Re: [PATCH 0/2] s390/cpum_cf: Check session cpu mask during hotplug

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

On Tue, Oct 06, 2026 at 01:13:02PM +0200, Sumanth Korikkar wrote:
> The cpu hotplug callbacks start and stop counter sets for every active
> hwctr session without checking the session cpu mask. This causes a
> reference count underflow when an unrelated cpu goes offline, and
> silently adds cpus to a session when they come online.
> 
> Patch 1 fixes the reference count underflow in cfset_offline_cpu() and
> is suitable for stable kernels. It does not change the online behavior,
> so no matching lshwc change is required.
> 
> Patch 2 extends the mask check to cfset_online_cpu() and keeps the
> userspace provided cpu mask unchanged across hotplug events. It requires
> a matching lshwc change and is therefore kept separate from patch 1.
> 
> Sumanth Korikkar (2):
>   s390/cpum_cf: Check session mask before cpu offline
>   s390/cpum_cf: Honor hwctr session cpu mask during hotplug
> 
>  arch/s390/kernel/perf_cpum_cf.c | 19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)

Series applied. Thanks!