Re: [Xen-devel] [PATCH] cpuidle: Check for dev before deregistering it.

[Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>]

On 12/04/2013 05:09 PM, Konrad Rzeszutek Wilk wrote:
> On Wed, Dec 04, 2013 at 10:10:28AM +0100, Daniel Lezcano wrote:
> > On 12/03/2013 10:33 PM, Rafael J. Wysocki wrote:
> > > On Tuesday, December 03, 2013 10:59:58 AM Konrad Rzeszutek Wilk wrote:
> > > > If not, we could end up in the unfortunate situation where
> > > > we dereference a NULL pointer b/c we have cpuidle disabled.
> > > >
> > > > This is the case when booting under Xen (which uses the
> > > > ACPI P/C states but disables the CPU idle driver) - and can
> > > > be easily reproduced when booting with cpuidle.off=1.
> > > >
> > > > BUG: unable to handle kernel NULL pointer dereference at           (null)
> > > > IP: [<ffffffff8156db4a>] cpuidle_unregister_device+0x2a/0x90
> > > > .. snip..
> > > > Call Trace:
> > > >   [<ffffffff813b15b4>] acpi_processor_power_exit+0x3c/0x5c
> > > >   [<ffffffff813af0a9>] acpi_processor_stop+0x61/0xb6
> > > >   [<ffffffff814215bf>] __device_release_driver+0fffff81421653>] 
> > > > device_release_driver+0x23/0x30
> > > >   [<ffffffff81420ed8>] bus_remove_device+0x108/0x180
> > > >   [<ffffffff8141d9d9>] device_del+0x129/0x1c0
> > > >   [<ffffffff813cb4b0>] ? unregister_xenbus_watch+0x1f0/0x1f0
> > > >   [<ffffffff8141da8e>] device_unregister+0x1e/0x60
> > > >   [<ffffffff814243e9>] unregister_cpu+0x39/0x60
> > > >   [<ffffffff81019e03>] arch_unregister_cpu+0x23/0x30
> > > >   [<ffffffff813c3c51>] handle_vcpu_hotplug_event+0xc1/0xe0
> > > >   [<ffffffff813cb4f5>] xenwatch_thread+0x45/0x120
> > > >   [<ffffffff810af010>] ? abort_exclusive_wait+0xb0/0xb0
> > > >   [<ffffffff8108ec42>] kthread+0xd2/0xf0
> > > >   [<ffffffff8108eb70>] ? kthread_create_on_node+0x180/0x180
> > > >   [<ffffffff816ce17c>] ret_from_fork+0x7c/0xb0
> > > >   [<ffffffff8108eb70>] ? kthread_create_on_node+0x180/0x180
> > > >
> > > > This problem also appears in 3.12 and could be a candidate for backport.
> > > >
> > > > CC: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>
> > > > CC: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> > > > CC: linux-pm@xxxxxxxxxxxxxxx
> > > > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > > Applied, thanks!
> > >
> > > > ---
> > > >   drivers/cpuidle/cpuidle.c | 2 +-
> > > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> > > > index 2a991e4..a55e68f 100644
> > > > --- a/drivers/cpuidle/cpuidle.c
> > > > +++ b/drivers/cpuidle/cpuidle.c
> > > > @@ -400,7 +400,7 @@ EXPORT_SYMBOL_GPL(cpuidle_register_device);
> > > >    */
> > > >   void cpuidle_unregister_device(struct cpuidle_device *dev)
> > > >   {
> > > > -       if (dev->registered == 0)
> > > > +       if (!dev || dev->registered == 0)
> > > >                 return;
> > > >
> > > >         cpuidle_pause_and_lock();
> > Oops, wait. Are we sure the problem is coming from cpuidle ?
> It is acpi_processor_power_exit assuming that the cpuidle is
> initialized. It could be fixed there too, but there are multiple
> entries in cpuidle where it does the : "if (!dev) return .."
> so I figured this should be done as well here.
I understand.

From my POV the bug is coming from the acpi processor idle driver.

The function acpi_processor_power_init registers the cpuidle driver and 
the cpuidle device when acpi_processor_registered is zero. Then it 
increments acpi_processor_registered preventing the next call to this 
function to register the driver but it will register the device.
As cpuidle is disabled, the cpuidle_register_driver fails, thus the 
device is not registered and acpi_processor_registered is not 
incremented. So all calls to acpi_processor_power_init prevents the 
driver and the device to be registered. No problem with that.
But the function acpi_processor_power_exit does not take care of the 
value of acpi_processor_registered and just unregister the device. Then 
it decrements acpi_processor_registered which is zero to -1.
Trying to be immune from a NULL pointer in cpuidle_unregister_device 
hides bogus code from the caller. So IMO, this check shouldn't be there 
and the acpi_processor_power_exit function should be fixed instead.

> > The cpuidle_unregister_device is called with a NULL pointer, that
> > shouldn't happen.
> It does :-)
> > Konrad, you say that could be easily reproduced. How do you produce
> > it ? Unplugging a cpu ?
> Yes.

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