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

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

On Wed, 30 Nov 2011 20:34:23 -0600 Dimitri Sivanich <sivanich@sgi.com> wrote:

> On Wed, Nov 30, 2011 at 06:12:05PM -0800, Andrew Morton wrote:
> > 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().
> 
> May be splitting hairs, but I think I like num_present_cpus even better.

Does that work correctly if the cpu-present map has holes?

> > 
> > > > 
> > > > 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:
> 
> Actually, I didn't have CONFIG_DEBUG_PER_CPU_MAPS turned on.

So why did it crash?  test_bit() far out of range?