From: Jeongjun Park <aha310510@gmail.com>
Date: Thu, 3 Jul 2025 06:56:00 +0000 (+0900)
Subject: mm/percpu: prevent concurrency problem for pcpu_nr_populated read with spin lock
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=b112a4e0a1af5745f987a1692553bf02c890babc;p=thirdparty%2Fkernel%2Flinux.git

mm/percpu: prevent concurrency problem for pcpu_nr_populated read with spin lock

pcpu_nr_pages() reads pcpu_nr_populated without any protection, which
causes a data race between read/write.

However, since this is an intended race, we should add a data_race
annotation instead of add a spin lock.

[akpm@linux-foundation.org: move pcpu_nr_units multiplication outside data_race(), per Vlastimil]
Link: https://lkml.kernel.org/r/20250703065600.132221-1-aha310510@gmail.com
Fixes: 7e8a6304d541 ("/proc/meminfo: add percpu populated pages count")
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
Reported-by: syzbot+e5bd32b79413e86f389e@syzkaller.appspotmail.com
Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Christoph Lameter (Ampere) <cl@gentwo.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

diff --git a/mm/percpu.c b/mm/percpu.c
index b35494c8ede28..d9cbaee92b605 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -3355,7 +3355,7 @@ void __init setup_per_cpu_areas(void)
  */
 unsigned long pcpu_nr_pages(void)
 {
-	return pcpu_nr_populated * pcpu_nr_units;
+	return data_race(READ_ONCE(pcpu_nr_populated)) * pcpu_nr_units;
 }
 
 /*