xen/netback: Wake dealloc thread after completing zerocopy work

Waking the dealloc thread before decrementing inflight_packets is racy
because it means the thread may go to sleep before inflight_packets is
decremented. If kthread_stop() has already been called, the dealloc
thread may wait forever with nothing to wake it. Instead, wake the
thread only after decrementing inflight_packets.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/interface.c

@@ -61,6 +61,12 @@ void xenvif_skb_zerocopy_prepare(struct xenvif_queue *queue,
 void xenvif_skb_zerocopy_complete(struct xenvif_queue *queue)
 {
 	atomic_dec(&queue->inflight_packets);
+
+	/* Wake the dealloc thread _after_ decrementing inflight_packets so
+	 * that if kthread_stop() has already been called, the dealloc thread
+	 * does not wait forever with nothing to wake it.
+	 */
+	wake_up(&queue->dealloc_wq);
 }
 
 int xenvif_schedulable(struct xenvif *vif)

drivers/net/xen-netback/netback.c

@@ -1541,7 +1541,6 @@ void xenvif_zerocopy_callback(struct ubuf_info *ubuf, bool zerocopy_success)
 		smp_wmb();
 		queue->dealloc_prod++;
 	} while (ubuf);
-	wake_up(&queue->dealloc_wq);
 	spin_unlock_irqrestore(&queue->callback_lock, flags);
 
 	if (likely(zerocopy_success))