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Re: [Xen-devel] [PATCHv1 net] xen-netfront: use napi_complete() correctly to prevent Rx stalling

From: David Vrabel <david.vrabel@xxxxxxxxxx>
Date: Tue, 16 Dec 2014 18:59:46 +0000

> After d75b1ade567ffab085e8adbbdacf0092d10cd09c (net: less interrupt
> masking in NAPI) the napi instance is removed from the per-cpu list
> prior to calling the n->poll(), and is only requeued if all of the
> budget was used.  This inadvertently broke netfront because netfront
> does not use NAPI correctly.
> If netfront had not used all of its budget it would do a final check
> for any Rx responses and avoid calling napi_complete() if there were
> more responses.  It would still return under budget so it would never
> be rescheduled.  The final check would also not re-enable the Rx
> interrupt.
> Additionally, xenvif_poll() would also 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 both of these by always calling napi_compete() if the budget was
> not all used, and then calling napi_schedule() if the final checks
> says there's more work.
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>

Applied, thanks.

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