block: move elv_register[unregister]_queue out of elevator_lock

Move elv_register[unregister]_queue out of ->elevator_lock & queue freezing,
so we can kill many lockdep warnings.

elv_register[unregister]_queue() is serialized, and just dealing with sysfs/
debugfs things, no need to be done with queue frozen:

- when it is called from adding disk, elevator switch isn't possible
  because ->queue_kobj isn't added yet

- when it is called from deleting disk, disable_elv_switch() is
  responsible for preventing new elevator switch and draining old
  elevator switch.

- when it is called from blk_mq_update_nr_hw_queues(), adding/removing
  disk and elevator switch can't be allowed or in-progress

With this change, elevator's ->exit() is called before calling
elv_unregister_queue, then user may call into ->show()/store() of elevator's
sysfs attributes, and we have covered this issue by adding `ELEVATOR_FLAG_DYNG`.

For blk-mq debugfs, hctx->sched_tags is always checked with ->elevator_lock by
debugfs code, meantime hctx->sched_tags is updated with ->elevator_lock, so
there isn't such issue.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250505141805.2751237-22-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -5041,11 +5041,10 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
 		blk_mq_debugfs_register_hctxs(q);
 	}
 
+	/* elv_update_nr_hw_queues() unfreeze queue for us */
 	list_for_each_entry(q, &set->tag_list, tag_set_list)
 		elv_update_nr_hw_queues(q);
 
-	list_for_each_entry(q, &set->tag_list, tag_set_list)
-		blk_mq_unfreeze_queue_nomemrestore(q);
 	memalloc_noio_restore(memflags);
 
 	/* Free the excess tags when nr_hw_queues shrink. */

block/elevator.c

@@ -49,6 +49,11 @@
 struct elv_change_ctx {
 	const char *name;
 	bool no_uevent;
+
+	/* for unregistering old elevator */
+	struct elevator_queue *old;
+	/* for registering new elevator */
+	struct elevator_queue *new;
 };
 
 static DEFINE_SPINLOCK(elv_list_lock);
@@ -158,14 +163,14 @@ static void elevator_exit(struct request_queue *q)
 {
 	struct elevator_queue *e = q->elevator;
 
+	lockdep_assert_held(&q->elevator_lock);
+
 	ioc_clear_queue(q);
 	blk_mq_sched_free_rqs(q);
 
 	mutex_lock(&e->sysfs_lock);
 	blk_mq_exit_sched(q, e);
 	mutex_unlock(&e->sysfs_lock);
-
-	kobject_put(&e->kobj);
 }
 
 static inline void __elv_rqhash_del(struct request *rq)
@@ -466,8 +471,6 @@ static int elv_register_queue(struct request_queue *q,
 {
 	int error;
 
-	lockdep_assert_held(&q->elevator_lock);
-
 	error = kobject_add(&e->kobj, &q->disk->queue_kobj, "iosched");
 	if (!error) {
 		const struct elv_fs_entry *attr = e->type->elevator_attrs;
@@ -494,8 +497,6 @@ static int elv_register_queue(struct request_queue *q,
 static void elv_unregister_queue(struct request_queue *q,
 				 struct elevator_queue *e)
 {
-	lockdep_assert_held(&q->elevator_lock);
-
 	if (e && test_and_clear_bit(ELEVATOR_FLAG_REGISTERED, &e->flags)) {
 		kobject_uevent(&e->kobj, KOBJ_REMOVE);
 		kobject_del(&e->kobj);
@@ -586,7 +587,7 @@ static int elevator_switch(struct request_queue *q, struct elv_change_ctx *ctx)
 	blk_mq_quiesce_queue(q);
 
 	if (q->elevator) {
-		elv_unregister_queue(q, q->elevator);
+		ctx->old = q->elevator;
 		elevator_exit(q);
 	}
 
@@ -594,11 +595,7 @@ static int elevator_switch(struct request_queue *q, struct elv_change_ctx *ctx)
 		ret = blk_mq_init_sched(q, new_e);
 		if (ret)
 			goto out_unfreeze;
-		ret = elv_register_queue(q, q->elevator, !ctx->no_uevent);
+		ctx->new = q->elevator;
-		if (ret) {
-			elevator_exit(q);
-			goto out_unfreeze;
-		}
 	} else {
 		blk_queue_flag_clear(QUEUE_FLAG_SQ_SCHED, q);
 		q->elevator = NULL;
@@ -619,6 +616,38 @@ static int elevator_switch(struct request_queue *q, struct elv_change_ctx *ctx)
 	return ret;
 }
 
+static void elv_exit_and_release(struct request_queue *q)
+{
+	struct elevator_queue *e;
+	unsigned memflags;
+
+	memflags = blk_mq_freeze_queue(q);
+	mutex_lock(&q->elevator_lock);
+	e = q->elevator;
+	elevator_exit(q);
+	mutex_unlock(&q->elevator_lock);
+	blk_mq_unfreeze_queue(q, memflags);
+	if (e)
+		kobject_put(&e->kobj);
+}
+
+static int elevator_change_done(struct request_queue *q,
+				struct elv_change_ctx *ctx)
+{
+	int ret = 0;
+
+	if (ctx->old) {
+		elv_unregister_queue(q, ctx->old);
+		kobject_put(&ctx->old->kobj);
+	}
+	if (ctx->new) {
+		ret = elv_register_queue(q, ctx->new, !ctx->no_uevent);
+		if (ret)
+			elv_exit_and_release(q);
+	}
+	return ret;
+}
+
 /*
  * Switch this queue to the given IO scheduler.
  */
@@ -645,6 +674,9 @@ static int elevator_change(struct request_queue *q, struct elv_change_ctx *ctx)
 		ret = elevator_switch(q, ctx);
 	mutex_unlock(&q->elevator_lock);
 	blk_mq_unfreeze_queue(q, memflags);
+	if (!ret)
+		ret = elevator_change_done(q, ctx);
+
 	return ret;
 }
 
@@ -654,18 +686,22 @@ static int elevator_change(struct request_queue *q, struct elv_change_ctx *ctx)
  */
 void elv_update_nr_hw_queues(struct request_queue *q)
 {
+	struct elv_change_ctx ctx = {};
+	int ret = -ENODEV;
+
 	WARN_ON_ONCE(q->mq_freeze_depth == 0);
 
 	mutex_lock(&q->elevator_lock);
 	if (q->elevator && !blk_queue_dying(q) && !blk_queue_registered(q)) {
-		struct elv_change_ctx ctx = {
+		ctx.name = q->elevator->type->elevator_name;
-			.name = q->elevator->type->elevator_name,
-		};
 
 		/* force to reattach elevator after nr_hw_queue is updated */
-		elevator_switch(q, &ctx);
+		ret = elevator_switch(q, &ctx);
 	}
 	mutex_unlock(&q->elevator_lock);
+	blk_mq_unfreeze_queue_nomemrestore(q);
+	if (!ret)
+		WARN_ON_ONCE(elevator_change_done(q, &ctx));
 }
 
 /*