Re: [PATCH 2/3] powerpc/powernv: fix preempt count leak in pnv_kexec_wait_secondaries_down

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi Aboorva.

On 5/18/26 10:38 AM, Aboorva Devarajan wrote:
> pnv_kexec_wait_secondaries_down() calls get_cpu() to obtain the current
> CPU id but never calls the matching put_cpu(), leaking one
> preempt_disable() nesting level on every invocation.
> 
> In practice the imbalance does not trigger a visible splat because the
> kexec teardown path is a one-way trip: IRQs are already disabled, no
> schedule() occurs after the leak, and default_machine_kexec() overwrites
> preempt_count with HARDIRQ_OFFSET before jumping into kexec_sequence()
> which never returns. However the bookkeeping is still wrong.
> 
> In the kexec teardown path IRQs are already disabled and the CPU is
> pinned, so get_cpu()'s preempt_disable() side-effect is unnecessary.
> Replace get_cpu() with raw_smp_processor_id() which returns the CPU id
> without touching preempt_count.
> 
> Fixes: 298b34d7d578 ("powerpc/powernv: Fix kexec races going back to OPAL")
> Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
> ---
>   arch/powerpc/platforms/powernv/setup.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/platforms/powernv/setup.c b/arch/powerpc/platforms/powernv/setup.c
> index 4dbb47ddbdcc4..177da0defcb36 100644
> --- a/arch/powerpc/platforms/powernv/setup.c
> +++ b/arch/powerpc/platforms/powernv/setup.c
> @@ -396,7 +396,7 @@ static void pnv_kexec_wait_secondaries_down(void)
>   {
>   	int my_cpu, i, notified = -1;
>   
> -	my_cpu = get_cpu();
> +	my_cpu = raw_smp_processor_id();
>   

Is it always with irq-disabled?
How about !CONFIG_SMP and in kexec_prepare_cpus. I see it disables interrupt later.
(though it is a less common config)

So use smp_processor_id()?? One could compile with CONFIG_DEBUG_PREEMPT=y and
see any reports.

>   	for_each_online_cpu(i) {
>   		uint8_t status;