[tip: timers/core] timers: Mark racy updates to hlist_node::pprev field

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     1159ad0a6aaa414dc726a33ccdcb702752d1b8eb
Gitweb:        https://git.kernel.org/tip/1159ad0a6aaa414dc726a33ccdcb702752d1b8eb
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Fri, 18 Sep 2026 17:25:22 -07:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxx>
CommitterDate: Tue, 29 Sep 2026 21:07:15 +02:00

timers: Mark racy updates to hlist_node::pprev field

hlist_unhashed_lockless() locklessly samples the hlist_node structure's
pprev field, but detach_timer(), hlist_move_list(), and hlist_splice_init()
all use plain C-language stores to update this field, despite the "The
READ_ONCE() is paired with the various WRITE_ONCE() in hlist helpers that
are defined below" in the hlist_unhashed_lockless() header comment.

Therefore use WRITE_ONCE() for these hlist_node::pprev updates.

KCSAN located this issue.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Link: https://patch.msgid.link/20260919002523.3133928-1-paulmck@xxxxxxxxxx
---
 include/linux/list.h | 6 +++---
 kernel/time/timer.c  | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/list.h b/include/linux/list.h
index 77fb62f..e375369 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -1172,7 +1172,7 @@ static inline void hlist_move_list(struct hlist_head *old,
 {
 	new->first = old->first;
 	if (new->first)
-		new->first->pprev = &new->first;
+		WRITE_ONCE(new->first->pprev, &new->first);
 	old->first = NULL;
 }
 
@@ -1189,10 +1189,10 @@ static inline void hlist_splice_init(struct hlist_head *from,
 				     struct hlist_head *to)
 {
 	if (to->first)
-		to->first->pprev = &last->next;
+		WRITE_ONCE(to->first->pprev, &last->next);
 	last->next = to->first;
 	to->first = from->first;
-	from->first->pprev = &to->first;
+	WRITE_ONCE(from->first->pprev, &to->first);
 	from->first = NULL;
 }
 
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index ae9abf1..42afdcb 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -890,7 +890,7 @@ static inline void detach_timer(struct timer_list *timer, bool clear_pending)
 
 	__hlist_del(entry);
 	if (clear_pending)
-		entry->pprev = NULL;
+		WRITE_ONCE(entry->pprev, NULL);
 	entry->next = LIST_POISON2;
 }