netdev - Re: [RFC] myri10ge: small rx

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Date:	Thu, 24 Mar 2011 16:59:37 +0100
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Stephen Hemminger <shemminger@...tta.com>,
	David Howells <dhowells@...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, Mar 24, 2011 at 08:15:39AM -0700, Stephen Hemminger 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.

Ok, so C code like:

code1
if (dev->features & NETIF_F_LRO) 
	branch1
code2;
if (dev->features & NETIF_F_LRO)
	branch2

and

bool lro_enabled = dev->features & NETIF_F_LRO;
code1
if (lro_enabled)
	branch1
code2
if (lro_enabled)
	branch2

can give the same assembly output.

It's really hard for me to understand that. I could
understand, if we would get global variable directly
like:
bool lro_enabled = dev->lro_enabled;
instead of:
bool lro_enabled = dev->features & NETIF_F_LRO;

David, can you confirm that Staphen is correct?

Also where this barrier() should go. Before  
"bool lro_enabled = dev->features & NETIF_F_LRO;"
or after?

Stanislaw
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