On Thu 2016-02-25 13:35:51, Peter Zijlstra wrote:

On Mon, Feb 22, 2016 at 03:56:54PM +0100, Petr Mladek wrote:

quoted

+/**
+ * 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);

Would it not make sense to set a flag here that inhibits (or warns)
queueing new work?

Otherwise this can, as you point out, last forever.

And I think its a logic fail if you both want to drain it and keeping
adding new work.

We must allow self-queuing because it might be needed to finish
the processing. We would need to detect it. Tejun suggested
to avoid this and make the code simple.

I do not have a strong opinion here. On one hand, such a check might
help with debugging. On the other hand, workqueues have happily lived
without it for years.

Thanks a lot for review,
Petr

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