block: protect read_ahead_kb using q->limits_lock

The bdi->ra_pages could be updated under q->limits_lock because it's
usually calculated from the queue limits by queue_limits_commit_update.
So protect reading/writing the sysfs attribute read_ahead_kb using
q->limits_lock instead of q->sysfs_lock.

Reviewed-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Hannes Reinecke <[e-mail]>
Reviewed-by: Ming Lei <[e-mail]>
Signed-off-by: Nilay Shroff <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

block/blk-sysfs.c

···
 {
 	ssize_t ret;
 
-	mutex_lock(&disk->queue->sysfs_lock);
+	mutex_lock(&disk->queue->limits_lock);
 	ret = queue_var_show(disk->bdi->ra_pages << (PAGE_SHIFT - 10), page);
-	mutex_unlock(&disk->queue->sysfs_lock);
+	mutex_unlock(&disk->queue->limits_lock);
 
 	return ret;
 }
···
 	ret = queue_var_store(&ra_kb, page, count);
 	if (ret < 0)
 		return ret;
-
-	mutex_lock(&q->sysfs_lock);
+	/*
+	 * ->ra_pages is protected by ->limits_lock because it is usually
+	 * calculated from the queue limits by queue_limits_commit_update.
+	 */
+	mutex_lock(&q->limits_lock);
 	memflags = blk_mq_freeze_queue(q);
 	disk->bdi->ra_pages = ra_kb >> (PAGE_SHIFT - 10);
+	mutex_unlock(&q->limits_lock);
 	blk_mq_unfreeze_queue(q, memflags);
-	mutex_unlock(&q->sysfs_lock);
 
 	return ret;
 }
···
 	&queue_dma_alignment_entry.attr,
 
 	/*
-	 * Attributes which are protected with q->sysfs_lock.
+	 * Attributes which require some form of locking other than
+	 * q->sysfs_lock.
 	 */
 	&queue_ra_entry.attr,
 

include/linux/blkdev.h

···
 	struct mutex		elevator_lock;
 
 	struct mutex		sysfs_lock;
+	/*
+	 * Protects queue limits and also sysfs attribute read_ahead_kb.
+	 */
 	struct mutex		limits_lock;
 
 	/*