[PATCH v8 06/12] kthread: Add kthread_drain_worker()

[Petr Mladek]

kthread_flush_worker() returns when the currently queued works are proceed.
But some other works might have been queued in the meantime.

This patch adds kthread_drain_worker() that is inspired by
drain_workqueue(). It returns when the queue is completely
empty and warns when it takes too long.

The initial implementation does not block queuing new works when
draining. It makes things much easier. The blocking would be useful
to debug potential problems but it is not clear if it is worth
the complication at the moment.

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
 include/linux/kthread.h |  1 +
 kernel/kthread.c        | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/include/linux/kthread.h b/include/linux/kthread.h
index 491ba721d549..6fc0940b9657 100644
--- a/include/linux/kthread.h
+++ b/include/linux/kthread.h
@@ -135,5 +135,6 @@ bool kthread_queue_work(struct kthread_worker *worker,
 			struct kthread_work *work);
 void kthread_flush_work(struct kthread_work *work);
 void kthread_flush_worker(struct kthread_worker *worker);
+void kthread_drain_worker(struct kthread_worker *worker);
 
 #endif /* _LINUX_KTHREAD_H */
diff --git a/kernel/kthread.c b/kernel/kthread.c
index b17f41059dcc..fa3c327be889 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -819,3 +819,37 @@ void kthread_flush_worker(struct kthread_worker *worker)
 	wait_for_completion(&fwork.done);
 }
 EXPORT_SYMBOL_GPL(kthread_flush_worker);
+
+/**
+ * kthread_drain_worker - drain a kthread worker
+ * @worker: worker to be drained
+ *
+ * Wait until there is no work queued for the given kthread worker.
+ * @worker is flushed repeatedly until it becomes empty.  The number
+ * of flushing is determined by the depth of chaining and should
+ * be relatively short.  Whine if it takes too long.
+ *
+ * The caller is responsible for blocking all users of this kthread
+ * worker from queuing new works. Also it is responsible for blocking
+ * the already queued works from an infinite re-queuing!
+ */
+void kthread_drain_worker(struct kthread_worker *worker)
+{
+	int flush_cnt = 0;
+
+	spin_lock_irq(&worker->lock);
+
+	while (!list_empty(&worker->work_list)) {
+		spin_unlock_irq(&worker->lock);
+
+		kthread_flush_worker(worker);
+		WARN_ONCE(flush_cnt++ > 10,
+			  "kthread worker %s: kthread_drain_worker() isn't complete after %u tries\n",
+			  worker->task->comm, flush_cnt);
+
+		spin_lock_irq(&worker->lock);
+	}
+
+	spin_unlock_irq(&worker->lock);
+}
+EXPORT_SYMBOL(kthread_drain_worker);
-- 
1.8.5.6