Subject: Re: [PATCH] xen-netback: fix race between napi_complete() and interrupt handler

From: Zoltan Kiss <[email-masked]>
Date: Tue, 25 Mar 2014 14:41:58 +0000

My idea was that the current code can't race with interrupt running on a 
different CPU, because if the interrupt was moved since the last 
napi_schedule (which scheduled NAPI on the same CPU as the interrupt), 
the kernel would make sure that the NAPI instance is moved along with 
it. However I couldn't find any trace of this in the kernel so far, but 
the current code actually works for me, even when I used a bash script 
to aggressively move the interrupts around while running.
I've added David and Eric to the mailing, maybe they can quickly shed 
some light on this: how does the kernel make sure that if the interrupt 
is moved away from a CPU (e.g. by irqbalance), the NAPI instance already 
scheduled there won't race with it?

Zoli

On 25/03/14 14:08, David Vrabel wrote:
> When the NAPI budget was not all used, xenvif_poll() would call
> napi_complete() /after/ enabling the interrupt.  This resulted in a
> race between the napi_complete() and the napi_schedule() in the
> interrupt handler.  The use of local_irq_save/restore() avoided by
> race iff the handler is running on the same CPU but not if it was
> running on a different CPU.
>
> Fix this properly by calling napi_complete() before reenabling
> interrupts (in the xenvif_check_rx_xenvif() call).
>
> Signed-off-by: David Vrabel <[email-masked]>
> ---
>   drivers/net/xen-netback/interface.c |   28 ++--------------------------
>   1 files changed, 2 insertions(+), 26 deletions(-)
>
> diff --git a/drivers/net/xen-netback/interface.c b/drivers/net/xen-netback/interface.c
> index 7669d49..ee322d9 100644
> --- a/drivers/net/xen-netback/interface.c
> +++ b/drivers/net/xen-netback/interface.c
> @@ -65,32 +65,8 @@ static int xenvif_poll(struct napi_struct *napi, int budget)
>   	work_done = xenvif_tx_action(vif, budget);
>
>   	if (work_done < budget) {
> -		int more_to_do = 0;
> -		unsigned long flags;
> -
> -		/* It is necessary to disable IRQ before calling
> -		 * RING_HAS_UNCONSUMED_REQUESTS. Otherwise we might
> -		 * lose event from the frontend.
> -		 *
> -		 * Consider:
> -		 *   RING_HAS_UNCONSUMED_REQUESTS
> -		 *   <frontend generates event to trigger napi_schedule>
> -		 *   __napi_complete
> -		 *
> -		 * This handler is still in scheduled state so the
> -		 * event has no effect at all. After __napi_complete
> -		 * this handler is descheduled and cannot get
> -		 * scheduled again. We lose event in this case and the ring
> -		 * will be completely stalled.
> -		 */
> -
> -		local_irq_save(flags);
> -
> -		RING_FINAL_CHECK_FOR_REQUESTS(&vif->tx, more_to_do);
> -		if (!more_to_do)
> -			__napi_complete(napi);
> -
> -		local_irq_restore(flags);
> +		napi_complete(napi);
> +		xenvif_check_rx_xenvif(vif);
>   	}
>
>   	return work_done;
>