[Xen-devel] [PATCH v2] xen/netback: Wake dealloc thread after completing zerocopy work

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
Changed in V2: Move wakeup into zerocopy_complete function.

 drivers/net/xen-netback/interface.c | 6 ++++++
 drivers/net/xen-netback/netback.c   | 1 -
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/net/xen-netback/interface.c 
b/drivers/net/xen-netback/interface.c
index 1a83e19..28577a3 100644
--- a/drivers/net/xen-netback/interface.c
+++ b/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)
diff --git a/drivers/net/xen-netback/netback.c 
b/drivers/net/xen-netback/netback.c
index 7d50711..09ffda4 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -1536,7 +1536,6 @@ 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))
-- 
2.1.0


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