When a CPU is an idle migrator, but another CPU wakes up before it,
becomes an active migrator and handles the queue, the initial idle
migrator may end up endlessly reprogramming its clockevent, chasing ghost
timers forever such as in the following scenario:

               [GRP0:0]
             migrator = 0
             active   = 0
             nextevt  = T1
              /         \
             0           1
          active        idle (T1)

0) CPU 1 is idle and has a timer queued (T1), CPU 0 is active and is
the active migrator.

               [GRP0:0]
             migrator = NONE
             active   = NONE
             nextevt  = T1
              /         \
             0           1
          idle        idle (T1)
          wakeup = T1

1) CPU 0 is now idle and is therefore the idle migrator. It has
programmed its next timer interrupt to handle T1.

                [GRP0:0]
             migrator = 1
             active   = 1
             nextevt  = KTIME_MAX
              /         \
             0           1
          idle        active
          wakeup = T1

2) CPU 1 has woken up, it is now active and it has just handled its own
timer T1.

3) CPU 0 gets a timer interrupt to handle T1 but tmigr_handle_remote()
realize it is not the migrator anymore. So it early returns without
observing that T1 has been expired already and therefore without
updating its ->wakeup value.

4) CPU 0 goes into tmigr_cpu_new_timer() which also early returns
because it doesn't queue a timer of its own. So ->wakeup is left
unchanged and the next timer is programmed to fire now.

5) goto 3) forever

This results in timer interrupt storms in idle and also in nohz_full (as
observed in rcutorture's TREE07 scenario).

Fix this with forcing a re-evaluation of tmc->wakeup while trying
remote timer handling when the CPU isn't the migrator anymmore. The
check is inherently racy but in the worst case the CPU just races setting
the KTIME_MAX value that a remote expiry also tries to set.

Fixes: 7ee98877 ("timers: Implement the hierarchical pull model")
Reported-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240318230729.15497-2-frederic@kernel.org