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

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Message-ID: <173902629795.10177.6552765671236398998.tip-bot2@tip-bot2>
Date: Sat, 08 Feb 2025 14:51:37 -0000
From: "tip-bot2 for Jann Horn" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Jann Horn <jannh@...gle.com>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject:
 [tip: sched/urgent] sched: Clarify wake_up_q()'s write to task->wake_q.next

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     bcc6244e13b4d4903511a1ea84368abf925031c0
Gitweb:        https://git.kernel.org/tip/bcc6244e13b4d4903511a1ea84368abf925031c0
Author:        Jann Horn <jannh@...gle.com>
AuthorDate:    Wed, 29 Jan 2025 20:53:03 +01:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Sat, 08 Feb 2025 15:43:12 +01:00

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20250129-sched-wakeup-prettier-v1-1-2f51f5f663fa@google.com
---
 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 3e5a6bf..8931d9b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1055,9 +1055,10 @@ 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