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

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Message-ID: <20100609104950.GB2599@dhcp-lab-161.englab.brq.redhat.com>
Date:	Wed, 9 Jun 2010 12:49:50 +0200
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Cong Wang <amwang@...hat.com>
Cc:	Ben Hutchings <bhutchings@...arflare.com>, netdev@...r.kernel.org,
	herbert.xu@...hat.com, nhorman@...hat.com, davem@...emloft.net
Subject: Re: [Patch 2/2] mlx4: add dynamic LRO disable support

Hi Amerigo

Sorry for being silent in this thread before.

On Wed, Jun 09, 2010 at 05:23:35PM +0800, Cong Wang wrote:
>>>> 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).
>
> I read this email again.
>
> I think you misunderstood the race condition here. Even read and write
> are atomic here, the race still exists. One can just set NETIF_F_LRO
> asynchronously right before mlx4 check this flag in mlx4_en_process_rx_cq()
> which doesn't take rtnl_lock.

If so, it's better to stop device before modify LRO settings. I suggest
something like that in mlx4_ethtool_op_set_flags:

if (!!(data & ETH_FLAG_LRO) != !!(dev->features & NETIF_F_LRO)) {
	/* Need to toggle LRO */

	if (netdev_running(dev)) {
               mutex_lock(&mdev->state_lock);
               mlx4_en_stop_port(dev);
               rc = mlx4_en_start_port(dev);
               if (rc)
                       en_err(priv, "Failed to restart port\n");
	}

	dev->features ^= NETIF_F_LRO;

	if (netdev_running(dev))
               mutex_unlock(&mdev->state_lock);
}

Stanislaw
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