netdev - Re: [PATCH net] amd-xgbe: Use wmb before updating current descriptor count

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Message-Id: <20151023.025943.873672852211823864.davem@davemloft.net>
Date:	Fri, 23 Oct 2015 02:59:43 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	thomas.lendacky@....com
Cc:	netdev@...r.kernel.org, christoffer.dall@...aro.org
Subject: Re: [PATCH net] amd-xgbe: Use wmb before updating current
 descriptor count

From: Tom Lendacky <thomas.lendacky@....com>
Date: Wed, 21 Oct 2015 15:37:05 -0500

> The code currently uses the lightweight dma_wmb barrier before updating
> the current descriptor count. Under heavy load, the Tx cleanup routine
> was seeing the updated current descriptor count before the updated
> descriptor information. As a result, the Tx descriptor was being cleaned
> up before it was used because it was not "owned" by the hardware yet,
> resulting in a Tx queue hang.
> 
> Using the wmb barrier insures that the descriptor is updated before the
> descriptor counter preventing the Tx queue hang. For extra insurance,
> the Tx cleanup routine is changed to grab the current decriptor count on
> entry and uses that initial value in the processing loop rather than
> trying to chase the current value.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
> Tested-by: Christoffer Dall <christoffer.dall@...aro.org>

Applied, thanks.
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