Re: [PATCH] cgroup: rstat: use LOCK CMPXCHG in css_rstat_updated

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Dec 05, 2025 at 07:35:30AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Thu, Dec 04, 2025 at 06:24:37PM -0800, Shakeel Butt wrote:
> ...
> > In Meta's fleet running the kernel with the commit 36df6e3dbd7e, we are
> > observing on some machines the memcg stats are getting skewed by more
> > than the actual memory on the system. On close inspection, we noticed
> > that lockless node for a workload for specific CPU was in the bad state
> > and thus all the updates on that CPU for that cgroup was being lost. At
> > the moment, we are not sure if this CMPXCHG without LOCK is the cause of
> > that but this needs to be fixed irrespective.
> 
> Is there a plausible theory of events that can explain the skew with the use
> of this_cpu_cmpxchg()? lnode.next being set to self but this_cpu_cmpxchg()
> returning something else? It may be useful to write a targeted repro for the
> particular combination - this_cpu_cmpxchg() vs. remote NULL clearing and see
> whether this_cpu_cmpxchg() can return a value that doesn't agree with what
> gets written in the memory.

Yes, I am working on creating a repro for this and will share the
results.

> 
> > @@ -113,9 +112,8 @@ __bpf_kfunc void css_rstat_updated(struct cgroup_subsys_state *css, int cpu)
> >  	 * successful and the winner will eventually add the per-cpu lnode to
> >  	 * the llist.
> >  	 */
> > -	self = &rstatc->lnode;
> > -	rstatc_pcpu = css->rstat_cpu;
> > -	if (this_cpu_cmpxchg(rstatc_pcpu->lnode.next, self, NULL) != self)
> > +	expected = &rstatc->lnode;
> > +	if (!try_cmpxchg(&rstatc->lnode.next, &expected, NULL))
> 
> Given that this is a relatively cold path, I don't see a problem with using
> locked op here even if this wasn't necessarily the culprit; however, can you
> please update the comment right above accordingly and explain why the locked
> op is used? After this patch, the commend and code disagree.

Thanks and yes I will update the comment in the next version.