Re: [PATCH v3 06/10] blk-wbt: fix incorrect lock order for rq_qos_mutex and freeze queue

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

On Sun, Nov 30, 2025 at 10:43:45AM +0800, Yu Kuai wrote:
> wbt_init() can be called from sysfs attribute and wbt_enable_default(),
> however the lock order are inversely.
> 
> - queue_wb_lat_store() freeze queue first, and then wbt_init() hold
>   rq_qos_mutex. In this case queue will be frozen again inside
>   rq_qos_add(), however, in this case freeze queue recursively is
>   inoperative;
> - wbt_enable_default() from elevator switch will hold rq_qos_mutex
>   first, and then rq_qos_add() will freeze queue;
> 
> Fix this problem by converting to use new helper rq_qos_add_frozen() in
> wbt_init(), and for wbt_enable_default(), freeze queue before calling
> wbt_init().
> 
> Fixes: a13bd91be223 ("block/rq_qos: protect rq_qos apis with a new lock")
> Signed-off-by: Yu Kuai <yukuai@fnnas.com>
> ---
>  block/blk-wbt.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-wbt.c b/block/blk-wbt.c
> index b1ab0f297f24..5e7e481103a1 100644
> --- a/block/blk-wbt.c
> +++ b/block/blk-wbt.c
> @@ -725,7 +725,11 @@ void wbt_enable_default(struct gendisk *disk)
>  		return;
>  
>  	if (queue_is_mq(q) && enable) {
> +		unsigned int memflags = blk_mq_freeze_queue(q);
> +
>  		wbt_init(disk);
> +		blk_mq_unfreeze_queue(q, memflags);
> +

This change causes new lockdep warning, see the report in:

https://lore.kernel.org/linux-block/692c17ca.a70a0220.d98e3.016c.GAE@google.com/


Thanks,
Ming