[PATCH][RFC] preempt_count corruption across H_CEDE call with CONFIG_PREEMPT on pseries

[Thomas Gleixner]

On Thu, 5 Aug 2010, Vaidyanathan Srinivasan wrote:

> * Darren Hart <dvhltc at us.ibm.com> [2010-08-04 21:45:51]:
> 
> > On 07/23/2010 12:07 AM, Vaidyanathan Srinivasan wrote:
> > >* Benjamin Herrenschmidt<benh at kernel.crashing.org>  [2010-07-23 15:11:00]:
> > >
> > >>On Fri, 2010-07-23 at 10:38 +0530, Vaidyanathan Srinivasan wrote:
> > >>>Yes.  extended_cede_processor() will return with interrupts enabled in
> > >>>the cpu. (This is done by the hypervisor).  Under normal cases we
> > >>>cannot be interrupted because no IO interrupts are routed to us after
> > >>>xics_teardown_cpu() and since the CPU is out of the map, nobody will
> > >>>send us IPIs.
> > >>
> > >>What about decrementer ?
> > >
> > >Decrementer expiry event handling is bit complex.  The event as such
> > >may not bring back the extended_cede_processor() cpu, but may be
> > >marked pending when we get out of this state eventually.  I will find
> > >more information on this event and update.
> > 
> > Hi Vaidy, have you been able to dig anything up about the
> > decrementer expiry?
> 
> Hi Darren,
> 
> Yes, it is possible that the cpu in extended_cede_processor() can be
> woken up by the decrementer.  But the expectation is that we will
> return back to this context since we will not have any pending timers.

The problem is that you might get woken _before_ the timers have been
migrated away. 
 
> Our stack should not change even if we get the decrementer interrupts.

But you should not execute kernel code when you got offlined either.

Thanks,

	tglx