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[PATCH v6 04/20] kthread: Add drain_kthread_worker()

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

This patch adds drain_kthread_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@suse.com>
---
 kernel/kthread.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 76364374ff98..6051aa9d93c6 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -818,3 +818,37 @@ void flush_kthread_worker(struct kthread_worker *worker)
 	wait_for_completion(&fwork.done);
 }
 EXPORT_SYMBOL_GPL(flush_kthread_worker);
+
+/**
+ * drain_kthread_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 drain_kthread_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);
+
+		flush_kthread_worker(worker);
+		WARN_ONCE(flush_cnt++ > 10,
+			  "kthread worker %s: drain_kthread_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(drain_kthread_worker);
-- 
1.8.5.6

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[PATCH v6 00/20] kthread: Use kthread worker API more widely Petr Mladek <pmladek@suse.com> - 2016-04-14 17:20 +0200
  [PATCH v6 18/20] memstick/r592: convert r592_io kthread into kthread worker API Petr Mladek <pmladek@suse.com> - 2016-04-14 17:20 +0200
  [PATCH v6 15/20] ipmi: Convert kipmi kthread into kthread worker API Petr Mladek <pmladek@suse.com> - 2016-04-14 17:20 +0200
  [PATCH v6 07/20] kthread: Initial support for delayed kthread work Petr Mladek <pmladek@suse.com> - 2016-04-14 17:20 +0200
  [PATCH v6 19/20] thermal/intel_powerclamp: Remove duplicated code that starts the kthread Petr Mladek <pmladek@suse.com> - 2016-04-14 17:20 +0200
  [PATCH v6 16/20] IB/fmr_pool: Convert the cleanup thread into kthread worker API Petr Mladek <pmladek@suse.com> - 2016-04-14 17:20 +0200
  [PATCH v6 12/20] ring_buffer: Convert benchmark kthreads into kthread worker API Petr Mladek <pmladek@suse.com> - 2016-04-14 17:20 +0200
  [PATCH v6 20/20] thermal/intel_powerclamp: Convert the kthread to kthread worker API Petr Mladek <pmladek@suse.com> - 2016-04-14 17:20 +0200
  [PATCH v6 02/20] kthread: Allow to call __kthread_create_on_node() with va_list args Petr Mladek <pmladek@suse.com> - 2016-04-14 17:20 +0200
  [PATCH v6 03/20] kthread: Add create_kthread_worker*() Petr Mladek <pmladek@suse.com> - 2016-04-14 17:20 +0200
  [PATCH v6 11/20] mm/huge_page: Convert khugepaged() into kthread worker API Petr Mladek <pmladek@suse.com> - 2016-04-14 17:20 +0200
  [PATCH v6 17/20] memstick/r592: Better synchronize debug messages in r592_io kthread Petr Mladek <pmladek@suse.com> - 2016-04-14 17:20 +0200
  [PATCH v6 05/20] kthread: Add destroy_kthread_worker() Petr Mladek <pmladek@suse.com> - 2016-04-14 17:20 +0200
  [PATCH v6 13/20] hung_task: Convert hungtaskd into kthread worker API Petr Mladek <pmladek@suse.com> - 2016-04-14 17:20 +0200
  [PATCH v6 01/20] kthread/smpboot: Do not park in kthread_create_on_cpu() Petr Mladek <pmladek@suse.com> - 2016-04-14 17:30 +0200
  [PATCH v6 06/20] kthread: Detect when a kthread work is used by more workers Petr Mladek <pmladek@suse.com> - 2016-04-14 17:30 +0200
  [PATCH v6 08/20] kthread: Allow to cancel kthread work Petr Mladek <pmladek@suse.com> - 2016-04-14 17:30 +0200
  [PATCH v6 04/20] kthread: Add drain_kthread_worker() Petr Mladek <pmladek@suse.com> - 2016-04-14 17:30 +0200

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