SLOW_WORK: Add support for cancellation of slow work

Add support for cancellation of queued slow work and delayed slow work items.
The cancellation functions will wait for items that are pending or undergoing
execution to be discarded by the slow work facility.

Attempting to enqueue work that is in the process of being cancelled will
result in ECANCELED.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 75 insertions, 6 deletions

diff --git a/kernel/slow-work.c b/kernel/slow-work.c
index fccf421..671cc43 100644
--- a/kernel/slow-work.c
+++ b/kernel/slow-work.c
@@ -236,12 +236,17 @@ static bool slow_work_execute(int id)
 	if (!test_and_clear_bit(SLOW_WORK_PENDING, &work->flags))
 		BUG();
 
-	work->ops->execute(work);
+	/* don't execute if the work is in the process of being cancelled */
+	if (!test_bit(SLOW_WORK_CANCELLING, &work->flags))
+		work->ops->execute(work);
 
 	if (very_slow)
 		atomic_dec(&vslow_work_executing_count);
 	clear_bit_unlock(SLOW_WORK_EXECUTING, &work->flags);
 
+	/* wake up anyone waiting for this work to be complete */
+	wake_up_bit(&work->flags, SLOW_WORK_EXECUTING);
+
 	/* if someone tried to enqueue the item whilst we were executing it,
 	 * then it'll be left unenqueued to avoid multiple threads trying to
 	 * execute it simultaneously
@@ -314,11 +319,16 @@ auto_requeue:
  * allowed to pick items to execute.  This ensures that very slow items won't
  * overly block ones that are just ordinarily slow.
  *
- * Returns 0 if successful, -EAGAIN if not.
+ * Returns 0 if successful, -EAGAIN if not (or -ECANCELED if cancelled work is
+ * attempted queued)
  */
 int slow_work_enqueue(struct slow_work *work)
 {
 	unsigned long flags;
+	int ret;
+
+	if (test_bit(SLOW_WORK_CANCELLING, &work->flags))
+		return -ECANCELED;
 
 	BUG_ON(slow_work_user_count <= 0);
 	BUG_ON(!work);
@@ -335,6 +345,9 @@ int slow_work_enqueue(struct slow_work *work)
 	if (!test_and_set_bit_lock(SLOW_WORK_PENDING, &work->flags)) {
 		spin_lock_irqsave(&slow_work_queue_lock, flags);
 
+		if (unlikely(test_bit(SLOW_WORK_CANCELLING, &work->flags)))
+			goto cancelled;
+
 		/* we promise that we will not attempt to execute the work
 		 * function in more than one thread simultaneously
 		 *
@@ -352,8 +365,9 @@ int slow_work_enqueue(struct slow_work *work)
 		if (test_bit(SLOW_WORK_EXECUTING, &work->flags)) {
 			set_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags);
 		} else {
-			if (slow_work_get_ref(work) < 0)
-				goto cant_get_ref;
+			ret = slow_work_get_ref(work);
+			if (ret < 0)
+				goto failed;
 			if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags))
 				list_add_tail(&work->link, &vslow_work_queue);
 			else
@@ -365,12 +379,67 @@ int slow_work_enqueue(struct slow_work *work)
 	}
 	return 0;
 
-cant_get_ref:
+cancelled:
+	ret = -ECANCELED;
+failed:
 	spin_unlock_irqrestore(&slow_work_queue_lock, flags);
-	return -EAGAIN;
+	return ret;
 }
 EXPORT_SYMBOL(slow_work_enqueue);
 
+static int slow_work_wait(void *word)
+{
+	schedule();
+	return 0;
+}
+
+/**
+ * slow_work_cancel - Cancel a slow work item
+ * @work: The work item to cancel
+ *
+ * This function will cancel a previously enqueued work item. If we cannot
+ * cancel the work item, it is guarenteed to have run when this function
+ * returns.
+ */
+void slow_work_cancel(struct slow_work *work)
+{
+	bool wait = true, put = false;
+
+	set_bit(SLOW_WORK_CANCELLING, &work->flags);
+
+	spin_lock_irq(&slow_work_queue_lock);
+
+	if (test_bit(SLOW_WORK_PENDING, &work->flags) &&
+	    !list_empty(&work->link)) {
+		/* the link in the pending queue holds a reference on the item
+		 * that we will need to release */
+		list_del_init(&work->link);
+		wait = false;
+		put = true;
+		clear_bit(SLOW_WORK_PENDING, &work->flags);
+
+	} else if (test_and_clear_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags)) {
+		/* the executor is holding our only reference on the item, so
+		 * we merely need to wait for it to finish executing */
+		clear_bit(SLOW_WORK_PENDING, &work->flags);
+	}
+
+	spin_unlock_irq(&slow_work_queue_lock);
+
+	/* the EXECUTING flag is set by the executor whilst the spinlock is set
+	 * and before the item is dequeued - so assuming the above doesn't
+	 * actually dequeue it, simply waiting for the EXECUTING flag to be
+	 * released here should be sufficient */
+	if (wait)
+		wait_on_bit(&work->flags, SLOW_WORK_EXECUTING, slow_work_wait,
+			    TASK_UNINTERRUPTIBLE);
+
+	clear_bit(SLOW_WORK_CANCELLING, &work->flags);
+	if (put)
+		slow_work_put_ref(work);
+}
+EXPORT_SYMBOL(slow_work_cancel);
+
 /*
  * Schedule a cull of the thread pool at some time in the near future
  */