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Date: Wed, 30 Nov 2011 16:16:10 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Dimitri Sivanich <sivanich@sgi.com>, linux-kernel@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] specific do_timer_cpu value for nohz off mode
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On Wed, 30 Nov 2011 16:11:31 -0800
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Wed, 30 Nov 2011 09:29:59 -0600
> Dimitri Sivanich <sivanich@sgi.com> wrote:
> 
> > +static ssize_t sysfs_store_do_timer_cpu(struct sys_device *dev,
> > +						struct sysdev_attribute *attr,
> > +						const char *buf, size_t size)
> > +{
> > +	struct sysdev_ext_attribute *ea = SYSDEV_TO_EXT_ATTR(attr);
> > +	unsigned int new;
> > +	int rv;
> > +
> > +#ifdef CONFIG_NO_HZ
> > +	/* nohz mode not supported */
> > +	if (tick_nohz_enabled)
> > +		return -EINVAL;
> > +#endif
> > +
> > +	rv = kstrtouint(buf, 0, &new);
> > +	if (rv)
> > +		return rv;
> > +
> > +	if (new >= NR_CPUS || !cpu_online(new))
> > +		return -ERANGE;
> > +
> > +	*(unsigned int *)(ea->var) = new;
> > +	return size;
> > +}
> 
> checkpatch tells us:
> 
> WARNING: usage of NR_CPUS is often wrong - consider using cpu_possible(), num_possible_cpus(), for_each_possible_cpu(), etc
> 
> I think the check can just be removed?  Surely cpu_online(1000000000)
> will return false?
> 

And the whole thing is racy, isn't it?  The "new" CPU can go offline a
nanosecond after we performed that test, so why perform it at all?