[PATCH] timekeeping: Use data_race() and READ_ONCE() in ktime_get_real_seconds()

[Dennis Moshegov]

From: Dennis Moshegov <dennis@xxxxxxxx>
Subject: [PATCH] timekeeping: Use data_race() and READ_ONCE() in
ktime_get_real_seconds()

On 64-bit architectures, ktime_get_real_seconds() directly returns
tk_core.timekeeper.xtime_sec without acquiring a seqlock. This lockless
read is an intentional design choice optimized for ultra-fast,
approximate timestamping paths where a minor 1-second discrepancy is
acceptable.

However, Kernel Concurrency Sanitizer (KCSAN) flags a data race here
because the timekeeper structure is updated concurrently via a bulk
structure copy inside timekeeping_update_from_shadow() during timer
interrupts.

Since the 64-bit read is naturally aligned and atomic at the hardware
level, this data race is benign and does not cause data tearing.
Annotate the read path using data_race() combined with READ_ONCE() to
prevent compiler optimization anomalies and suppress the KCSAN warning.

Reported-by: syzbot+72789cd1697965e714ca@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspotmail.com/bug?extid=72789cd1697965e714ca
Assisted-by: Gemini:3.5-flash
Signed-off-by: Dennis Moshegov <dennis@xxxxxxxx>
---
 kernel/time/timekeeping.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 4fd3c70c1..b2e5a87da 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1153,7 +1153,7 @@ time64_t ktime_get_real_seconds(void)
 time64_t ktime_get_real_seconds(void)
 {
 #if BITS_PER_LONG == 64
- return tk_core.timekeeper.xtime_sec;
+ return data_race(READ_ONCE(tk_core.timekeeper.xtime_sec));
 #else
  unsigned int seq;
  time64_t seconds;
-- 
2.43.0