linux-kernel - Re: CONFIG_PREEMPT

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Message-ID: <Pine.LNX.4.64.0908092139090.3194@sister.anvils>
Date:	Sun, 9 Aug 2009 21:53:53 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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, 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 ;-)

Hugh
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