block: fix potential deadlock while running nr_hw_queue update

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>

block/blk-mq-sched.c

@@ -427,6 +427,32 @@ void blk_mq_free_sched_tags(struct elevator_tags *et,
 	kfree(et);
 }
 
+void blk_mq_free_sched_tags_batch(struct xarray *et_table,
+		struct blk_mq_tag_set *set)
+{
+	struct request_queue *q;
+	struct elevator_tags *et;
+
+	lockdep_assert_held_write(&set->update_nr_hwq_lock);
+
+	list_for_each_entry(q, &set->tag_list, tag_set_list) {
+		/*
+		 * Accessing q->elevator without holding q->elevator_lock is
+		 * safe because we're holding here set->update_nr_hwq_lock in
+		 * the writer context. So, scheduler update/switch code (which
+		 * acquires the same lock but in the reader context) can't run
+		 * concurrently.
+		 */
+		if (q->elevator) {
+			et = xa_load(et_table, q->id);
+			if (unlikely(!et))
+				WARN_ON_ONCE(1);
+			else
+				blk_mq_free_sched_tags(et, set);
+		}
+	}
+}
+
 struct elevator_tags *blk_mq_alloc_sched_tags(struct blk_mq_tag_set *set,
 		unsigned int nr_hw_queues)
 {
@@ -477,6 +503,45 @@ out:
 	return NULL;
 }
 
+int blk_mq_alloc_sched_tags_batch(struct xarray *et_table,
+		struct blk_mq_tag_set *set, unsigned int nr_hw_queues)
+{
+	struct request_queue *q;
+	struct elevator_tags *et;
+	gfp_t gfp = GFP_NOIO | __GFP_ZERO | __GFP_NOWARN | __GFP_NORETRY;
+
+	lockdep_assert_held_write(&set->update_nr_hwq_lock);
+
+	list_for_each_entry(q, &set->tag_list, tag_set_list) {
+		/*
+		 * Accessing q->elevator without holding q->elevator_lock is
+		 * safe because we're holding here set->update_nr_hwq_lock in
+		 * the writer context. So, scheduler update/switch code (which
+		 * acquires the same lock but in the reader context) can't run
+		 * concurrently.
+		 */
+		if (q->elevator) {
+			et = blk_mq_alloc_sched_tags(set, nr_hw_queues);
+			if (!et)
+				goto out_unwind;
+			if (xa_insert(et_table, q->id, et, gfp))
+				goto out_free_tags;
+		}
+	}
+	return 0;
+out_free_tags:
+	blk_mq_free_sched_tags(et, set);
+out_unwind:
+	list_for_each_entry_continue_reverse(q, &set->tag_list, tag_set_list) {
+		if (q->elevator) {
+			et = xa_load(et_table, q->id);
+			if (et)
+				blk_mq_free_sched_tags(et, set);
+		}
+	}
+	return -ENOMEM;
+}
+
 /* caller must have a reference to @e, will grab another one if successful */
 int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e,
 		struct elevator_tags *et)

block/blk-mq-sched.h

@@ -25,8 +25,12 @@ void blk_mq_sched_free_rqs(struct request_queue *q);
 
 struct elevator_tags *blk_mq_alloc_sched_tags(struct blk_mq_tag_set *set,
 		unsigned int nr_hw_queues);
+int blk_mq_alloc_sched_tags_batch(struct xarray *et_table,
+		struct blk_mq_tag_set *set, unsigned int nr_hw_queues);
 void blk_mq_free_sched_tags(struct elevator_tags *et,
 		struct blk_mq_tag_set *set);
+void blk_mq_free_sched_tags_batch(struct xarray *et_table,
+		struct blk_mq_tag_set *set);
 
 static inline void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
 {

block/blk-mq.c

@@ -4974,12 +4974,13 @@ int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
  * Switch back to the elevator type stored in the xarray.
  */
 static void blk_mq_elv_switch_back(struct request_queue *q,
-		struct xarray *elv_tbl)
+		struct xarray *elv_tbl, struct xarray *et_tbl)
 {
 	struct elevator_type *e = xa_load(elv_tbl, q->id);
+	struct elevator_tags *t = xa_load(et_tbl, q->id);
 
 	/* The elv_update_nr_hw_queues unfreezes the queue. */
-	elv_update_nr_hw_queues(q, e);
+	elv_update_nr_hw_queues(q, e, t);
 
 	/* Drop the reference acquired in blk_mq_elv_switch_none. */
 	if (e)
@@ -5031,7 +5032,7 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
 	int prev_nr_hw_queues = set->nr_hw_queues;
 	unsigned int memflags;
 	int i;
-	struct xarray elv_tbl;
+	struct xarray elv_tbl, et_tbl;
 
 	lockdep_assert_held(&set->tag_list_lock);
 
@@ -5044,6 +5045,10 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
 
 	memflags = memalloc_noio_save();
 
+	xa_init(&et_tbl);
+	if (blk_mq_alloc_sched_tags_batch(&et_tbl, set, nr_hw_queues) < 0)
+		goto out_memalloc_restore;
+
 	xa_init(&elv_tbl);
 
 	list_for_each_entry(q, &set->tag_list, tag_set_list) {
@@ -5087,7 +5092,7 @@ fallback:
 switch_back:
 	/* The blk_mq_elv_switch_back unfreezes queue for us. */
 	list_for_each_entry(q, &set->tag_list, tag_set_list)
-		blk_mq_elv_switch_back(q, &elv_tbl);
+		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);
@@ -5098,7 +5103,8 @@ switch_back:
 	}
 
 	xa_destroy(&elv_tbl);
-
+	xa_destroy(&et_tbl);
+out_memalloc_restore:
 	memalloc_noio_restore(memflags);
 
 	/* Free the excess tags when nr_hw_queues shrink. */

block/blk.h

@@ -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);
 

block/elevator.c

@@ -705,7 +705,8 @@ static int elevator_change(struct request_queue *q, struct elv_change_ctx *ctx)
  * The I/O scheduler depends on the number of hardware queues, this forces a
  * reattachment when nr_hw_queues changes.
  */
-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)
 {
 	struct blk_mq_tag_set *set = q->tag_set;
 	struct elv_change_ctx ctx = {};
@@ -715,25 +716,21 @@ void elv_update_nr_hw_queues(struct request_queue *q, struct elevator_type *e)
 
 	if (e && !blk_queue_dying(q) && blk_queue_registered(q)) {
 		ctx.name = e->elevator_name;
-		ctx.et = blk_mq_alloc_sched_tags(set, set->nr_hw_queues);
+		ctx.et = t;
-		if (!ctx.et) {
+
-			WARN_ON_ONCE(1);
-			goto unfreeze;
-		}
 		mutex_lock(&q->elevator_lock);
 		/* force to reattach elevator after nr_hw_queue is updated */
 		ret = elevator_switch(q, &ctx);
 		mutex_unlock(&q->elevator_lock);
 	}
-unfreeze:
 	blk_mq_unfreeze_queue_nomemrestore(q);
 	if (!ret)
 		WARN_ON_ONCE(elevator_change_done(q, &ctx));
 	/*
 	 * Free sched tags if it's allocated but we couldn't switch elevator.
 	 */
-	if (ctx.et && !ctx.new)
+	if (t && !ctx.new)
-		blk_mq_free_sched_tags(ctx.et, set);
+		blk_mq_free_sched_tags(t, set);
 }
 
 /*