From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jarod Wilson <jarod@redhat.com>
Subject: Re: [PATCH v2 net-next] net/core: generic support for disabling netdev
 features down stack
Date: Tue, 03 Nov 2015 10:18:02 -0500
Message-ID: <5638D02A.8030403@redhat.com>
References: <1446486818-26166-1-git-send-email-jarod@redhat.com> <1446519359-21400-1-git-send-email-jarod@redhat.com> <5638865D.2000804@cumulusnetworks.com> <CAMuHMdXrs2fkHw+DGgutM1y71Y=MTsxSqJ+J78e_Q7oZjaNesA@mail.gmail.com> <5638BD2E.8010508@redhat.com> <5638BF29.7030601@cumulusnetworks.com>
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Cc: Geert Uytterhoeven <geert@linux-m68k.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jay Vosburgh <j.vosburgh@gmail.com>,
	Veaceslav Falico <vfalico@gmail.com>,
	Andy Gospodarek <gospo@cumulusnetworks.com>,
	Jiri Pirko <jiri@resnulli.us>,
	Nikolay Aleksandrov <razor@blackwall.org>,
	Michal Kubecek <mkubecek@suse.cz>,
	Alexander Duyck <alexander.duyck@gmail.com>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>
To: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <5638BF29.7030601@cumulusnetworks.com>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: netdev.vger.kernel.org

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@cumulusnetworks.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@redhat.com