cgroup: rstat: relax NMI guard after switch to try_cmpxchg

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@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index ed60ba119c687b6dfeab3f45063880ce7f78a707..de816a43db9f0a309911a50e80f5dbaa7af4c855 100644 (file)
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -81,11 +81,10 @@ void __css_rstat_updated(struct cgroup_subsys_state *css, int cpu)
        lockdep_assert_preemption_disabled();
 
        /*
-        * For archs withnot nmi safe cmpxchg or percpu ops support, ignore
-        * the requests from nmi context.
+        * The lockless insertion below relies on NMI-safe cmpxchg;
+        * bail out in NMI on archs that don't provide it.
         */
-       if ((!IS_ENABLED(CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG) ||
-            !IS_ENABLED(CONFIG_ARCH_HAS_NMI_SAFE_THIS_CPU_OPS)) && in_nmi())
+       if (!IS_ENABLED(CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG) && in_nmi())
                return;
 
        rstatc = css_rstat_cpu(css, cpu);