On Wed, Apr 24, 2024 at 05:59:39AM -0700, Breno Leitao wrote: > 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> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
