[PATCH 1/2] s390/cpum_cf: Check session mask before cpu offline

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

cfset_offline_cpu() stops counter sets for every active session without
checking whether the outgoing cpu belongs to the session mask.

For example, run an hwctr session on cpu0 and a cpu bound perf event on
cpu1:

lshwc -l 9999 -i 5 0:b
perf stat -C 1 -e cpum_cf/CPU_CYCLES/ sleep 120
chcpu -d 1
chcpu -e 1

Although the hwctr session does not include cpu1, cfset_offline_cpu()
decrements its counter set reference count. The subsequent perf teardown
decrements it again, causing it to underflow.

The perf event keeps the percpu state alive across hotplug. When cpu1
comes online, the hwctr session is started there and the reference count
moves from -1 to 0. Stopping the hwctr session then decrements it to -1
again, so the zero triggered counter set disable path is skipped.

Check the session mask before stopping counter sets on the outgoing cpu.
This prevents the unmatched decrement and the resulting reference count
underflow.

Fixes: a029a4eab39e ("s390/cpumf: Allow concurrent access for CPU Measurement Counter Facility")
Reported-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: stable@vger.kernel.org # v5.14+
Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
---
 arch/s390/kernel/perf_cpum_cf.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/s390/kernel/perf_cpum_cf.c b/arch/s390/kernel/perf_cpum_cf.c
index 69fa303cd479..1d63dc3b0ab9 100644
--- a/arch/s390/kernel/perf_cpum_cf.c
+++ b/arch/s390/kernel/perf_cpum_cf.c
@@ -1797,6 +1797,9 @@ static int cfset_offline_cpu(unsigned int cpu)
 
 	if (!list_empty(&cfset_session.head)) {
 		list_for_each_entry(rp, &cfset_session.head, node) {
+			if (!cpumask_test_cpu(cpu, &rp->mask))
+				continue;
+
 			p.sets = rp->ctrset;
 			cfset_ioctl_off(&p);
 			cpumask_clear_cpu(cpu, &rp->mask);
-- 
2.53.0