3 files changed, 132 insertions, 9 deletions

diff --git a/Documentation/slow-work.txt b/Documentation/slow-work.txt
index 0169c9d9dd16..52bc31433723 100644
--- a/Documentation/slow-work.txt
+++ b/Documentation/slow-work.txt
@@ -158,6 +158,50 @@ with a requeue pending).  This can be used to work out whether an item on which
 another depends is on the queue, thus allowing a dependent item to be queued
 after it.
 
+If the above shows an item on which another depends not to be queued, then the
+owner of the dependent item might need to wait.  However, to avoid locking up
+the threads unnecessarily be sleeping in them, it can make sense under some
+circumstances to return the work item to the queue, thus deferring it until
+some other items have had a chance to make use of the yielded thread.
+
+To yield a thread and defer an item, the work function should simply enqueue
+the work item again and return.  However, this doesn't work if there's nothing
+actually on the queue, as the thread just vacated will jump straight back into
+the item's work function, thus busy waiting on a CPU.
+
+Instead, the item should use the thread to wait for the dependency to go away,
+but rather than using schedule() or schedule_timeout() to sleep, it should use
+the following function:
+
+	bool requeue = slow_work_sleep_till_thread_needed(
+			struct slow_work *work,
+			signed long *_timeout);
+
+This will add a second wait and then sleep, such that it will be woken up if
+either something appears on the queue that could usefully make use of the
+thread - and behind which this item can be queued, or if the event the caller
+set up to wait for happens.  True will be returned if something else appeared
+on the queue and this work function should perhaps return, of false if
+something else woke it up.  The timeout is as for schedule_timeout().
+
+For example:
+
+	wq = bit_waitqueue(&my_flags, MY_BIT);
+	init_wait(&wait);
+	requeue = false;
+	do {
+		prepare_to_wait(wq, &wait, TASK_UNINTERRUPTIBLE);
+		if (!test_bit(MY_BIT, &my_flags))
+			break;
+		requeue = slow_work_sleep_till_thread_needed(&my_work,
+							     &timeout);
+	} while (timeout > 0 && !requeue);
+	finish_wait(wq, &wait);
+	if (!test_bit(MY_BIT, &my_flags)
+		goto do_my_thing;
+	if (requeue)
+		return; // to slow_work
+
 
 ===============
 ITEM OPERATIONS
diff --git a/include/linux/slow-work.h b/include/linux/slow-work.h
index bfd3ab4c8898..5035a2691739 100644
--- a/include/linux/slow-work.h
+++ b/include/linux/slow-work.h
@@ -152,6 +152,9 @@ static inline void delayed_slow_work_cancel(struct delayed_slow_work *dwork)
 	slow_work_cancel(&dwork->work);
 }
 
+extern bool slow_work_sleep_till_thread_needed(struct slow_work *work,
+					       signed long *_timeout);
+
 #ifdef CONFIG_SYSCTL
 extern ctl_table slow_work_sysctls[];
 #endif
diff --git a/kernel/slow-work.c b/kernel/slow-work.c
index b763bc2d2670..da94f3c101af 100644
--- a/kernel/slow-work.c
+++ b/kernel/slow-work.c
@@ -133,6 +133,15 @@ LIST_HEAD(vslow_work_queue);
 DEFINE_SPINLOCK(slow_work_queue_lock);
 
 /*
+ * The following are two wait queues that get pinged when a work item is placed
+ * on an empty queue.  These allow work items that are hogging a thread by
+ * sleeping in a way that could be deferred to yield their thread and enqueue
+ * themselves.
+ */
+static DECLARE_WAIT_QUEUE_HEAD(slow_work_queue_waits_for_occupation);
+static DECLARE_WAIT_QUEUE_HEAD(vslow_work_queue_waits_for_occupation);
+
+/*
  * The thread controls.  A variable used to signal to the threads that they
  * should exit when the queue is empty, a waitqueue used by the threads to wait
  * for signals, and a completion set by the last thread to exit.
@@ -306,6 +315,50 @@ auto_requeue:
 }
 
 /**
+ * slow_work_sleep_till_thread_needed - Sleep till thread needed by other work
+ * work: The work item under execution that wants to sleep
+ * _timeout: Scheduler sleep timeout
+ *
+ * Allow a requeueable work item to sleep on a slow-work processor thread until
+ * that thread is needed to do some other work or the sleep is interrupted by
+ * some other event.
+ *
+ * The caller must set up a wake up event before calling this and must have set
+ * the appropriate sleep mode (such as TASK_UNINTERRUPTIBLE) and tested its own
+ * condition before calling this function as no test is made here.
+ *
+ * False is returned if there is nothing on the queue; true is returned if the
+ * work item should be requeued
+ */
+bool slow_work_sleep_till_thread_needed(struct slow_work *work,
+					signed long *_timeout)
+{
+	wait_queue_head_t *wfo_wq;
+	struct list_head *queue;
+
+	DEFINE_WAIT(wait);
+
+	if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags)) {
+		wfo_wq = &vslow_work_queue_waits_for_occupation;
+		queue = &vslow_work_queue;
+	} else {
+		wfo_wq = &slow_work_queue_waits_for_occupation;
+		queue = &slow_work_queue;
+	}
+
+	if (!list_empty(queue))
+		return true;
+
+	add_wait_queue_exclusive(wfo_wq, &wait);
+	if (list_empty(queue))
+		*_timeout = schedule_timeout(*_timeout);
+	finish_wait(wfo_wq, &wait);
+
+	return !list_empty(queue);
+}
+EXPORT_SYMBOL(slow_work_sleep_till_thread_needed);
+
+/**
  * slow_work_enqueue - Schedule a slow work item for processing
  * @work: The work item to queue
  *
@@ -335,6 +388,8 @@ auto_requeue:
  */
 int slow_work_enqueue(struct slow_work *work)
 {
+	wait_queue_head_t *wfo_wq;
+	struct list_head *queue;
 	unsigned long flags;
 	int ret;
 
@@ -354,6 +409,14 @@ int slow_work_enqueue(struct slow_work *work)
 	 * maintaining our promise
 	 */
 	if (!test_and_set_bit_lock(SLOW_WORK_PENDING, &work->flags)) {
+		if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags)) {
+			wfo_wq = &vslow_work_queue_waits_for_occupation;
+			queue = &vslow_work_queue;
+		} else {
+			wfo_wq = &slow_work_queue_waits_for_occupation;
+			queue = &slow_work_queue;
+		}
+
 		spin_lock_irqsave(&slow_work_queue_lock, flags);
 
 		if (unlikely(test_bit(SLOW_WORK_CANCELLING, &work->flags)))
@@ -380,11 +443,13 @@ int slow_work_enqueue(struct slow_work *work)
 			if (ret < 0)
 				goto failed;
 			slow_work_mark_time(work);
-			if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags))
-				list_add_tail(&work->link, &vslow_work_queue);
-			else
-				list_add_tail(&work->link, &slow_work_queue);
+			list_add_tail(&work->link, queue);
 			wake_up(&slow_work_thread_wq);
+
+			/* if someone who could be requeued is sleeping on a
+			 * thread, then ask them to yield their thread */
+			if (work->link.prev == queue)
+				wake_up(wfo_wq);
 		}
 
 		spin_unlock_irqrestore(&slow_work_queue_lock, flags);
@@ -487,9 +552,19 @@ EXPORT_SYMBOL(slow_work_cancel);
  */
 static void delayed_slow_work_timer(unsigned long data)
 {
+	wait_queue_head_t *wfo_wq;
+	struct list_head *queue;
 	struct slow_work *work = (struct slow_work *) data;
 	unsigned long flags;
-	bool queued = false, put = false;
+	bool queued = false, put = false, first = false;
+
+	if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags)) {
+		wfo_wq = &vslow_work_queue_waits_for_occupation;
+		queue = &vslow_work_queue;
+	} else {
+		wfo_wq = &slow_work_queue_waits_for_occupation;
+		queue = &slow_work_queue;
+	}
 
 	spin_lock_irqsave(&slow_work_queue_lock, flags);
 	if (likely(!test_bit(SLOW_WORK_CANCELLING, &work->flags))) {
@@ -502,17 +577,18 @@ static void delayed_slow_work_timer(unsigned long data)
 			put = true;
 		} else {
 			slow_work_mark_time(work);
-			if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags))
-				list_add_tail(&work->link, &vslow_work_queue);
-			else
-				list_add_tail(&work->link, &slow_work_queue);
+			list_add_tail(&work->link, queue);
 			queued = true;
+			if (work->link.prev == queue)
+				first = true;
 		}
 	}
 
 	spin_unlock_irqrestore(&slow_work_queue_lock, flags);
 	if (put)
 		slow_work_put_ref(work);
+	if (first)
+		wake_up(wfo_wq);
 	if (queued)
 		wake_up(&slow_work_thread_wq);
 }