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Subject: Re: [PATCH 1/5] user_hooks: New user hooks subsystem
From: Steven Rostedt <rostedt@goodmis.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Alessio Igor Bogani <abogani@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>, Avi Kivity <avi@redhat.com>,
        Chris Metcalf <cmetcalf@tilera.com>, Christoph Lameter <cl@linux.com>,
        Geoff Levand <geoff@infradead.org>,
        Gilad Ben Yossef <gilad@benyossef.com>,
        Hakan Akkan <hakanakkan@gmail.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Ingo Molnar <mingo@kernel.org>, Kevin Hilman <khilman@ti.com>,
        Max Krasnyansky <maxk@qualcomm.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Stephen Hemminger <shemminger@vyatta.com>,
        Sven-Thorsten Dietrich <thebigcorporation@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>
Date: Mon, 30 Jul 2012 11:27:54 -0400
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On Mon, 2012-07-30 at 17:08 +0200, Peter Zijlstra wrote:
> On Fri, 2012-07-27 at 17:40 +0200, Frederic Weisbecker wrote:
> > +++ b/kernel/user_hooks.c
> > @@ -0,0 +1,56 @@
> > +#include <linux/user_hooks.h>
> > +#include <linux/rcupdate.h>
> > +#include <linux/sched.h>
> > +#include <linux/percpu.h>
> > +
> > +struct user_hooks {
> > +       bool hooking;
> > +       bool in_user;
> > +};
> 
> I really detest using bool in structures.. but that's just me. Also this
> really wants a comment as to wtf 'hooking' means. in_user I can just
> about guess.

I'm curious to what you have against bool in structures? Would you
prefer a:

struct user_hooks {
	unsigned int hooking:1;
	unsigned int in_user:1;
};

instead? I haven't checked, but I would hope that gcc would optimize the
struct into a single word.

But I could see that it can cause races as that would make modifying
hooking and in_user dependent on each other. That is, if one CPU updates
hooking as another CPU updates in_user, that could cause a
read-modify-write race. At least in this case the modification is only
done on local cpu variables.

-- Steve