[PATCH] ARM: Fix subtle race in CPU pen_release hotplug code

[Catalin Marinas]

On 18 December 2010 11:04, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> There is a subtle race in the CPU hotplug code, where a CPU which has
> been offlined can online itself before being requested, which results
> in things going astray on the next online/offline cycle.
[...]
> --- a/arch/arm/mach-realview/platsmp.c
> +++ b/arch/arm/mach-realview/platsmp.c
> @@ -36,6 +36,19 @@ extern void realview_secondary_startup(void);
>  */
>  volatile int __cpuinitdata pen_release = -1;
>
> +/*
> + * Write pen_release in a way that is guaranteed to be visible to all
> + * observers, irrespective of whether they're taking part in coherency
> + * or not.  This is necessary for the hotplug code to work reliably.
> + */
> +static void write_pen_release(int val)
> +{
> +       pen_release = val;
> +       smp_wmb();
> +       __cpuc_flush_dcache_area((void *)&pen_release, sizeof(pen_release));
> +       outer_clean_range(__pa(&pen_release), __pa(&pen_release + 1));
> +}

Just a minor thing - I don't think we need any barrier here. According
to the ARM ARM B2.2.7:

"Any data cache or unified cache maintenance operation by MVA must be
executed in program order
relative to any explicit load or store on the same processor to an
address covered by the MVA of the
cache operation."

We also have a corresponding smp_rmb() in boot_secondary(), I don't
think it has any use either.

-- 
Catalin