linux-kernel - Re: CONFIG_PREEMPT

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Date:	Sun, 9 Aug 2009 14:16:29 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: CONFIG_PREEMPT_RCU in next/mmotm

On Sun, Aug 09, 2009 at 09:53:53PM +0100, Hugh Dickins wrote:
> On Sun, 9 Aug 2009, Paul E. McKenney wrote:
> > 
> > I introduced the problem in commit 7fe616c5dd50a50f334edec1ea0580b90b7af0d9
> > by changing from register_cpu_notifier() to hotcpu_notifier().  The former
> > lets you know when CPUs come on line unconditionally, the latter only
> > when CONFIG_HOTPLUG_CPU is in effect.
> > 
> > But hotcpu_notifier() is much nicer to use, so I propose introducing
> > a cpu_notifier() that is invoked like hotcpu_notifier() is, but is
> > unconditional in the same way that register_cpu_notifier().
> > 
> > Something like the following (untested, probably does not compile):
> > 
> > diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> > index 4d668e0..d5dfc1f 100644
> > --- a/include/linux/cpu.h
> > +++ b/include/linux/cpu.h
> > @@ -48,6 +48,15 @@ struct notifier_block;
> >  
> >  #ifdef CONFIG_SMP
> >  /* Need to know about CPUs going up/down? */
> > +#if defined(CONFIG_HOTPLUG_CPU) || !defined(MODULE)
> > +#define cpu_notifier(fn, pri) {					\
> > +	static struct notifier_block fn##_nb __cpuinitdata =	\
> > +		{ .notifier_call = fn, .priority = pri };	\
> > +	register_cpu_notifier(&fn##_nb);			\
> > +}
> > +#else /* #if defined(CONFIG_HOTPLUG_CPU) || !defined(MODULE) */
> > +#define cpu_notifier(fn, pri)	do { (void)(fn); } while (0)
> > +#endif /* #else #if defined(CONFIG_HOTPLUG_CPU) || !defined(MODULE) */
> >  #ifdef CONFIG_HOTPLUG_CPU
> >  extern int register_cpu_notifier(struct notifier_block *nb);
> >  extern void unregister_cpu_notifier(struct notifier_block *nb);
> > @@ -99,11 +108,7 @@ extern struct sysdev_class cpu_sysdev_class;
> >  
> >  extern void get_online_cpus(void);
> >  extern void put_online_cpus(void);
> > -#define hotcpu_notifier(fn, pri) {				\
> > -	static struct notifier_block fn##_nb __cpuinitdata =	\
> > -		{ .notifier_call = fn, .priority = pri };	\
> > -	register_cpu_notifier(&fn##_nb);			\
> > -}
> > +#define hotcpu_notifier(fn, pri)	cpu_notifier(fn, pri)
> >  #define register_hotcpu_notifier(nb)	register_cpu_notifier(nb)
> >  #define unregister_hotcpu_notifier(nb)	unregister_cpu_notifier(nb)
> >  int cpu_down(unsigned int cpu);
> > diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
> > index 9f0584e..c1bbfd5 100644
> > --- a/kernel/rcupdate.c
> > +++ b/kernel/rcupdate.c
> > @@ -251,7 +251,7 @@ void __init rcu_init(void)
> >  	int i;
> >  
> >  	__rcu_init();
> > -	hotcpu_notifier(rcu_barrier_cpu_hotplug, 0);
> > +	cpu_notifier(rcu_barrier_cpu_hotplug, 0);
> >  
> >  	/*
> >  	 * We don't need protection against CPU-hotplug here because
> > 
> [ removed repeat of rcupdate.c patch ]
> > 
> > Thoughts?
> 
> That builds and works for me, with or without CONFIG_HOTPLUG_CPU.
> 
> But I didn't get what you're achieving with the MODULE part of it;
> and (I'm not a notifier buff at all) it does seems rather baroque to
> me - a single callsite, why not stick with register_cpu_notifier()?
> 
> Ah, perhaps it's your ambition to move others over to this
> (or perhaps it's your ambition to leave that to someone else ;-)

Actually, nothing quite that clearly thought out.  I was just following
the pattern set for register_cpu_notifier().  My guess at the reasoning
is that when !HOTPLUG_CPU, modules cannot be loaded until all the CPUs
are online, so there is no point in letting a module set itself up for
notification.

But whatever their reasoning, mine was that there is no point in
creating a struct notifier_block that wasn't going to be used.  ;-)

							Thanx, Paul
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