netdev - Re: [PATCH 1/4] [RFC] net: Explicitly initialize u64_stats

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Date:	Fri, 27 Sep 2013 07:44:55 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	John Stultz <john.stultz@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Mirko Lindner <mlindner@...vell.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Roger Luethi <rl@...lgate.ch>,
	Patrick McHardy <kaber@...sh.net>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Wensong Zhang <wensong@...ux-vs.org>,
	Simon Horman <horms@...ge.net.au>,
	Julian Anastasov <ja@....bg>, Jesse Gross <jesse@...ira.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: [PATCH 1/4] [RFC] net: Explicitly initialize u64_stats_sync
 structures for lockdep


* Eric Dumazet <eric.dumazet@...il.com> wrote:

> On Thu, 2013-09-26 at 11:34 -0700, John Stultz wrote:
> > In order to enable lockdep on seqcount/seqlock structures, we
> > must explicitly initialize any locks.
> > 
> 
> > diff --git a/include/linux/u64_stats_sync.h b/include/linux/u64_stats_sync.h
> > index 8da8c4e..c450e11 100644
> > --- a/include/linux/u64_stats_sync.h
> > +++ b/include/linux/u64_stats_sync.h
> > @@ -67,6 +67,13 @@ struct u64_stats_sync {
> >  #endif
> >  };
> >  
> > +
> > +#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
> > +#define u64_stats_init(syncp)	seqcount_init(syncp.seq)
> > +#else
> > +#define u64_stats_init(syncp)
> > +#endif
> > +
> 
> I would prefer a function.

C cannot pass along symbolic names, unfortunately, so we are stuck with 
1970's tech and the C preprocessor.

There's a way to make such macros look a tiny bit more structured and thus 
be more palatable:

#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
# define u64_stats_init(syncp) seqcount_init(syncp.seq)
#else
# define u64_stats_init(syncp)
#endif

Note, the 'else' branch should probably be:

# define u64_stats_init(syncp) do { } while (0)

Thanks,

	Ingo
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