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

[Sumanth Korikkar <sumanthk@linux.ibm.com>]

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(-)

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2.53.0