Re: [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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/