block: protect nr_requests update using q->elevator_lock

The sysfs attribute nr_requests could be simultaneously updated from
elevator switch/update or nr_hw_queue update code path. The update to
nr_requests for each of those code paths runs holding q->elevator_lock.
So we should protect access to sysfs attribute nr_requests using q->
elevator_lock instead of q->sysfs_lock.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Link: https://lore.kernel.org/r/20250304102551.2533767-6-nilay@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-sysfs.c

@@ -55,9 +55,9 @@ static ssize_t queue_requests_show(struct gendisk *disk, char *page)
 {
 	ssize_t ret;
 
-	mutex_lock(&disk->queue->sysfs_lock);
+	mutex_lock(&disk->queue->elevator_lock);
 	ret = queue_var_show(disk->queue->nr_requests, page);
-	mutex_unlock(&disk->queue->sysfs_lock);
+	mutex_unlock(&disk->queue->elevator_lock);
 	return ret;
 }
 
@@ -76,16 +76,16 @@ queue_requests_store(struct gendisk *disk, const char *page, size_t count)
 	if (ret < 0)
 		return ret;
 
-	mutex_lock(&q->sysfs_lock);
 	memflags = blk_mq_freeze_queue(q);
+	mutex_lock(&q->elevator_lock);
 	if (nr < BLKDEV_MIN_RQ)
 		nr = BLKDEV_MIN_RQ;
 
 	err = blk_mq_update_nr_requests(disk->queue, nr);
 	if (err)
 		ret = err;
+	mutex_unlock(&q->elevator_lock);
 	blk_mq_unfreeze_queue(q, memflags);
-	mutex_unlock(&q->sysfs_lock);
 	return ret;
 }
 
@@ -692,7 +692,6 @@ static struct attribute *blk_mq_queue_attrs[] = {
 	/*
 	 * Attributes which are protected with q->sysfs_lock.
 	 */
-	&queue_requests_entry.attr,
 #ifdef CONFIG_BLK_WBT
 	&queue_wb_lat_entry.attr,
 #endif
@@ -701,6 +700,7 @@ static struct attribute *blk_mq_queue_attrs[] = {
 	 * q->sysfs_lock.
 	 */
 	&elv_iosched_entry.attr,
+	&queue_requests_entry.attr,
 
 	/*
 	 * Attributes which don't require locking.

include/linux/blkdev.h

@@ -561,10 +561,12 @@ struct request_queue {
 	struct list_head	flush_list;
 
 	/*
-	 * Protects against I/O scheduler switching, specifically when
-	 * updating q->elevator. To ensure proper locking order during
-	 * an elevator update, first freeze the queue, then acquire
-	 * ->elevator_lock.
+	 * Protects against I/O scheduler switching, particularly when
+	 * updating q->elevator. Since the elevator update code path may
+	 * also modify q->nr_requests, this lock also protects the sysfs
+	 * attribute nr_requests.
+	 * To ensure proper locking order during an elevator update, first
+	 * freeze the queue, then acquire ->elevator_lock.
 	 */
 	struct mutex		elevator_lock;