netdev - Re: [PATCH] net: force dst_default

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Message-ID: <1344379474.2688.89.camel@bwh-desktop.uk.solarflarecom.com>
Date:	Tue, 7 Aug 2012 23:44:34 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: force dst_default_metrics to const section

On Wed, 2012-08-08 at 00:34 +0200, Eric Dumazet wrote:
> On Tue, 2012-08-07 at 23:12 +0100, Ben Hutchings wrote:
> > On Tue, 2012-08-07 at 22:55 +0200, Eric Dumazet wrote:
> > > From: Eric Dumazet <edumazet@...gle.com>
> > > 
> > > 
> > > > Some day the compiler may be smart enough to ignore the different
> > > > between explicit and implicit zero-initialisation, and put it back in
> > > > BSS.  Declaring this __cache_aligned_in_smp might be a better option.
> > > 
> > > __cache_aligned_in_smp aligns start of the structure, but can be
> > > followed by another var in same cache line. Yes, this is bad.
> > 
> > Oh, that's unexpected.
> > 
> > > By the way we dont care of cache alignment on this structure, only it
> > > should be const. Its a soft requirement, machine wont crash if it is not
> > > the case.
> > 
> > Right.
> > 
> > > If compiler is smart one day as you say (it should first be non buggy
> > > IMHO), then we can add a non zero field like this :
> > [...]
> > 
> > That would work, but it's ugly!  How about defining and using a
> > meaningfully-named macro that expands to __section(.rodata)?
> 
> You are kidding. I prefer plain C and not having to mess with all
> arches.

Any consideration of implementation details like BSS and cache line
sharing is already outside of 'plain C'.  And you don't have to 'mess
with all arches'; just look at what <linux/init.h> and <linux/module.h>
do.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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