netdev - Re: [PATCH] ipv6: Fix preferred_lft not updating in some cases

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Message-ID: <CAHaKRvJDZJjuv4sALmQAotk5EUMfYPiLN=8_noWCRQYOW+bxSA@mail.gmail.com>
Date:	Fri, 27 Sep 2013 13:28:06 -0700
From:	Paul Marks <pmarks@...gle.com>
To:	Paul Marks <pmarks@...gle.com>, netdev@...r.kernel.org,
	davem@...emloft.net, yoshfuji@...ux-ipv6.org,
	Lorenzo Colitti <lorenzo@...gle.com>
Subject: Re: [PATCH] ipv6: Fix preferred_lft not updating in some cases

On Fri, Sep 27, 2013 at 1:16 AM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> On Wed, Sep 25, 2013 at 03:12:55PM -0700, Paul Marks wrote:
>> -                                     if (prefered_lft != ifp->prefered_lft) {
>
> Wouldn't the easiest solution be to just drop this if and execute the two
> lines below unconditionally?

Yes, that's also correct.  But is it not better to have simpler code
than shorter diffs?  Should we transliterate English to C, or think
about what the algorithm is actually doing?  The fact that this bug
has gone unnoticed provides some evidence that the code may have been
too complicated.

>> +                             const u32 minimum_lft = min(
>> +                                     stored_lft, (u32)MIN_VALID_LIFETIME);
>> +                             valid_lft = max(valid_lft, minimum_lft);
>
> Quick question: Don't we need a prefered_lft = min(preferred_lft, valid_lft)
> here?

The invariant is (preferred_lft <= valid_lft), and valid_lft can only
get bigger, so I don't think there's a problem.
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