Re: [iptables PATCH] list: fix prefetch dummy

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Wed, Apr 08, 2015 at 05:18:25PM -0700, Stephen Hemminger wrote:
> On Mon, 06 Apr 2015 20:05:41 +0200
> Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com> wrote:
> 
> > linux_list.h:381:59: warning: right-hand operand of comma expression has no effect [-Wunused-value]
> >   for (pos = list_entry((head)->next, typeof(*pos), member), \
> >                                                            ^
> > libiptc.c:552:2: note: in expansion of macro 'list_for_each_entry'
> >   list_for_each_entry(c, &h->chains, list) {
> >   ^
> > 
> > [ Patch copied from one similar of Patrick McHardy on libnftnl ]
> > 
> > Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
> > ---
> >  libiptc/linux_list.h |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/libiptc/linux_list.h b/libiptc/linux_list.h
> > index abdcf88..559e33c 100644
> > --- a/libiptc/linux_list.h
> > +++ b/libiptc/linux_list.h
> > @@ -27,7 +27,7 @@
> >  	1; \
> >  })
> >  
> > -#define prefetch(x)		1
> > +#define prefetch(x)		((void)0)
> >  
> >  /* empty define to make this work in userspace -HW */
> >  #define smp_wmb()
> > 
> > --
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> 
> The kernel dropped the prefetch() in the list macros a couple
> of years ago. The prefetch() had no performance gain and hurt
> in several cases.

We have an old copy of linux's list.h in our userspace trees. We can
probably refresh those to get them in sync with current at some point.