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

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Date:	Tue, 3 Nov 2015 15:05:29 +0100
From:	Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:	Jarod Wilson <jarod@...hat.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	"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

On 11/03/2015 02:57 PM, Jarod Wilson wrote:
> 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.
> 

Yeah, sorry for not elaborating, I wrote it in a hurry. :-)
Thanks Geert!

By the way since you'll be changing this code, I don't know if it's okay to
declare caller-visible hidden local variables in a macro like this, at the very
least please consider renaming it to something that's much less common, I can see
"bit" being used here and there. IMO either try to find a way to avoid it
altogether or add another argument to the macro so it's explicitly passed.

Cheers,
 Nik


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