mm/memcontrol.c | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-)
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@linux.dev>
Signed-off-by: Breno Leitao <leitao@debian.org>
---
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
On Wed 24-04-24 05:59:39, 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@linux.dev> > Signed-off-by: Breno Leitao <leitao@debian.org> Acked-by: Michal Hocko <mhocko@suse.com> It is worth mentioning that the race is harmless. Thanks! > --- > 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 > -- Michal Hocko SUSE Labs
On Wed, Apr 24, 2024 at 6:00 AM Breno Leitao <leitao@debian.org> 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@linux.dev> > Signed-off-by: Breno Leitao <leitao@debian.org> Reviewed-by: Yosry Ahmed <yosryahmed@google.com> , and or posterity: Fixes: 9cee7e8ef3e3 ("mm: memcg: optimize parent iteration in memcg_rstat_updated()")
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@linux.dev> > Signed-off-by: Breno Leitao <leitao@debian.org> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
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@linux.dev> > Signed-off-by: Breno Leitao <leitao@debian.org> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
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