Re: [PATCH-block] blk-cgroup: Use css_tryget() in blkcg_destroy_blkgs()

Waiman Long
2022-11-28T15:32:42Z

On 11/28/22 09:06, Michal Koutný wrote:
> Hello.
>
> On Sun, Nov 27, 2022 at 10:30:57PM -0500, Waiman Long <longman@redhat.com> wrote:
>> That may not be true if there is no blkg associated with the blkcg. If
>> css_get() fails, the subsequent css_put() call may lead to data
>> corruption as was illustrated in a test system that it crashed on
>> bootup when that commit was included.
> Do you have a stacktrace of the underflowing css_put() in
> blkcg_destroy_blkgs()?
>
> It looks to me slightly as a mistake of the caller site that it passes
> struct blkcg * without any references.
>
> By a cursory look, could it be cgwb_release_workfn?
>
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -390,11 +390,11 @@ static void cgwb_release_workfn(struct work_struct *work)
> wb_shutdown(wb);
>
> css_put(wb->memcg_css);
> - css_put(wb->blkcg_css);
> mutex_unlock(&wb->bdi->cgwb_release_mutex);
>
> /* triggers blkg destruction if no online users left */
> blkcg_unpin_online(wb->blkcg_css);
> + css_put(wb->blkcg_css);
>
> fprop_local_destroy_percpu(&wb->memcg_completions);
>
> Does your crash involve this stack?

That looks like a possible cause for the system crash that we are
seeing. In my testing, I do see one case out of more than a dozen calls
to blkcg_destroy_blkgs() where css_tryget() fail in the reproducing
system during bootup. However, I didn't force a stack dump at that point
and so I am not sure if that is the place, though it looks likely. One
of the crashes that was reported does involve blkcg_unpin_online(). So
maybe that is it.

Cheers,
Longman