Re: [Xen-devel] [PATCH net v2 2/3] xen-netback: don't stop dealloc kthread too early

On 11/08/14 15:31, Wei Liu wrote:

On Mon, Aug 11, 2014 at 02:58:13PM +0100, David Vrabel wrote:

quoted

On 11/08/14 14:48, Wei Liu wrote:

quoted

On Mon, Aug 11, 2014 at 01:10:12PM +0100, David Vrabel wrote:

quoted

On 08/08/14 17:37, Wei Liu wrote:

[...]

quoted

quoted

 	if (tx_evtchn == rx_evtchn) {
 		/* feature-split-event-channels == 0 */

@@ -687,6 +700,9 @@ void xenvif_disconnect(struct xenvif *vif)
 			queue->task = NULL;
 		}
 
+		wait_event(queue->inflight_wq,
+			   atomic_read(&queue->inflight_packets) == 0);

Just make the dealloc task not stop unless it has deallocated all
outstanding requests.  There's no need for another wait queue here.

Are you suggesting something like

  while (atomic_read(&queue->inflight_packets) !=0)
  	schedule_timeout(SOME_TIMEOUT);

No. That would be awful!

Add the test to the kthread itself:

int xenvif_dealloc_kthread(void *data)
{
	struct xenvif_queue *queue = data;

	while (atomic_read(&queue->inflight_packets) > 0
               || !kthread_should_stop()) {
            [...]

etc.

Although, the main loop is a bit confused, so I suggest adding:

static bool xenvif_dealloc_thread_should_stop(struct xenvif_queue *q)
{
    /* Dealloc thread must remain running if there are any inflight
     * packets so it can properly dealloc them when they complete.
     */
    return atomic_read(&queue->inflight_packets) == 0
        && kthread_should_stop();
}

And cleaning it up a bit (the while() could be a for(;;)).

Unfortunately this approach is bogus. If xenbus thread is not blocked it
can free up various resources while dealloc thread is running -- queue
can be gone under dealloc thread's feet.

kthread_stop() waits until the thread exits (like pthread_join()).

/**
 * kthread_stop - stop a thread created by kthread_create().
 * @k: thread created by kthread_create().
 *
 * Sets kthread_should_stop() for @k to return true, wakes it, and
 * waits for it to exit.

David