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

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Message-Id: <20140205.162422.857987635200528036.davem@redhat.com>
Date:	Wed, 05 Feb 2014 16:24:22 -0800 (PST)
From:	David Miller <davem@...hat.com>
To:	zoltan.kiss@...rix.com
Cc:	ian.campbell@...rix.com, wei.liu2@...rix.com,
	xen-devel@...ts.xenproject.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, jonathan.davies@...rix.com
Subject: Re: [PATCH net v3] xen-netback: Fix Rx stall due to race condition 

From: Zoltan Kiss <zoltan.kiss@...rix.com>
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@...rix.com>
> Reviewed-by: Paul Durrant <paul.durrant@...rix.com>
> Acked-by: Wei Liu <wei.liu2@...rix.com>

Applied, thanks.
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