cpu/hotplug: Don't offline the last non-isolated CPU

In the Linux kernel, the following vulnerability has been resolved:

cpu/hotplug: Don't offline the last non-isolated CPU

If a system has isolated CPUs via the "isolcpus=" command line parameter,

then an attempt to offline the last housekeeping CPU will result in a

WARN_ON() when rebuilding the scheduler domains and a subsequent panic due

to and unhandled empty CPU mas in partition_sched_domains_locked().

cpuset_hotplug_workfn()

rebuild_sched_domains_locked()

ndoms = generate_sched_domains(&doms, &attr);

cpumask_and(doms[0], top_cpuset.effective_cpus, housekeeping_cpumask(HK_FLAG_DOMAIN));

Thus results in an empty CPU mask which triggers the warning and then the

subsequent crash:

WARNING: CPU: 4 PID: 80 at kernel/sched/topology.c:2366 build_sched_domains+0x120c/0x1408

Call trace:

build_sched_domains+0x120c/0x1408

partition_sched_domains_locked+0x234/0x880

rebuild_sched_domains_locked+0x37c/0x798

rebuild_sched_domains+0x30/0x58

cpuset_hotplug_workfn+0x2a8/0x930

Unable to handle kernel paging request at virtual address fffe80027ab37080

partition_sched_domains_locked+0x318/0x880

rebuild_sched_domains_locked+0x37c/0x798

Aside of the resulting crash, it does not make any sense to offline the last

last housekeeping CPU.

Prevent this by masking out the non-housekeeping CPUs when selecting a

target CPU for initiating the CPU unplug operation via the work queue.