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().

Cunlong Li committed 5 months ago

Commit 22572db1 parent 88170051 file · +3 −4Browse files

Unified · Split

kernel/cgroup/rstat.c+3−4

@@ -81,11 +81,10 @@ void __css_rstat_updated(struct cgroup_subsys_state *css, int cpu) · 80 unmodified lines
81	81	lockdep_assert_preemption_disabled();
82	82
83	83	/*
84		− * For archs withnot nmi safe cmpxchg or percpu ops support, ignore
85		− * the requests from nmi context.
	84	+ * The lockless insertion below relies on NMI-safe cmpxchg;
	85	+ * bail out in NMI on archs that don't provide it.
86	86	*/
87		− if ((!IS_ENABLED(CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG) \|\|
88		− !IS_ENABLED(CONFIG_ARCH_HAS_NMI_SAFE_THIS_CPU_OPS)) && in_nmi())
	87	+ if (!IS_ENABLED(CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG) && in_nmi())
89	88	return;
90	89
91	90	rstatc = css_rstat_cpu(css, cpu);