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

[sashiko-bot@kernel.org]

> 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>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261006111304.4023017-1-sumanthk@linux.ibm.com?part=1