[PATCHv6 7/7] block: protect read_ahead_kb using q->limits_lock

[Nilay Shroff <nilay@linux.ibm.com>]

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 <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>
---
 block/blk-sysfs.c      | 16 ++++++++++------
 include/linux/blkdev.h |  3 +++
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 223da196a548..d584461a1d84 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -93,9 +93,9 @@ static ssize_t queue_ra_show(struct gendisk *disk, char *page)
 {
 	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;
 }
@@ -111,12 +111,15 @@ queue_ra_store(struct gendisk *disk, const char *page, size_t count)
 	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;
 }
@@ -670,7 +673,8 @@ static struct attribute *queue_attrs[] = {
 	&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,
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 2977a583d740..4daef0408683 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -573,6 +573,9 @@ struct request_queue {
 	struct mutex		elevator_lock;
 
 	struct mutex		sysfs_lock;
+	/*
+	 * Protects queue limits and also sysfs attribute read_ahead_kb.
+	 */
 	struct mutex		limits_lock;
 
 	/*
-- 
2.47.1