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

[Jarod Wilson]

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@xxxxxxxxxxxxxxxxxxx>   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@xxxxxxxxxx


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/