Re: [PATCH 0/5 cgroup/for-6.16-fixes] harden css_create() for safe placement of call to css_rstat_init()

[Michal Koutný]

On Mon, Jul 28, 2025 at 11:04:56AM -0700, JP Kobryn <inwardvessel@xxxxxxxxx> wrote:
> I did consider adding an "initialized" flag to the css but since there can
> be multiple css's per
> cgroup it felt like it would be adding overhead. So I went the path of
> getting the call
> sequence right. I'm open to feedback on this, though.

An implicit flag that builds upon the assumption that css_rstat_init()
must only succeed after it allocates ->rstat_cpu (didn't check gotchas
of this approach with !CONFIG_SMP)

--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -488,6 +488,10 @@ void css_rstat_exit(struct cgroup_subsys_state *css)
        if (!css_uses_rstat(css))
                return;

+       /* Incomplete css whose css_rstat_init failed */
+       if (!css->rstat_cpu)
+               return;
+
        css_rstat_flush(css);

        /* sanity check */

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