2 files changed, 92 insertions, 0 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4adc536a3b03..254d88e80f65 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -921,6 +921,50 @@ enum cpu_idle_type {
 #define SCHED_CAPACITY_SCALE	(1L << SCHED_CAPACITY_SHIFT)
 
 /*
+ * Wake-queues are lists of tasks with a pending wakeup, whose
+ * callers have already marked the task as woken internally,
+ * and can thus carry on. A common use case is being able to
+ * do the wakeups once the corresponding user lock as been
+ * released.
+ *
+ * We hold reference to each task in the list across the wakeup,
+ * thus guaranteeing that the memory is still valid by the time
+ * the actual wakeups are performed in wake_up_q().
+ *
+ * One per task suffices, because there's never a need for a task to be
+ * in two wake queues simultaneously; it is forbidden to abandon a task
+ * in a wake queue (a call to wake_up_q() _must_ follow), so if a task is
+ * already in a wake queue, the wakeup will happen soon and the second
+ * waker can just skip it.
+ *
+ * The WAKE_Q macro declares and initializes the list head.
+ * wake_up_q() does NOT reinitialize the list; it's expected to be
+ * called near the end of a function, where the fact that the queue is
+ * not used again will be easy to see by inspection.
+ *
+ * Note that this can cause spurious wakeups. schedule() callers
+ * must ensure the call is done inside a loop, confirming that the
+ * wakeup condition has in fact occurred.
+ */
+struct wake_q_node {
+	struct wake_q_node *next;
+};
+
+struct wake_q_head {
+	struct wake_q_node *first;
+	struct wake_q_node **lastp;
+};
+
+#define WAKE_Q_TAIL ((struct wake_q_node *) 0x01)
+
+#define WAKE_Q(name)					\
+	struct wake_q_head name = { WAKE_Q_TAIL, &name.first }
+
+extern void wake_q_add(struct wake_q_head *head,
+		       struct task_struct *task);
+extern void wake_up_q(struct wake_q_head *head);
+
+/*
  * sched-domains (multiprocessor balancing) declarations:
  */
 #ifdef CONFIG_SMP
@@ -1532,6 +1576,8 @@ struct task_struct {
 	/* Protection of the PI data structures: */
 	raw_spinlock_t pi_lock;
 
+	struct wake_q_node wake_q;
+
 #ifdef CONFIG_RT_MUTEXES
 	/* PI waiters blocked on a rt_mutex held by this task */
 	struct rb_root pi_waiters;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 22b53c863ef3..355f9538ca33 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -541,6 +541,52 @@ static bool set_nr_if_polling(struct task_struct *p)
 #endif
 #endif
 
+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.
+	 *
+	 * This cmpxchg() implies a full barrier, which pairs with the write
+	 * barrier implied by the wakeup in wake_up_list().
+	 */
+	if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL))
+		return;
+
+	get_task_struct(task);
+
+	/*
+	 * The head is context local, there can be no concurrency.
+	 */
+	*head->lastp = node;
+	head->lastp = &node->next;
+}
+
+void wake_up_q(struct wake_q_head *head)
+{
+	struct wake_q_node *node = head->first;
+
+	while (node != WAKE_Q_TAIL) {
+		struct task_struct *task;
+
+		task = container_of(node, struct task_struct, wake_q);
+		BUG_ON(!task);
+		/* task can safely be re-inserted now */
+		node = node->next;
+		task->wake_q.next = NULL;
+
+		/*
+		 * wake_up_process() implies a wmb() to pair with the queueing
+		 * in wake_q_add() so as not to miss wakeups.
+		 */
+		wake_up_process(task);
+		put_task_struct(task);
+	}
+}
+
 /*
  * resched_curr - mark rq's current task 'to be rescheduled now'.
  *