request_queue *q, struct list_head *list,
bool blk_insert_flush(struct request *rq);
void elv_update_nr_hw_queues(struct request_queue *q, struct elevator_type *e);
void elv_update_nr_hw_queues(struct request_queue *q, struct elevator_type *e,
struct elevator_tags *t);
void elevator_set_default(struct request_queue *q);
void elevator_set_none(struct request_queue *q);