[PATCH] memcg: Fix data-race KCSAN bug in rstats

[Breno Leitao <leitao@xxxxxxxxxx>]

A data-race issue in memcg rstat occurs when two distinct code paths
access the same 4-byte region concurrently. KCSAN detection triggers the
following BUG as a result.

	BUG: KCSAN: data-race in __count_memcg_events / mem_cgroup_css_rstat_flush

	write to 0xffffe8ffff98e300 of 4 bytes by task 5274 on cpu 17:
	mem_cgroup_css_rstat_flush (mm/memcontrol.c:5850)
	cgroup_rstat_flush_locked (kernel/cgroup/rstat.c:243 (discriminator 7))
	cgroup_rstat_flush (./include/linux/spinlock.h:401 kernel/cgroup/rstat.c:278)
	mem_cgroup_flush_stats.part.0 (mm/memcontrol.c:767)
	memory_numa_stat_show (mm/memcontrol.c:6911)
<snip>

	read to 0xffffe8ffff98e300 of 4 bytes by task 410848 on cpu 27:
	__count_memcg_events (mm/memcontrol.c:725 mm/memcontrol.c:962)
	count_memcg_event_mm.part.0 (./include/linux/memcontrol.h:1097 ./include/linux/memcontrol.h:1120)
	handle_mm_fault (mm/memory.c:5483 mm/memory.c:5622)
<snip>

	value changed: 0x00000029 -> 0x00000000

The race occurs because two code paths access the same "stats_updates"
location. Although "stats_updates" is a per-CPU variable, it is remotely
accessed by another CPU at
cgroup_rstat_flush_locked()->mem_cgroup_css_rstat_flush(), leading to
the data race mentioned.

Considering that memcg_rstat_updated() is in the hot code path, adding
a lock to protect it may not be desirable, especially since this
variable pertains solely to statistics.

Therefore, annotating accesses to stats_updates with READ/WRITE_ONCE()
can prevent KCSAN splats and potential partial reads/writes.

Suggested-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 mm/memcontrol.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index fabce2b50c69..3c99457b36a1 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -715,6 +715,7 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
 {
 	struct memcg_vmstats_percpu *statc;
 	int cpu = smp_processor_id();
+	unsigned int stats_updates;
 
 	if (!val)
 		return;
@@ -722,8 +723,9 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
 	cgroup_rstat_updated(memcg->css.cgroup, cpu);
 	statc = this_cpu_ptr(memcg->vmstats_percpu);
 	for (; statc; statc = statc->parent) {
-		statc->stats_updates += abs(val);
-		if (statc->stats_updates < MEMCG_CHARGE_BATCH)
+		stats_updates = READ_ONCE(statc->stats_updates) + abs(val);
+		WRITE_ONCE(statc->stats_updates, stats_updates);
+		if (stats_updates < MEMCG_CHARGE_BATCH)
 			continue;
 
 		/*
@@ -731,9 +733,9 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
 		 * redundant. Avoid the overhead of the atomic update.
 		 */
 		if (!memcg_vmstats_needs_flush(statc->vmstats))
-			atomic64_add(statc->stats_updates,
+			atomic64_add(stats_updates,
 				     &statc->vmstats->stats_updates);
-		statc->stats_updates = 0;
+		WRITE_ONCE(statc->stats_updates, 0);
 	}
 }
 
@@ -5845,7 +5847,7 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu)
 			}
 		}
 	}
-	statc->stats_updates = 0;
+	WRITE_ONCE(statc->stats_updates, 0);
 	/* We are in a per-cpu loop here, only do the atomic write once */
 	if (atomic64_read(&memcg->vmstats->stats_updates))
 		atomic64_set(&memcg->vmstats->stats_updates, 0);
-- 
2.43.0