RE: [RFC] csum experts, csum_replace2() is too expensive

[David Laight]

From: Eric Dumazet
> On Fri, 2014-03-21 at 14:52 -0400, David Miller wrote:
> > From: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> > Date: Fri, 21 Mar 2014 05:50:50 -0700
> >
> > > It looks like a barrier() would be more appropriate.
> >
> > barrier() == __asm__ __volatile__(:::"memory")
> 
> Indeed, but now you mention it, ip_fast_csum() do not uses volatile
> keyword on x86_64, and has no "m" constraint either.

Adding 'volatile' isn't sufficient to force gcc to write data
into the area being checksummed.
ip_fast_csum() either needs an explicit "m" constraint for the actual
buffer (and target) bytes, or the stronger "memory" constraint.
The 'volatile' is then not needed.

	David