linux-kernel - [PATCH AUTOSEL 6.6 09/17] sched: Clarify wake_up_q()'s write to task->wake

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Message-Id: <20250218202743.3593296-9-sashal@kernel.org>
Date: Tue, 18 Feb 2025 15:27:33 -0500
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Jann Horn <jannh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Sasha Levin <sashal@...nel.org>,
	mingo@...hat.com,
	juri.lelli@...hat.com,
	vincent.guittot@...aro.org
Subject: [PATCH AUTOSEL 6.6 09/17] sched: Clarify wake_up_q()'s write to task->wake_q.next

From: Jann Horn <jannh@...gle.com>

[ Upstream commit bcc6244e13b4d4903511a1ea84368abf925031c0 ]

Clarify that wake_up_q() does an atomic write to task->wake_q.next, after
which a concurrent __wake_q_add() can immediately overwrite
task->wake_q.next again.

Signed-off-by: Jann Horn <jannh@...gle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20250129-sched-wakeup-prettier-v1-1-2f51f5f663fa@google.com
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 kernel/sched/core.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c686d826a91cf..e29746cd11afe 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1019,9 +1019,10 @@ void wake_up_q(struct wake_q_head *head)
 		struct task_struct *task;
 
 		task = container_of(node, struct task_struct, wake_q);
-		/* Task can safely be re-inserted now: */
 		node = node->next;
-		task->wake_q.next = NULL;
+		/* pairs with cmpxchg_relaxed() in __wake_q_add() */
+		WRITE_ONCE(task->wake_q.next, NULL);
+		/* Task can safely be re-inserted now. */
 
 		/*
 		 * wake_up_process() executes a full barrier, which pairs with
-- 
2.39.5