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


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