On Sat, Dec 10, 2022 at 08:33:10AM -1000, Tejun Heo wrote:
> When a gendisk is successfully initialized but add_disk() fails such as when
> a loop device has invalid number of minor device numbers specified,
> blkcg_init_disk() is called during init and then blkcg_exit_disk() during
> error handling. Unfortunately, iolatency gets initialized in the former but
> doesn't get cleaned up in the latter.
> 
> This is because, in non-error cases, the cleanup is performed by
> del_gendisk() calling rq_qos_exit(), the assumption being that rq_qos
> policies, iolatency being one of them, can only be activated once the disk
> is fully registered and visible. That assumption is true for wbt and iocost,
> but not so for iolatency as it gets initialized before add_disk() is called.
> 
> It is desirable to lazy-init rq_qos policies because they are optional
> features and add to hot path overhead once initialized - each IO has to walk
> all the registered rq_qos policies. So, we want to switch iolatency to lazy
> init too. However, that's a bigger change. As a fix for the immediate
> problem, let's just add an extra call to rq_qos_exit() in blkcg_exit_disk().
> This is safe because duplicate calls to rq_qos_exit() become noop's.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Reported-by: darklight2357@icloud.com

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>