netdev - Re: [RFC] myri10ge: small rx

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Date:	Thu, 24 Mar 2011 09:16:21 +0100
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	netdev@...r.kernel.org, Andrew Gallatin <gallatin@...i.com>,
	Brice Goglin <brice@...i.com>
Subject: Re: [RFC] myri10ge: small rx_done refactoring

On Wed, Mar 23, 2011 at 08:33:57AM -0700, Stephen Hemminger wrote:
> On Wed, 23 Mar 2011 13:52:04 +0100
> Stanislaw Gruszka <sgruszka@...hat.com> wrote:
> 
> > Add lro_enable variable to read NETIF_F_LRO flag only once per napi poll
> > call. This should fix theoretical race condition with
> > myri10ge_set_rx_csum() and myri10ge_set_flags() where flag NETIF_F_LRO
> > can be changed.
> 
> You may need a barrier or the race may still be there.

I don't understand why barrier in that case is need.

What I tried to avoid is.

myri10ge_clean_rx_done():

if (dev->features & NETIF_F_LRO) 
	setup lro 
					myri10ge_set_flags()

if (dev->features & NETIF_F_LRO)
	flush lro

Now we read dev->features & NETIF_F_LRO only once to local
lro_enabled variable. So we can not flush without setup
or setup without flush. No idea why memory barries is still
needed.

> The driver seems to use mb() where wmb() is intended, and never use rmb()?

Yes, I think we can have some optimalization here.

Stanislaw
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