[PATCH 1/2] timers: Mark updates to hlist_node ->pprev field

[Paul E. McKenney]

The hlist_unhashed_lockless() function 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 ->pprev updates.

KCSAN located this issue.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Anna-Maria Behnsen <anna-maria@xxxxxxxxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxx>
Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
---
 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 77fb62f79928..e3753695e76c 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 ae9abf14688e..42afdcb229d8 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;
 }
 
-- 
2.40.1