Re: [PATCH] blk-cgroup: Flush stats before releasing blkcg_gq

[Waiman Long <longman@redhat.com>]

On 5/23/23 22:37, Ming Lei wrote:
> Hi Yosry,
>
> On Tue, May 23, 2023 at 07:06:38PM -0700, Yosry Ahmed wrote:
>> Hi Ming,
>>
>> On Tue, May 23, 2023 at 6:21 PM Ming Lei <ming.lei@redhat.com> wrote:
>>> As noted by Michal, the blkg_iostat_set's in the lockless list
>>> hold reference to blkg's to protect against their removal. Those
>>> blkg's hold reference to blkcg. When a cgroup is being destroyed,
>>> cgroup_rstat_flush() is only called at css_release_work_fn() which
>>> is called when the blkcg reference count reaches 0. This circular
>>> dependency will prevent blkcg and some blkgs from being freed after
>>> they are made offline.
>> I am not at all familiar with blkcg, but does calling
>> cgroup_rstat_flush() in offline_css() fix the problem?
> Except for offline, this list needs to be flushed after the associated disk
> is deleted.
>
>> or can items be
>> added to the lockless list(s) after the blkcg is offlined?
> Yeah.
>
> percpu_ref_*get(&blkg->refcnt) still can succeed after the percpu refcnt
> is killed in blkg_destroy() which is called from both offline css and
> removing disk.

As suggested by Tejun, we can use percpu_ref_tryget(&blkg->refcnt) to 
make sure that we can only take a reference when the blkg is online. I 
think it is a bit safer to take a percpu refcnt to avoid use after free. 
My other concern about your patch is that the per cpu list iterations 
will be done multiple times when a blkcg is destroyed if many blkgs are 
attached to the blkcg. I still prefer to do it once in 
blkcg_destroy_blkgs(). I am going to post an updated version tomorrow 
after some more testings.

Cheers,
Longman