netdev - Re: [net-next-2.6 PATCH] ixgbe: prevent speculative processing of descriptors before ready

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Message-Id: <20100222.154913.144379311.davem@davemloft.net>
Date:	Mon, 22 Feb 2010 15:49:13 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	jeffrey.t.kirsher@...el.com
Cc:	netdev@...r.kernel.org, gospo@...hat.com, miltonm@....com,
	anton@...ba.org, donald.c.skidmore@...el.com
Subject: Re: [net-next-2.6 PATCH] ixgbe: prevent speculative processing of
 descriptors before ready

From: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Date: Fri, 19 Feb 2010 19:44:42 -0800

> From: Milton Miller <miltonm@....com>
> 
> The PowerPC architecture does not require loads to independent bytes to be
> ordered without adding an explicit barrier.
> 
> In ixgbe_clean_rx_irq we load the status bit then load the packet data.
> With packet split disabled if these loads go out of order we get a
> stale packet, but we will notice the bad sequence numbers and drop it.
> 
> The problem occurs with packet split enabled where the TCP/IP header and data
> are in different descriptors. If the reads go out of order we may have data
> that doesn't match the TCP/IP header. Since we use hardware checksumming this
> bad data is never verified and it makes it all the way to the application.
> 
> This bug was found during stress testing and adding this barrier has been shown
> to fix it.
> 
> Signed-off-by: Milton Miller <miltonm@....com>
> Signed-off-by: Anton Blanchard <anton@...ba.org>
> Acked-by: Don Skidmore <donald.c.skidmore@...el.com>
> Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@...el.com>

Applied.
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