diff mbox series

[PATCHv4,7/7] block: protect read_ahead_kb using q->limits_lock

Message ID 20250225133110.1441035-8-nilay@linux.ibm.com (mailing list archive)
State New
Headers show
Series block: fix lock order and remove redundant locking | expand

Commit Message

Nilay Shroff Feb. 25, 2025, 1:30 p.m. UTC 
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: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 block/blk-sysfs.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

Comments

Christoph Hellwig Feb. 25, 2025, 3:14 p.m. UTC | #1 
On Tue, Feb 25, 2025 at 07:00:42PM +0530, 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.

Please add a comment near limits_lock that mentions that the lock
protects the limits and read_ahead_kb field.  I should have probably
done the former myself when adding it, but now that it also protects
a non-obvious field we really need it.
Nilay Shroff Feb. 26, 2025, 12:10 p.m. UTC | #2 
On 2/25/25 8:44 PM, Christoph Hellwig wrote:
> On Tue, Feb 25, 2025 at 07:00:42PM +0530, 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.
> 
> Please add a comment near limits_lock that mentions that the lock
> protects the limits and read_ahead_kb field.  I should have probably
> done the former myself when adding it, but now that it also protects
> a non-obvious field we really need it.
> 
Okay this will be addressed in the next patchset.

Thanks,
--Nilay
diff mbox series

Patch

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 31bfd6c92b2c..8d078c7f0347 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,