netdev - Re: [Patch 2/2] mlx4: add dynamic LRO disable support

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Message-Id: <1275661552.2095.13.camel@achroite.uk.solarflarecom.com>
Date:	Fri, 04 Jun 2010 15:25:52 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Cong Wang <amwang@...hat.com>
Cc:	netdev@...r.kernel.org, herbert.xu@...hat.com, nhorman@...hat.com,
	sgruszka@...hat.com, davem@...emloft.net
Subject: Re: [Patch 2/2] mlx4: add dynamic LRO disable support

On Fri, 2010-06-04 at 09:56 +0800, Cong Wang wrote:
> On 06/03/10 20:37, Ben Hutchings wrote:
> > On Wed, 2010-06-02 at 23:39 -0400, Amerigo Wang wrote:
> >> This patch adds dynamic LRO diable support for mlx4 net driver.
> >> It also fixes a bug of mlx4, which checks NETIF_F_LRO flag in rx
> >> path without rtnl lock.
> > [...]
> >
> > Is that flag test actually unsafe - and if so, how is testing num_lro
> > any better?  Perhaps access to net_device::features should be wrapped
> > with ACCESS_ONCE() to ensure that reads and writes are atomic.
> >
> 
> At least, I don't find there is any race with 'num_lro', thus
> no lock is needed.

In both cases there is a race condition but it is harmless so long as
the read and the write are atomic.  There is a general assumption in
networking code that this is the case for int and long.  Personally I
would prefer to see this made explicit using ACCESS_ONCE(), but I don't
see any specific problem in mlx4 (not that I'm familiar with this driver
either).

Now that I look at the patch again, I see you're using a static (i.e.
global) variable to 'back up' the non-zero (enabled) value of num_lro.
This is introducing a bug!  The correct value is apparently set in
mlx4_en_get_profile(); you would need to replicate that.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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