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

[Steven Rostedt]

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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