Re: [PATCH v3] s390/cpum_cf: Handle CPU hotplug via prepare/dead callbacks

[Heiko Carstens <hca@linux.ibm.com>]

On Tue, Aug 11, 2026 at 03:39:01PM +0200, Thomas Richter wrote:
> The command 'perf stat -e cycles -- <command>' crashes the kernel
> when CPUs are hotplug added during that run.
...
> Fixes: 9b9cf3c77e7e ("s390/cpum_cf: rework PER_CPU_DEFINE of struct cpu_cf_events")
> Cc: <stable@vger.kernel.org> # v6.5+
> Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
> Suggested-by: Heiko Carstens <hca@linux.ibm.com>
> Suggested-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> ---
>  arch/s390/kernel/perf_cpum_cf.c | 215 +++++++++++++++++++++-----------
>  1 file changed, 142 insertions(+), 73 deletions(-)
...
> @@ -167,12 +170,14 @@ static void cpum_cf_reset_cpu(void *flags)
>  }
>  
>  /* Free per CPU data when the last event is removed. */
> -static void cpum_cf_free_root(void)
> +static void cpum_cf_free_root(unsigned int num)
>  {
> -	if (!refcount_dec_and_test(&cpu_cf_root.refcnt))
> +	struct cpu_cf_ptr __percpu *p = cpu_cf_root.cfptr;
> +
> +	if (!refcount_sub_and_test(num, &cpu_cf_root.refcnt))
>  		return;
> -	free_percpu(cpu_cf_root.cfptr);
>  	cpu_cf_root.cfptr = NULL;
> +	free_percpu(p);

This appears to be an unrelated fix. But anyway:

Acked-by: Heiko Carstens <hca@linux.ibm.com>