Re: [PATCH 1/2] bus: arm-ccn: Fix use of smp_processor_id() in preemptible context

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Sep 04, 2017 at 06:30:40PM +0100, Marc Zyngier wrote:
> Booting a DEBUG_PREEMPT enabled kernel on a CCN-based system
> results in the following splat:
> 
> [...]
> arm-ccn e8000000.ccn: No access to interrupts, using timer.
> BUG: using smp_processor_id() in preemptible [00000000] code: swapper/0/1
> caller is debug_smp_processor_id+0x1c/0x28
> CPU: 1 PID: 1 Comm: swapper/0 Not tainted 4.13.0 #6111
> Hardware name: AMD Seattle/Seattle, BIOS 17:08:23 Jun 26 2017
> Call trace:
> [<ffff000008089e78>] dump_backtrace+0x0/0x278
> [<ffff00000808a22c>] show_stack+0x24/0x30
> [<ffff000008bc3bc4>] dump_stack+0x8c/0xb0
> [<ffff00000852b534>] check_preemption_disabled+0xfc/0x100
> [<ffff00000852b554>] debug_smp_processor_id+0x1c/0x28
> [<ffff000008551bd8>] arm_ccn_probe+0x358/0x4f0
> [...]
> 
> as we use smp_processor_id() in the wrong context. Turning this
> into a get_cpu()/put_cpu() does the trick.
> 
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
>  drivers/bus/arm-ccn.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/bus/arm-ccn.c b/drivers/bus/arm-ccn.c
> index e8c6946fed9d..8502f1767024 100644
> --- a/drivers/bus/arm-ccn.c
> +++ b/drivers/bus/arm-ccn.c
> @@ -1297,7 +1297,8 @@ static int arm_ccn_pmu_init(struct arm_ccn *ccn)
>  	}
>  
>  	/* Pick one CPU which we will use to collect data from CCN... */
> -	cpumask_set_cpu(smp_processor_id(), &ccn->dt.cpu);
> +	cpumask_set_cpu(get_cpu(), &ccn->dt.cpu);
> +	put_cpu();

I think this isn't quite right, and we need to keep a CPU online until
we've registerd the hotplug callbacks. Otherwise, we could race with a
concurrent hotplug, and not migrate events appropriately.

Thanks,
Mark.