Re: [PATCH] cgroup: rstat: relax NMI guard after switch to try_cmpxchg

[Shakeel Butt]

On Wed, May 20, 2026 at 11:30:54AM +0800, Cunlong Li wrote:
> Commit 36df6e3dbd7e ("cgroup: make css_rstat_updated nmi safe") used
> this_cpu_cmpxchg() for the lockless insertion, and therefore required
> both ARCH_HAVE_NMI_SAFE_CMPXCHG and ARCH_HAS_NMI_SAFE_THIS_CPU_OPS in
> the NMI guard: on archs without the latter, this_cpu_cmpxchg() falls
> back to "local_irq_save() + plain cmpxchg", and local_irq_save()
> cannot mask NMIs.
> 
> Commit 3309b63a2281 ("cgroup: rstat: use LOCK CMPXCHG in
> css_rstat_updated") later replaced this_cpu_cmpxchg() with plain
> try_cmpxchg() to fix cross-CPU lockless-list corruption, but left the
> NMI guard untouched.  After that switch, css_rstat_updated() no longer
> performs any this_cpu_*() RMW operations and only relies on the arch
> having NMI-safe cmpxchg, so ARCH_HAS_NMI_SAFE_THIS_CPU_OPS is no
> longer required in the guard.
> 
> Relax the guard accordingly so that archs which have HAVE_NMI and
> ARCH_HAVE_NMI_SAFE_CMPXCHG but not ARCH_HAS_NMI_SAFE_THIS_CPU_OPS
> (e.g. sparc, powerpc on PPC64/BOOK3S) can benefit from the existing
> CONFIG_MEMCG_NMI_SAFETY_REQUIRES_ATOMIC path.  Without this, the css
> is never queued in NMI on those archs, and the atomics staged by
> account_{slab,kmem}_nmi_safe() are not drained by flush_nmi_stats().
> 
> Fixes: 3309b63a2281 ("cgroup: rstat: use LOCK CMPXCHG in css_rstat_updated")
> Signed-off-by: Cunlong Li <shenxiaogll@xxxxxxxxx>

Looks fine but how did you find this? AI?

Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>