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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Aug 04, 2015 at 03:40:59PM +0100, Ross Lagerwall wrote:
> 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>

Acked-by: Wei Liu <wei.liu2@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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