netdev - Re: [PATCH] sky2: Avoid race in sky2_change

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Date:	Mon, 3 May 2010 16:26:34 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Mike McCormack <mikem@...g3k.org>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] sky2: Avoid race in sky2_change_mtu

On Mon, 03 May 2010 23:18:56 +0900
Mike McCormack <mikem@...g3k.org> wrote:

> netif_stop_queue does not ensure all in-progress transmits are complete,
>  so use netif_tx_disable() instead.
> 
> Make sure NAPI polls are disabled, otherwise NAPI might trigger a TX
>  restart between when we stop the queue and NAPI is disabled.

How could NAPI trigger a TX restart? Restart is a timer, not controlled
by NAPI. There is a different race that your patch isn't covering.

If a large MTU send is in progress while mtu is changing, the changes
to the GMAC store and forward (sky2_set_tx_stfwd) are not synchronized
with hardware queued transmits.

So I think a there needs to be another spinloop.

--- a/drivers/net/sky2.c	2010-05-03 16:19:58.828106182 -0700
+++ b/drivers/net/sky2.c	2010-05-03 16:24:37.850232475 -0700
@@ -2274,8 +2274,12 @@ static int sky2_change_mtu(struct net_de
 	imask = sky2_read32(hw, B0_IMSK);
 	sky2_write32(hw, B0_IMSK, 0);
 
-	dev->trans_start = jiffies;	/* prevent tx timeout */
 	netif_stop_queue(dev);
+
+	/* Wait for ongoing Tx to complete */
+	while (sky2->tx_cons != sky2->tx_prod)
+		udelay(10);
+
 	napi_disable(&hw->napi);
 
 	synchronize_irq(hw->pdev->irq);
--
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