Re: [PATCH 3/3] powerpc/kexec: fix double get_cpu() imbalance in kexec_prepare_cpus

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

Hi Aboorva.

On 5/18/26 10:38 AM, Aboorva Devarajan wrote:
> kexec_prepare_cpus_wait() calls get_cpu() internally to obtain the
> current CPU id. kexec_prepare_cpus() calls kexec_prepare_cpus_wait()
> twice -- once for KEXEC_STATE_IRQS_OFF and once for
> KEXEC_STATE_REAL_MODE -- but only issues a single put_cpu() at the end,
> leaving preempt_count elevated by one extra nesting level.
> 
> In practice the imbalance does not trigger a 'scheduling while atomic'
> splat because the kexec 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.
> 
> Lift the get_cpu()/put_cpu() pair into kexec_prepare_cpus() so it is
> called exactly once, and pass the CPU id to kexec_prepare_cpus_wait()
> as a parameter. This keeps preempt_count correctly balanced.
> 
> Fixes: 1fc711f7ffb01 ("powerpc/kexec: Fix race in kexec shutdown")
> Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
> ---
>   arch/powerpc/kexec/core_64.c | 15 ++++++++-------
>   1 file changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/kexec/core_64.c b/arch/powerpc/kexec/core_64.c
> index 825ab8a88f18e..9d7e5a1e6e5b8 100644
> --- a/arch/powerpc/kexec/core_64.c
> +++ b/arch/powerpc/kexec/core_64.c
> @@ -164,12 +164,11 @@ static void kexec_smp_down(void *arg)
>   	/* NOTREACHED */
>   }
>   
> -static void kexec_prepare_cpus_wait(int wait_state)
> +static void kexec_prepare_cpus_wait(int wait_state, int my_cpu)
>   {
> -	int my_cpu, i, notified=-1;
> +	int i, notified = -1;
>   
>   	hw_breakpoint_disable();
> -	my_cpu = get_cpu();
>   	/* Make sure each CPU has at least made it to the state we need.
>   	 *
>   	 * FIXME: There is a (slim) chance of a problem if not all of the CPUs
> @@ -246,6 +245,8 @@ static void wake_offline_cpus(void)
>   
>   static void kexec_prepare_cpus(void)
>   {
> +	int my_cpu;
> +
>   	wake_offline_cpus();
>   	smp_call_function(kexec_smp_down, NULL, /* wait */0);
>   	local_irq_disable();
> @@ -254,7 +255,8 @@ static void kexec_prepare_cpus(void)
>   	mb(); /* make sure IRQs are disabled before we say they are */
>   	get_paca()->kexec_state = KEXEC_STATE_IRQS_OFF;
>   
> -	kexec_prepare_cpus_wait(KEXEC_STATE_IRQS_OFF);
> +	my_cpu = get_cpu();

raw_smp_processor_id() is better here. All it needs is get current cpu?
caller does irq_disable above and that renders call for get_cpu un-necessary.


> +	kexec_prepare_cpus_wait(KEXEC_STATE_IRQS_OFF, my_cpu);
>   	/* we are sure every CPU has IRQs off at this point */
>   	kexec_all_irq_disabled = 1;
>   
> @@ -262,13 +264,12 @@ static void kexec_prepare_cpus(void)
>   	 * Before removing MMU mappings make sure all CPUs have entered real
>   	 * mode:
>   	 */
> -	kexec_prepare_cpus_wait(KEXEC_STATE_REAL_MODE);
> +	kexec_prepare_cpus_wait(KEXEC_STATE_REAL_MODE, my_cpu);
> +	put_cpu();
>   
>   	/* after we tell the others to go down */
>   	if (ppc_md.kexec_cpu_down)
>   		ppc_md.kexec_cpu_down(0, 0);
> -
> -	put_cpu();
>   }
>   
>   #else /* ! SMP */