[RFC PATCH] ARM: smp: Fix the CPU hotplug race with scheduler.

[Santosh Shilimkar]

On 6/20/2011 4:05 PM, Russell King - ARM Linux wrote:
> On Mon, Jun 20, 2011 at 03:58:03PM +0530, Santosh Shilimkar wrote:
>> On 6/20/2011 3:44 PM, Russell King - ARM Linux wrote:
>>> On Mon, Jun 20, 2011 at 10:50:53AM +0100, Russell King - ARM Linux wrote:
>>>> On Mon, Jun 20, 2011 at 02:53:59PM +0530, Santosh Shilimkar wrote:
>>>>> The current ARM CPU hotplug code suffers from couple of race conditions
>>>>> in CPU online path with scheduler.
>>>>> The ARM CPU hotplug code doesn't wait for hot-plugged CPU to be marked
>>>>> active as part of cpu_notify() by the CPU which brought it up before
>>>>> enabling interrupts.
>>>>
>>>> Hmm, why not just move the set_cpu_online() call before notify_cpu_starting()
>>>> and add the wait after the set_cpu_online() ?
>>>
>>> Actually, the race is caused by the CPU being marked online (and therefore
>>> available for the scheduler) but not yet active (the CPU asking this one
>>> to boot hasn't run the online notifiers yet.)
>>>
>> Scheduler uses the active mask and not online mask. For schedules CPU
>> is ready for migration as soon as it is marked as active and that's
>> the reason, interrupts should never be enabled before CPU is marked
>> as active in online path.
>>
>>> This, I feel, is a fault of generic code.  If the CPU is not ready to have
>>> processes scheduled on it (because migration is not initialized) then we
>>> shouldn't be scheduling processes on the new CPU yet.
>>>
>>> In any case, this should close the window by ensuring that we don't receive
>>> an interrupt in the online-but-not-active case.  Can you please test?
>>>
>> No it doesn't work. I still get the crash. The important point
>> here is not to enable interrupts before CPU is marked
>> as online and active.
>
> But we can't do that.
Why is that ?
Is it because of calibration or the hotplug start notifies needs to
be called with interrupts enabled ?

Regards
Santosh