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

[Hannes Reinecke <hare@suse.de>]

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@linux.ibm.com>
> ---
>   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@suse.de                                +49 911 74053 688
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