The patch titled
Subject: memcg: fix data-race KCSAN bug in rstats
has been added to the -mm mm-unstable branch. Its filename is
memcg-fix-data-race-kcsan-bug-in-rstats.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/memcg-fix-data-race-kcsan-bug-in-rstats.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
b) Prefer to cc a suitable mailing list as well
c) Ideally: find the original patch on the mailing list and do a
reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days
------------------------------------------------------
From: Breno Leitao <leitao@xxxxxxxxxx>
Subject: memcg: fix data-race KCSAN bug in rstats
Date: Wed, 24 Apr 2024 05:59:39 -0700
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.
Link: https://lkml.kernel.org/r/20240424125940.2410718-1-leitao@xxxxxxxxxx
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
Suggested-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/memcontrol.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)
--- a/mm/memcontrol.c~memcg-fix-data-race-kcsan-bug-in-rstats
+++ a/mm/memcontrol.c
@@ -715,6 +715,7 @@ static inline void memcg_rstat_updated(s
{
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(s
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(s
* 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);
}
}
@@ -5887,7 +5889,7 @@ static void mem_cgroup_css_rstat_flush(s
}
}
}
- 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);
_
Patches currently in -mm which might be from leitao@xxxxxxxxxx are
memcg-fix-data-race-kcsan-bug-in-rstats.patch
