[PATCH RFC] arm64: Mark set_preempt_need_resched() access to .need_resched

[Paul E. McKenney]

The .need_resched field can be accessed from both task level and
from interrrupt handlers, so apply WRITE_ONCE() to the update in
set_preempt_need_resched().  This also brings arm64 in line with s390
(which uses atomic operations) and x86 (which uses inline assembly).
Other architectures avoid this issue via the empty definition in
include/asm-generic/preempt.h.

KCSAN located this issue.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
Cc: Ada Couprie Diaz <ada.coupriediaz@xxxxxxx>
Cc: <linux-arm-kernel@xxxxxxxxxxxxxxxxxxx>

---
 preempt.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/preempt.h b/arch/arm64/include/asm/preempt.h
index 932ea4b6204289..610853da140a78 100644
--- a/arch/arm64/include/asm/preempt.h
+++ b/arch/arm64/include/asm/preempt.h
@@ -28,7 +28,7 @@ static inline void preempt_count_set(u64 pc)
 
 static inline void set_preempt_need_resched(void)
 {
-	current_thread_info()->preempt.need_resched = 0;
+	WRITE_ONCE(current_thread_info()->preempt.need_resched, 0);
 }
 
 static inline void clear_preempt_need_resched(void)