[PATCH -tip 2/3] sched/wake_q: Relax to acquire semantics

[Davidlohr Bueso <dave@stgolabs.net>]

The barrier parings for wake-queues are very straightforward, and thus
we can ease the barrier requirements, for archs that support it, for
wake_q_add by relying on acquire semantics. As such, (i) we keep the
pairing structure/logic and (ii) users, such as mqueues, can continue to
rely on a full barrier after the successful [Rmw].

[Another alternative could be to just not try at all and fully downgrade
to cmpxchg_relaxed() and rely on users to enable their own synchronization.
But controlling this ourselves makes me sleep better at night.]

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 kernel/sched/core.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 6ab415a..7567603 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -523,14 +523,14 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task)
 	struct wake_q_node *node = &task->wake_q;
 
 	/*
-	 * Atomically grab the task, if ->wake_q is !nil already it means
-	 * its already queued (either by us or someone else) and will get the
-	 * wakeup due to that.
+	 * Atomically grab the task. If ->wake_q is non-nil (failed cmpxchg)
+	 * then the task is already queued (by us or someone else) and will
+	 * get the wakeup due to that.
 	 *
-	 * This cmpxchg() implies a full barrier, which pairs with the write
-	 * barrier implied by the wakeup in wake_up_list().
+	 * Use acquire semantics to add the next pointer, which pairs with the
+	 * write barrier implied by the wakeup in wake_up_list().
 	 */
-	if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL))
+	if (cmpxchg_acquire(&node->next, NULL, WAKE_Q_TAIL))
 		return;
 
 	get_task_struct(task);
-- 
2.1.4