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

[Aboorva Devarajan <aboorvad@linux.ibm.com>]

Hi Shrikanth,

On Mon, 2026-05-18 at 11:32 +0530, Shrikanth Hegde wrote:
> 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.

Agreed, get_cpu() is not needed here. kexec_prepare_cpus() already does
local_irq_disable()/hard_irq_disable() before calling
kexec_prepare_cpus_wait(), so we only need the current cpu id.

I will go ahead with smp_processor_id() rather than
raw_smp_processor_id() to stay consistent with Patch 2 and to keep the
CONFIG_DEBUG_PREEMPT check.

> >   
> > @@ -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 */

Regards,
Aboorva