Re: [PATCHv3 7/7] block: protect read_ahead_kb using q->limits_lock

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On 2/24/25 14:30, Nilay Shroff wrote:
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.

Signed-off-by: Nilay Shroff <nilay@xxxxxxxxxxxxx>
---
  block/blk-sysfs.c | 16 ++++++++++------
  1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 8f47d9f30fbf..228f81a9060f 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);

Cf my comments to the previous patch: Ordering.

Here we take the lock _before_ 'freeze', with the previous patch we took
the lock _after_ 'freeze'.
Why?

Cheers,

Hannes
--
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@xxxxxxx                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich




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