Re: [PATCH 0/2] Protect cpu map updates from ACPI processor

[Toshi Kani <toshi.kani@hp.com>]

On Tue, 2013-08-13 at 02:43 +0200, Rafael J. Wysocki wrote:
> On Monday, August 12, 2013 09:45:52 AM Toshi Kani wrote:
> > CPU system maps are protected with reader/writer locks.  The reader
> > lock, put_online_cpus(), assures that the maps are not updated while
> > holding the lock.  The writer lock, cpu_hotplug_begin(), is used to
> > udpate the cpu maps along with cpu_maps_update_begin().
> > 
> > However, the ACPI processor handler updates the cpu maps without
> > holding the the writer lock.  This patchset fixes this problem.
> > 
> > ---
> > The patchset is based on linux-next of the pm tree.
> 
> Basically looks OK to me, but I'd just merge the patches together,
> because the [2/2] is the very reason for the change made by [1/2].

I separated for reviewing purpose, but yes, they can be together.  Let
me know if you need me to re-send it together.

> Also it would be good to mention the actual scenario that may lead
> to problems with that.

For example, the get_online_cpus() below is supposed to assure that
cpu_possible_mask is not changed while the code is iterating with
for_each_possible_cpu().

        get_online_cpus();
        for_each_possible_cpu(cpu) {
		:
        }
        put_online_cpus();

However, this lock has no protection with CPU hotplug since the ACPI
processor handler does not use the writer lock when it updates
cpu_possible_mask.  The reader lock does not serialize within the
readers.

Thanks,
-Toshi