Re: [PATCH net v3] xen-netback: Fix Rx stall due to race condition

From: David Miller
Date: Wed Feb 05 2014 - 19:24:37 EST

From: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
Date: Tue, 4 Feb 2014 19:54:37 +0000

> The recent patch to fix receive side flow control
> (11b57f90257c1d6a91cee720151b69e0c2020cf6: xen-netback: stop vif thread
> spinning if frontend is unresponsive) solved the spinning thread problem,
> however caused an another one. The receive side can stall, if:
> - [THREAD] xenvif_rx_action sets rx_queue_stopped to true
> - [INTERRUPT] interrupt happens, and sets rx_event to true
> - [THREAD] then xenvif_kthread sets rx_event to false
> - [THREAD] rx_work_todo doesn't return true anymore
> Also, if interrupt sent but there is still no room in the ring, it take quite a
> long time until xenvif_rx_action realize it. This patch ditch that two variable,
> and rework rx_work_todo. If the thread finds it can't fit more skb's into the
> ring, it saves the last slot estimation into rx_last_skb_slots, otherwise it's
> kept as 0. Then rx_work_todo will check if:
> - there is something to send to the ring (like before)
> - there is space for the topmost packet in the queue
> I think that's more natural and optimal thing to test than two bool which are
> set somewhere else.
> Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
> Reviewed-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

Applied, thanks.
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