Re: [PATCH] specific do_timer_cpu value for nohz off mode

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 30 Nov 2011 20:06:31 -0600 Dimitri Sivanich <sivanich@sgi.com> wrote:

> On Wed, Nov 30, 2011 at 04:11:31PM -0800, Andrew Morton 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 a check against num_possible_cpus() should be OK.

You want cpu_possible().  Or maybe cpu_present().

> > 
> > I think the check can just be removed?  Surely cpu_online(1000000000)
> > will return false?
> 
> A value > NR_CPUS and < MAX_INT caused a panic in sysfs_store_do_timer_cpu,
> presumably from the cpu_online() check.  The check against NR_CPUS avoided
> the panic.

OK.  Well, it's not a panic:

static inline unsigned int cpumask_check(unsigned int cpu)
{
#ifdef CONFIG_DEBUG_PER_CPU_MAPS
	WARN_ON_ONCE(cpu >= nr_cpumask_bits);
#endif /* CONFIG_DEBUG_PER_CPU_MAPS */
	return cpu;
}

so we can't do cpu_online(insane number)