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

["Michal Koutný" <mkoutny@suse.com>]

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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?

Thanks,
Michal

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