sched: Clarify wake_up_q()'s write to task->wake_q.next

JIRA: https://issues.redhat.com/browse/RHEL-96250

commit bcc6244
Author: Jann Horn <jannh@google.com>
Date:   Wed Jan 29 20:53:03 2025 +0100

    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@google.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Link: https://lkml.kernel.org/r/20250129-sched-wakeup-prettier-v1-1-2f51f5f663fa@google.com

Signed-off-by: Phil Auld <pauld@redhat.com>

Author: auldp

kernel/sched/core.c

@@ -1021,9 +1021,10 @@ 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