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

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Date:	Tue, 03 Nov 2015 10:18:02 -0500
From:	Jarod Wilson <jarod@...hat.com>
To:	Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
CC:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	"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

Nikolay Aleksandrov wrote:
> 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.

Just posted a follow-up that removes the macro-internal use of bit and 
doesn't botch up assigning feature. It's not as pretty, but it works 
correctly with multiple feature bits.

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


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