Re: [PATCH v3 07/22] kthread: Detect when a kthread work is used by more workers

On Tue 2015-11-24 09:49:42, Tejun Heo wrote:

Hello, Petr.

On Tue, Nov 24, 2015 at 11:06:50AM +0100, Petr Mladek wrote:

quoted

quoted

quoted

@@ -610,6 +625,12 @@ repeat:
 	if (work) {
 		__set_current_state(TASK_RUNNING);
 		work->func(work);
+
+		spin_lock_irq(&worker->lock);
+		/* Allow to queue the work into another worker */
+		if (!kthread_work_pending(work))
+			work->worker = NULL;
+		spin_unlock_irq(&worker->lock);

Doesn't this mean that the work item can't be freed from its callback?
That pattern tends to happen regularly.

I am not sure if I understand your question. Do you mean switching
work->func during the life time of the struct kthread_work? This
should not be affected by the above code.

IOW, you can't expect the work
item to remain accessible once the work function starts executing.

I see, I was not aware of this pattern.

quoted

The above code allows to queue an _unused_ kthread_work into any
kthread_worker. For example, it is needed for khugepaged,
see http://marc.info/?l=linux-kernel&m=144785344924871&w=2
The work is static but the worker can be started/stopped
(allocated/freed) repeatedly. It means that the work need
to be usable with many workers. But it is associated only
with one worker when being used.

It can just re-init work items when it restarts workers, right?

Yes, this would work. It might be slightly inconvenient but
it looks like a good compromise. It helps to keep the API
implementation rather simple and rather secure.

Alternatively, we could allow to queue the work on another worker
if it is not pending. But then we would need to check the pending
status without the worker->lock because work->worker might point
to an already freed worker. We need to check the pending
status in many situations. It might open a can of worms that
I probably do not want to catch.

Thank you and PeterZ for explanation,
Petr

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>