linux-kernel - Re: [PATCH 1/5] user_hooks: New user hooks subsystem

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Message-ID: <1343662074.27983.4.camel@gandalf.stny.rr.com>
Date:	Mon, 30 Jul 2012 11:27:54 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Kevin Hilman <khilman@...com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/5] user_hooks: New user hooks subsystem

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


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