linux-kernel - [PATCH] sched: Clarify wake_up_q()'s write to task->wake

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Message-Id: <20250129-sched-wakeup-prettier-v1-1-2f51f5f663fa@google.com>
Date: Wed, 29 Jan 2025 20:53:03 +0100
From: Jann Horn <jannh@...gle.com>
To: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
 Juri Lelli <juri.lelli@...hat.com>, 
 Vincent Guittot <vincent.guittot@...aro.org>, 
 Dietmar Eggemann <dietmar.eggemann@....com>, 
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, 
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>
Cc: linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>
Subject: [PATCH] sched: Clarify wake_up_q()'s write to task->wake_q.next

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>
---
a minor change; I figured we should at least be using WRITE_ONCE() here,
and I might as well change the comments a little bit while I'm touching
this
---
 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 9142a0394d46605e96e10cef97cce02d741f6c93..ce64652858703826dca510479f563a28c2fb2405 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1063,9 +1063,10 @@ 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

---
base-commit: 05dbaf8dd8bf537d4b4eb3115ab42a5fb40ff1f5
change-id: 20250129-sched-wakeup-prettier-c28762d8deee

-- 
Jann Horn <jannh@...gle.com>