netdev - Re: [Patch net-next v3] net: clean up skb headers code

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Message-ID: <20130531001841.GB26684@verge.net.au>
Date:	Fri, 31 May 2013 09:18:41 +0900
From:	Simon Horman <horms@...ge.net.au>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	David Miller <davem@...emloft.net>, David.Laight@...LAB.COM,
	amwang@...hat.com, netdev@...r.kernel.org
Subject: Re: [Patch net-next v3] net: clean up skb headers code

On Thu, May 30, 2013 at 10:05:18PM +0100, Ben Hutchings wrote:
> On Thu, 2013-05-30 at 13:16 -0700, David Miller wrote:
> > From: "David Laight" <David.Laight@...LAB.COM>
> > Date: Thu, 30 May 2013 12:18:13 +0100
> > 
> > >> commit 1a37e412a0225fcba5587 (net: Use 16bits for *_headers
> > >> fields of struct skbuff) converts skb->*_header to u16,
> > >> some #if NET_SKBUFF_DATA_USES_OFFSET are now useless,
> > >> and to be safe, we could just use "X = ~(typeof(X))0;"
> > >> as suggested by David and Ben.
> > > 
> > > IIRC ~(unsigned short)0 is the same as ~0 and will be -1.
> > > define an actual constant with value 0xffff
> > 
> > The problem with doing that is that if this type is changed again,
> > we'lll go through this same circus changing the constant everywhere
> > and missing some cases.
> > 
> > I absolutely want to see a version of this fix that makes sure that if
> > the type changes, either the constantly automatically change (that's
> > the "typeof(X)" approach) or the build breaks spectacularly until the
> > value is fixed up.
> 
> Let's just give the 'invalid' value a name and define it right next to
> those fields.  It might be worth reintroducing sk_buff_data_t too, since
> there was at least one place which wanted to save and restore offsets in
> local variables.

sk_buff_data_t was not removed, its just used for less fields.
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