Re: [PATCH] blk-cgroup: delay calling blkcg_exit_disk until disk_release

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

On Wed, Feb 08, 2023 at 07:35:14AM +0100, Christoph Hellwig wrote:
> While del_gendisk ensures there is no outstanding I/O on the queue,
> it can't prevent block layer users from building new I/O.
> 
> This leads to a NULL ->root_blkg reference in bio_associate_blkg when
> allocating a new bio on a shut down file system.  Delay freeing the
> blk-cgroup subsystems from del_gendisk until disk_release to make
> sure the blkg and throttle information is still avaіlable for bio
> submitters, even if those bios will immediately fail.
> 
> This now can cause a case where disk_release is called on a disk
> that hasn't been added.  That's mostly harmless, except for a case
> in blk_throttl_exit that now needs to check for a NULL ->td pointer.
> 
> Fixes: 178fa7d49815 ("blk-cgroup: delay blk-cgroup initialization until add_disk")
> Reported-by: Ming Lei <ming.lei@redhat.com>
> Signed-off-by: Christoph Hellwig <hch@lst.de>

hammmmmm, this patch actually causes bigger trouble.

After commit 84d7d462b16d ("blk-cgroup: pin the gendisk in struct blkcg_gq"),
blkcg_gq instance grabs disk's reference, so moving blkcg_exit_disk
into disk_release() just causes reference cross-dependency, both are
leaked.

Thanks,
Ming