netdev - Re: [PATCH v2 net-next] net/core: generic support for disabling netdev features down stack

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Date:	Tue, 03 Nov 2015 08:57:02 -0500
From:	Jarod Wilson <jarod@...hat.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
CC:	Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jay Vosburgh <j.vosburgh@...il.com>,
	Veaceslav Falico <vfalico@...il.com>,
	Andy Gospodarek <gospo@...ulusnetworks.com>,
	Jiri Pirko <jiri@...nulli.us>,
	Nikolay Aleksandrov <razor@...ckwall.org>,
	Michal Kubecek <mkubecek@...e.cz>,
	Alexander Duyck <alexander.duyck@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 net-next] net/core: generic support for disabling netdev
 features down stack

Geert Uytterhoeven wrote:
> On Tue, Nov 3, 2015 at 11:03 AM, Nikolay Aleksandrov
> <nikolay@...ulusnetworks.com>  wrote:
>> On 11/03/2015 03:55 AM, Jarod Wilson wrote:
>> [snip]
>>> +#define for_each_netdev_feature(mask_addr, feature)                          \
>>> +     int bit;                                                                \
>>> +     for_each_set_bit(bit, (unsigned long *)mask_addr, NETDEV_FEATURE_COUNT) \
>>> +             feature = __NETIF_F_BIT(bit);
>>> +
>> ^
>> This is broken, it will not work for more than a single feature.
>
> Indeed it is.
>
> This is used as:
>
>          for_each_netdev_feature(&upper_disables, feature) {
>          ...
>          }
>
> which expands to:
>
>          int bit;
>          for_each_set_bit(bit, (unsigned long *)mask_addr, NETDEV_FEATURE_COUNT)
>                  feature = __NETIF_F_BIT(bit);
>          {
>                  ...
>          }
>
> Note the assignment to "feature" happens outside the {}-delimited block.
> And the block is always executed once.

Bah, crap, I was still staring at the code not seeing it, thank you for 
the detailed cluebat. I'll fix that up right now.

-- 
Jarod Wilson
jarod@...hat.com


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