Re: [PATCH 05/10] block: fix queue freeze vs limits lock order in sysfs store methods

[Ming Lei <ming.lei@redhat.com>]

On Wed, Jan 08, 2025 at 10:25:02AM +0100, Christoph Hellwig wrote:
> queue_attr_store() always freezes a device queue before calling the
> attribute store operation. For attributes that control queue limits, the
> store operation will also lock the queue limits with a call to
> queue_limits_start_update(). However, some drivers (e.g. SCSI sd) may
> need to issue commands to a device to obtain limit values from the
> hardware with the queue limits locked. This creates a potential ABBA
> deadlock situation if a user attempts to modify a limit (thus freezing
> the device queue) while the device driver starts a revalidation of the
> device queue limits.
> 
> Avoid such deadlock by not freezing the queue before calling the
> ->store_limit() method in struct queue_sysfs_entry and instead use the
> queue_limits_commit_update_frozen helper to freeze the queue after taking
> the limits lock.
> 
> (commit log adapted from a similar patch from  Damien Le Moal)
> 
> Fixes: ff956a3be95b ("block: use queue_limits_commit_update in queue_discard_max_store")
> Fixes: 0327ca9d53bf ("block: use queue_limits_commit_update in queue_max_sectors_store")
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
> ---
>  block/blk-sysfs.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index f36356cbde0b..2de405cb5f10 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -691,22 +691,24 @@ queue_attr_store(struct kobject *kobj, struct attribute *attr,
>  	if (entry->load_module)
>  		entry->load_module(disk, page, length);
>  
> -	mutex_lock(&q->sysfs_lock);
> -	blk_mq_freeze_queue(q);
>  	if (entry->store_limit) {
>  		struct queue_limits lim = queue_limits_start_update(q);
>  
>  		res = entry->store_limit(disk, page, length, &lim);

Looks fine, but now ->store_limit() is called without holding
->sysfs_lock, maybe it should be documented.

Reviewed-by: Ming Lei <ming.lei@redhat.com>


thanks,
Ming