Kuyo reported sporadic failures on a sched_setaffinity() vs CPU
hotplug stress-test -- notably affine_move_task() remains stuck in
wait_for_completion(), leading to a hung-task detector warning.

Specifically, it was reported that stop_one_cpu_nowait(.fn =
migration_cpu_stop) returns false -- this stopper is responsible for
the matching complete().

The race scenario is:

	CPU0					CPU1

					// doing _cpu_down()

  __set_cpus_allowed_ptr()
    task_rq_lock();
					takedown_cpu()
					  stop_machine_cpuslocked(take_cpu_down..)

					<PREEMPT: cpu_stopper_thread()
					  MULTI_STOP_PREPARE
					  ...
    __set_cpus_allowed_ptr_locked()
      affine_move_task()
        task_rq_unlock();

  <PREEMPT: cpu_stopper_thread()\>
    ack_state()
					  MULTI_STOP_RUN
					    take_cpu_down()
					      __cpu_disable();
					      stop_machine_park();
						stopper->enabled = false;
					 />
   />
	stop_one_cpu_nowait(.fn = migration_cpu_stop);
          if (stopper->enabled) // false!!!

That is, by doing stop_one_cpu_nowait() after dropping rq-lock, the
stopper thread gets a chance to preempt and allows the cpu-down for
the target CPU to complete.

OTOH, since stop_one_cpu_nowait() / cpu_stop_queue_work() needs to
issue a wakeup, it must not be ran under the scheduler locks.

Solve this apparent contradiction by keeping preemption disabled over
the unlock + queue_stopper combination:

	preempt_disable();
	task_rq_unlock(...);
	if (!stop_pending)
	  stop_one_cpu_nowait(...)
	preempt_enable();

This respects the lock ordering contraints while still avoiding the
above race. That is, if we find the CPU is online under rq-lock, the
targeted stop_one_cpu_nowait() must succeed.

Apply this pattern to all similar stop_one_cpu_nowait() invocations.

Fixes: 6d337eab ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
Reported-by: "Kuyo Chang (張建文)" <Kuyo.Chang@mediatek.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: "Kuyo Chang (張建文)" <Kuyo.Chang@mediatek.com>
Link: https://lkml.kernel.org/r/20231010200442.GA16515@noisy.programming.kicks-ass.net