netdev - Re: [RFC] myri10ge: small rx

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Message-ID: <20110324081539.47ad0972@nehalam>
Date:	Thu, 24 Mar 2011 08:15:39 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Stanislaw Gruszka <sgruszka@...hat.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 Thu, 24 Mar 2011 09:16:21 +0100
Stanislaw Gruszka <sgruszka@...hat.com> wrote:

> 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.
> 

Without barrier there is no guarantee that compiler read the flags
into a local variable. It is free to do the same thing as the original
code.

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