[PATCH 21/22] ARM: CPU hotplug: ensure correct ordering of unplug

[Russell King - ARM Linux]

On Mon, Dec 06, 2010 at 06:13:37PM +0000, Catalin Marinas wrote:
> On 3 December 2010 20:26, Russell King - ARM Linux
> <linux at arm.linux.org.uk> wrote:
> > Don't call idle_task_exit() with interrupts disabled, and ensure
> > that we have a memory barrier after interrupts are disabled but
> > before signalling that this CPU has shut down.
> >
> > Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>
> > ---
> >  arch/arm/kernel/smp.c |    4 +++-
> >  1 files changed, 3 insertions(+), 1 deletions(-)
> >
> > diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> > index bebe3bd..8cc9c03 100644
> > --- a/arch/arm/kernel/smp.c
> > +++ b/arch/arm/kernel/smp.c
> > @@ -269,9 +269,11 @@ void __ref cpu_die(void)
> >  {
> >        unsigned int cpu = smp_processor_id();
> >
> > -       local_irq_disable();
> >        idle_task_exit();
> >
> > +       local_irq_disable();
> > +       mb();
> > +
> >        /* Tell __cpu_die() that this CPU is now safe to dispose of */
> >        complete(&cpu_died);
> 
> I don't fully understand the point of the mb() here. If you want to
> drain the write buffer that would be a dsb().

It's there to ensure that writes prior to the IRQ disable do not
cross the complete() call, that is all.  local_irq_disable() does
not have any built-in barrier semantics.