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

From: Toshi Kani
Date: Mon Aug 12 2013 - 20:56:51 EST

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(cpu) {

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


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