Re: [PATCH v2] block: restore default wbt enablement

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

On Tue, Aug 12, 2025 at 11:42:57PM +0800, Julian Sun wrote:
> The commit 245618f8e45f ("block: protect wbt_lat_usec using
> q->elevator_lock") protected wbt_enable_default() with
> q->elevator_lock; however, it also placed wbt_enable_default()
> before blk_queue_flag_set(QUEUE_FLAG_REGISTERED, q);, resulting
> in wbt failing to be enabled.
> 
> Moreover, the protection of wbt_enable_default() by q->elevator_lock
> was removed in commit 78c271344b6f ("block: move wbt_enable_default()
> out of queue freezing from sched ->exit()"), so we can directly fix
> this issue by placing wbt_enable_default() after
> blk_queue_flag_set(QUEUE_FLAG_REGISTERED, q);.
> 
> Additionally, this issue also causes the inability to read the
> wbt_lat_usec file, and the scenario is as follows:
> 
> root@q:/sys/block/sda/queue# cat wbt_lat_usec
> cat: wbt_lat_usec: Invalid argument
> 
> root@q:/data00/sjc/linux# ls /sys/kernel/debug/block/sda/rqos
> cannot access '/sys/kernel/debug/block/sda/rqos': No such file or directory
> 
> root@q:/data00/sjc/linux# find /sys -name wbt
> /sys/kernel/debug/tracing/events/wbt
> 
> After testing with this patch, wbt can be enabled normally.
> 
> Signed-off-by: Julian Sun <sunjunchao@bytedance.com>
> Cc: stable@vger.kernel.org
> Fixes: 245618f8e45f ("block: protect wbt_lat_usec using q->elevator_lock")

Looks fine,

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

Thanks,
Ming