The struct clocksource callbacks enable() and disable() are described as a
way to allow clock sources to enter a power save mode. See commit
4614e6ad ("clocksource: add enable() and disable() callbacks")

But using runtime PM from these callbacks triggers a cyclic lockdep warning when
switching clock source using change_clocksource().

  # echo e60f0000.timer > /sys/devices/system/clocksource/clocksource0/current_clocksource
   ======================================================
   WARNING: possible circular locking dependency detected
   ------------------------------------------------------
   migration/0/11 is trying to acquire lock:
   ffff0000403ed220 (&dev->power.lock){-...}-{2:2}, at: __pm_runtime_resume+0x40/0x74

   but task is already holding lock:
   ffff8000113c8f88 (tk_core.seq.seqcount){----}-{0:0}, at: multi_cpu_stop+0xa4/0x190

   which lock already depends on the new lock.

   the existing dependency chain (in reverse order) is:

   -> #2 (tk_core.seq.seqcount){----}-{0:0}:
          ktime_get+0x28/0xa0
          hrtimer_start_range_ns+0x210/0x2dc
          generic_sched_clock_init+0x70/0x88
          sched_clock_init+0x40/0x64
          start_kernel+0x494/0x524

   -> #1 (hrtimer_bases.lock){-.-.}-{2:2}:
          hrtimer_start_range_ns+0x68/0x2dc
          rpm_suspend+0x308/0x5dc
          rpm_idle+0xc4/0x2a4
          pm_runtime_work+0x98/0xc0
          process_one_work+0x294/0x6f0
          worker_thread+0x70/0x45c
          kthread+0x154/0x160
          ret_from_fork+0x10/0x20

   -> #0 (&dev->power.lock){-...}-{2:2}:
          _raw_spin_lock_irqsave+0x7c/0xc4
          __pm_runtime_resume+0x40/0x74
          sh_cmt_start+0x1c4/0x260
          sh_cmt_clocksource_enable+0x28/0x50
          change_clocksource+0x9c/0x160
          multi_cpu_stop+0xa4/0x190
          cpu_stopper_thread+0x90/0x154
          smpboot_thread_fn+0x244/0x270
          kthread+0x154/0x160
          ret_from_fork+0x10/0x20

   other info that might help us debug this:

   Chain exists of:
     &dev->power.lock --> hrtimer_bases.lock --> tk_core.seq.seqcount

    Possible unsafe locking scenario:

          CPU0                    CPU1
          ----                    ----
     lock(tk_core.seq.seqcount);
                                  lock(hrtimer_bases.lock);
                                  lock(tk_core.seq.seqcount);
     lock(&dev->power.lock);

    *** DEADLOCK ***

   2 locks held by migration/0/11:
    #0: ffff8000113c9278 (timekeeper_lock){-.-.}-{2:2}, at: change_clocksource+0x2c/0x160
    #1: ffff8000113c8f88 (tk_core.seq.seqcount){----}-{0:0}, at: multi_cpu_stop+0xa4/0x190

Rework change_clocksource() so it enables the new clocksource and disables
the old clocksource outside of the timekeeper_lock and seqcount write held
region. There is no requirement that these callbacks are invoked from the
lock held region.
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20210211134318.323910-1-niklas.soderlund+renesas@ragnatech.se