Re: [PATCH] blk-mq: fix lockdep warning in __blk_mq_update_nr_hw_queues

[Yu Kuai <yukuai1@xxxxxxxxxxxxxxx>]

Hi,

在 2025/08/15 17:15, Yu Kuai 写道:
> Will it be simpler if we move blk_mq_freeze_queue_nomemsave() into
> blk_mq_elv_switch_none(), after elevator is succeed switching to none
> then freeze the queue.
>
> Later in blk_mq_elv_switch_back we'll know if xa_load() return valid
> elevator_type, related queue is already freezed.

Like following:

diff --git a/block/blk-mq.c b/block/blk-mq.c
index e9f037a25fe3..3640fae5707b 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -5010,7 +5010,13 @@ static int blk_mq_elv_switch_none(struct 
request_queue *q,
                __elevator_get(q->elevator->type);

                elevator_set_none(q);
+       } else {
+               ret = xa_insert(elv_tbl, q->id, xa_mk_value(1), GFP_KERNEL);
+               if (WARN_ON_ONCE(ret))
+                       return ret;
        }
+
+       blk_mq_freeze_queue_nomemsave(q);
        return ret;
 }

@@ -5045,9 +5051,6 @@ 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
diff --git a/block/elevator.c b/block/elevator.c
index e2ebfbf107b3..9400ea9ec024 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -715,16 +715,21 @@ void elv_update_nr_hw_queues(struct request_queue 
*q, struct elevator_type *e,

        WARN_ON_ONCE(q->mq_freeze_depth == 0);

-       if (e && !blk_queue_dying(q) && blk_queue_registered(q)) {
-               ctx.name = e->elevator_name;
-               ctx.et = t;
-
-               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);
+       if (e) {
+               if (!xa_is_value(e) && !blk_queue_dying(q) &&
+                   blk_queue_registered(q)) {
+                       ctx.name = e->elevator_name;
+                       ctx.et = t;
+
+                       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);
+               }
+
+               blk_mq_unfreeze_queue_nomemrestore(q);
        }
-       blk_mq_unfreeze_queue_nomemrestore(q);
+
        if (!ret)
                WARN_ON_ONCE(elevator_change_done(q, &ctx));
        /*