linux-kernel - [PATCH] seqlock: Use WRITE_ONCE() when updating sequence

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Message-ID: <0eaea03ecc9df536649763cfecda356fc38b6938.1734477414.git.dxu@dxuuu.xyz>
Date: Tue, 17 Dec 2024 15:17:36 -0800
From: Daniel Xu <dxu@...uu.xyz>
To: mingo@...hat.com,
	will@...nel.org,
	peterz@...radead.org
Cc: longman@...hat.com,
	boqun.feng@...il.com,
	linux-kernel@...r.kernel.org,
	paulmck@...nel.org
Subject: [PATCH] seqlock: Use WRITE_ONCE() when updating sequence

`sequence` is a concurrently accessed shared variable on the reader
side. Therefore, it needs to be wrapped in WRITE_ONCE() in order to
prevent unwanted compiler optimizations like store tearing.

Signed-off-by: Daniel Xu <dxu@...uu.xyz>
---
 include/linux/seqlock.h | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 5298765d6ca4..f4c6f2507742 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -45,7 +45,7 @@ static inline void __seqcount_init(seqcount_t *s, const char *name,
 	 * Make sure we are not reinitializing a held lock:
 	 */
 	lockdep_init_map(&s->dep_map, name, key, 0);
-	s->sequence = 0;
+	WRITE_ONCE(s->sequence, 0);
 }
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
@@ -405,7 +405,7 @@ do {									\
 static inline void do_raw_write_seqcount_begin(seqcount_t *s)
 {
 	kcsan_nestable_atomic_begin();
-	s->sequence++;
+	WRITE_ONCE(s->sequence, READ_ONCE(s->sequence) + 1);
 	smp_wmb();
 }
 
@@ -426,7 +426,7 @@ do {									\
 static inline void do_raw_write_seqcount_end(seqcount_t *s)
 {
 	smp_wmb();
-	s->sequence++;
+	WRITE_ONCE(s->sequence, READ_ONCE(s->sequence) + 1);
 	kcsan_nestable_atomic_end();
 }
 
@@ -548,9 +548,9 @@ static inline void do_write_seqcount_end(seqcount_t *s)
 static inline void do_raw_write_seqcount_barrier(seqcount_t *s)
 {
 	kcsan_nestable_atomic_begin();
-	s->sequence++;
+	WRITE_ONCE(s->sequence, READ_ONCE(s->sequence) + 1);
 	smp_wmb();
-	s->sequence++;
+	WRITE_ONCE(s->sequence, READ_ONCE(s->sequence) + 1);
 	kcsan_nestable_atomic_end();
 }
 
@@ -569,7 +569,7 @@ static inline void do_write_seqcount_invalidate(seqcount_t *s)
 {
 	smp_wmb();
 	kcsan_nestable_atomic_begin();
-	s->sequence+=2;
+	WRITE_ONCE(s->sequence, READ_ONCE(s->sequence) + 2);
 	kcsan_nestable_atomic_end();
 }
 
@@ -673,7 +673,7 @@ read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
 static __always_inline void raw_write_seqcount_latch(seqcount_latch_t *s)
 {
 	smp_wmb();	/* prior stores before incrementing "sequence" */
-	s->seqcount.sequence++;
+	WRITE_ONCE(s->seqcount.sequence, READ_ONCE(s->seqcount.sequence) + 1);
 	smp_wmb();      /* increment "sequence" before following stores */
 }
 
-- 
2.47.0