1 files changed, 16 insertions, 3 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a674c7db2f29..d8d76a65cfdd 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -396,6 +396,18 @@ static bool set_nr_if_polling(struct task_struct *p)
 #endif
 #endif
 
+/**
+ * wake_q_add() - queue a wakeup for 'later' waking.
+ * @head: the wake_q_head to add @task to
+ * @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.
+ *
+ * This function must be used as-if it were wake_up_process(); IOW the task
+ * must be ready to be woken at this location.
+ */
 void wake_q_add(struct wake_q_head *head, struct task_struct *task)
 {
         struct wake_q_node *node = &task->wake_q;
@@ -405,10 +417,11 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task)
          * its already queued (either by us or someone else) and will get the
          * wakeup due to that.
          *
-         * This cmpxchg() executes a full barrier, which pairs with the full
-         * barrier executed by the wakeup in wake_up_q().
+         * In order to ensure that a pending wakeup will observe our pending
+         * state, even in the failed case, an explicit smp_mb() must be used.
          */
-        if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL))
+        smp_mb__before_atomic();
+        if (cmpxchg_relaxed(&node->next, NULL, WAKE_Q_TAIL))
                 return;
 
         get_task_struct(task);