[RFC v3 02/11] kthread: Introduce __kthread_queue_work()

[Lyude Paul]

While kthread_queue_work() is fine for basic kthread_worker usecases,
it's a little limiting if you want to create your own delayed work
implementations that delay off things other than a clock. Looking at
kthread_delayed_works for instance, all of the code shares the lock in
kthread_work so that that both the timer_list and actual kthread_worker
can be inspected and modified together atomically.

Currently, we want to be able to implement a type of delayed
kthread_work in DRM that delays until a specific vblank sequence has
passed, which we refer to as a drm_vblank_work, as opposed to using a
simple time-based delay. Some of the requirements we have for this are
the ability to allow for rescheduling and flushing on drm_vblank_works,
which become a lot harder to do properly if we can't re-queue work under
lock. Additionally, being able to specify a custom position in the
kthread_worker's work_list (which requires being under lock) is
important to be able to do since it's needed for implementing a custom
work flushing mechanism that waits for both the vblank sequence and
kthread_worker to complete once.

So - let's expose an unlocked version of kthread_queue_work() called
__kthread_queue_work(), which also allows for specifying a custom list
position in which to insert the work before.

Cc: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
---
 include/linux/kthread.h |  3 +++
 kernel/kthread.c        | 34 ++++++++++++++++++++++++++++++----
 2 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/include/linux/kthread.h b/include/linux/kthread.h
index 8bbcaad7ef0f..02e0c1c157bf 100644
--- a/include/linux/kthread.h
+++ b/include/linux/kthread.h
@@ -179,6 +179,9 @@ __printf(3, 4) struct kthread_worker *
 kthread_create_worker_on_cpu(int cpu, unsigned int flags,
 			     const char namefmt[], ...);
 
+bool __kthread_queue_work(struct kthread_worker *worker,
+			  struct kthread_work *work,
+			  struct list_head *pos);
 bool kthread_queue_work(struct kthread_worker *worker,
 			struct kthread_work *work);
 
diff --git a/kernel/kthread.c b/kernel/kthread.c
index bfbfa481be3a..46de56142593 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -816,6 +816,35 @@ static void kthread_insert_work(struct kthread_worker *worker,
 		wake_up_process(worker->task);
 }
 
+/**
+ * __kthread_queue_work - queue a kthread_work while under lock
+ * @worker: target kthread_worker
+ * @work: kthread_work to queue
+ * @pos: The position in @worker.work_list to insert @work before
+ *
+ * This is the same as kthread_queue_work(), except that it already expects
+ * the caller to be holding &kthread_worker.lock and allows for specifying a
+ * custom position in @worker.work_list to insert @work before.
+ *
+ * This function is mostly useful for users which might need to create their
+ * own delayed kthread_worker implementations.
+ *
+ * Returns: %true if @work was successfully queued, %false if it was already
+ * pending.
+ */
+bool __kthread_queue_work(struct kthread_worker *worker,
+			  struct kthread_work *work,
+			  struct list_head *pos)
+{
+	if (!queuing_blocked(worker, work)) {
+		kthread_insert_work(worker, work, pos);
+		return true;
+	}
+
+	return false;
+}
+EXPORT_SYMBOL_GPL(__kthread_queue_work);
+
 /**
  * kthread_queue_work - queue a kthread_work
  * @worker: target kthread_worker
@@ -835,10 +864,7 @@ bool kthread_queue_work(struct kthread_worker *worker,
 	unsigned long flags;
 
 	raw_spin_lock_irqsave(&worker->lock, flags);
-	if (!queuing_blocked(worker, work)) {
-		kthread_insert_work(worker, work, &worker->work_list);
-		ret = true;
-	}
+	ret = __kthread_queue_work(worker, work, &worker->work_list);
 	raw_spin_unlock_irqrestore(&worker->lock, flags);
 	return ret;
 }
-- 
2.25.1