[PATCH 1/6] wake_q: Cleanup + Documentation update.

[Manfred Spraul <manfred@colorfullife.com>]

1) wake_q_add() contains a memory barrier, and callers such as
ipc/mqueue.c rely on this barrier.
Unfortunately, this is documented in ipc/mqueue.c, and not in the
description of wake_q_add().
Therefore: Update the documentation.
Removing/updating ipc/mqueue.c will happen with the next patch in the
series.

2) wake_q_add() ends with get_task_struct(), which is an
unordered refcount increase. Add a clear comment that the callers
are responsible for a barrier: most likely spin_unlock() or
smp_store_release().

3) wake_up_q() relies on the memory barrier in try_to_wake_up().
Add a comment, to simplify searching.

4) wake_q.next is accessed without synchroniyation by wake_q_add(),
using cmpxchg_relaxed(), and by wake_up_q().
Therefore: Use WRITE_ONCE in wake_up_q(), to ensure that the
compiler doesn't perform any tricks.

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
---
 kernel/sched/core.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index dd05a378631a..60ae574317fd 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -440,8 +440,16 @@ static bool __wake_q_add(struct wake_q_head *head, struct task_struct *task)
  * @task: the task to queue for 'later' wakeup
  *
  * Queue a task for later wakeup, most likely by the wake_up_q() call in the
- * same context, _HOWEVER_ this is not guaranteed, the wakeup can come
- * instantly.
+ * same context, _HOWEVER_ this is not guaranteed. Especially, the wakeup
+ * may happen before the function returns.
+ *
+ * What is guaranteed is that there is a memory barrier before the wakeup,
+ * callers may rely on this barrier.
+ *
+ * On the other hand, the caller must guarantee that @task does not disappear
+ * before wake_q_add() completed. wake_q_add() does not contain any memory
+ * barrier to ensure ordering, thus the caller may need to use
+ * smp_store_release().
  *
  * This function must be used as-if it were wake_up_process(); IOW the task
  * must be ready to be woken at this location.
@@ -486,11 +494,14 @@ void wake_up_q(struct wake_q_head *head)
 		BUG_ON(!task);
 		/* Task can safely be re-inserted now: */
 		node = node->next;
-		task->wake_q.next = NULL;
+
+		WRITE_ONCE(task->wake_q.next, NULL);
 
 		/*
 		 * wake_up_process() executes a full barrier, which pairs with
 		 * the queueing in wake_q_add() so as not to miss wakeups.
+		 * The barrier is the smp_mb__after_spinlock() in
+		 * try_to_wake_up().
 		 */
 		wake_up_process(task);
 		put_task_struct(task);
-- 
2.21.0