block: protect debugfs attrs using elevator_lock instead of sysfs_lock

Currently, the block debugfs attributes (tags, tags_bitmap, sched_tags,
and sched_tags_bitmap) are protected using q->sysfs_lock. However, these
attributes are updated in multiple scenarios:
- During driver probe method
- During an elevator switch/update
- During an nr_hw_queues update
- When writing to the sysfs attribute nr_requests

All these update paths (except driver probe method, which doesn't
require any protection) are already protected using q->elevator_lock. To
ensure consistency and proper synchronization, replace q->sysfs_lock
with q->elevator_lock for protecting these debugfs attributes.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250313115235.3707600-2-nilay@linux.ibm.com
[axboe: some commit message rewording/fixes]
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq-debugfs.c

@@ -400,12 +400,12 @@ static int hctx_tags_show(void *data, struct seq_file *m)
 	struct request_queue *q = hctx->queue;
 	int res;
 
-	res = mutex_lock_interruptible(&q->sysfs_lock);
+	res = mutex_lock_interruptible(&q->elevator_lock);
 	if (res)
 		goto out;
 	if (hctx->tags)
 		blk_mq_debugfs_tags_show(m, hctx->tags);
-	mutex_unlock(&q->sysfs_lock);
+	mutex_unlock(&q->elevator_lock);
 
 out:
 	return res;
@@ -417,12 +417,12 @@ static int hctx_tags_bitmap_show(void *data, struct seq_file *m)
 	struct request_queue *q = hctx->queue;
 	int res;
 
-	res = mutex_lock_interruptible(&q->sysfs_lock);
+	res = mutex_lock_interruptible(&q->elevator_lock);
 	if (res)
 		goto out;
 	if (hctx->tags)
 		sbitmap_bitmap_show(&hctx->tags->bitmap_tags.sb, m);
-	mutex_unlock(&q->sysfs_lock);
+	mutex_unlock(&q->elevator_lock);
 
 out:
 	return res;
@@ -434,12 +434,12 @@ static int hctx_sched_tags_show(void *data, struct seq_file *m)
 	struct request_queue *q = hctx->queue;
 	int res;
 
-	res = mutex_lock_interruptible(&q->sysfs_lock);
+	res = mutex_lock_interruptible(&q->elevator_lock);
 	if (res)
 		goto out;
 	if (hctx->sched_tags)
 		blk_mq_debugfs_tags_show(m, hctx->sched_tags);
-	mutex_unlock(&q->sysfs_lock);
+	mutex_unlock(&q->elevator_lock);
 
 out:
 	return res;
@@ -451,12 +451,12 @@ static int hctx_sched_tags_bitmap_show(void *data, struct seq_file *m)
 	struct request_queue *q = hctx->queue;
 	int res;
 
-	res = mutex_lock_interruptible(&q->sysfs_lock);
+	res = mutex_lock_interruptible(&q->elevator_lock);
 	if (res)
 		goto out;
 	if (hctx->sched_tags)
 		sbitmap_bitmap_show(&hctx->sched_tags->bitmap_tags.sb, m);
-	mutex_unlock(&q->sysfs_lock);
+	mutex_unlock(&q->elevator_lock);
 
 out:
 	return res;

include/linux/blkdev.h

@@ -566,9 +566,9 @@ struct request_queue {
 	 * nr_requests and wbt latency, this lock also protects the sysfs attrs
 	 * nr_requests and wbt_lat_usec. Additionally the nr_hw_queues update
 	 * may modify hctx tags, reserved-tags and cpumask, so this lock also
-	 * helps protect the hctx attrs. To ensure proper locking order during
-	 * an elevator or nr_hw_queue update, first freeze the queue, then
-	 * acquire ->elevator_lock.
+	 * helps protect the hctx sysfs/debugfs attrs. To ensure proper locking
+	 * order during an elevator or nr_hw_queue update, first freeze the
+	 * queue, then acquire ->elevator_lock.
 	 */
 	struct mutex		elevator_lock;