powerpc/smp: Call rcu_cpu_starting() earlier

The call to rcu_cpu_starting() in start_secondary() is not early
enough in the CPU-hotplug onlining process, which results in lockdep
splats as follows (with CONFIG_PROVE_RCU_LIST=y):

  WARNING: suspicious RCU usage
  -----------------------------
  kernel/locking/lockdep.c:3497 RCU-list traversed in non-reader section!!

  other info that might help us debug this:

  RCU used illegally from offline CPU!
  rcu_scheduler_active = 1, debug_locks = 1
  no locks held by swapper/1/0.

  Call Trace:
  dump_stack+0xec/0x144 (unreliable)
  lockdep_rcu_suspicious+0x128/0x14c
  __lock_acquire+0x1060/0x1c60
  lock_acquire+0x140/0x5f0
  _raw_spin_lock_irqsave+0x64/0xb0
  clockevents_register_device+0x74/0x270
  register_decrementer_clockevent+0x94/0x110
  start_secondary+0x134/0x800
  start_secondary_prolog+0x10/0x14

This is avoided by adding a call to rcu_cpu_starting() near the
beginning of the start_secondary() function. Note that the
raw_smp_processor_id() is required in order to avoid calling into
lockdep before RCU has declared the CPU to be watched for readers.

It's safe to call rcu_cpu_starting() in the arch code as well as later
in generic code, as explained by Paul:

  It uses a per-CPU variable so that RCU pays attention only to the
  first call to rcu_cpu_starting() if there is more than one of them.
  This is even intentional, due to there being a generic
  arch-independent call to rcu_cpu_starting() in
  notify_cpu_starting().

  So multiple calls to rcu_cpu_starting() are fine by design.

Fixes: 4d004099 ("lockdep: Fix lockdep recursion")
Signed-off-by: Qian Cai <cai@redhat.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
[mpe: Add Fixes tag, reword slightly & expand change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201028182334.13466-1-cai@redhat.com

arch/powerpc/kernel/smp.c

@@ -1393,13 +1393,14 @@ static void add_cpu_to_masks(int cpu)
 /* Activate a secondary processor. */
 void start_secondary(void *unused)
 {
-	unsigned int cpu = smp_processor_id();
+	unsigned int cpu = raw_smp_processor_id();
 
 	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 
 	smp_store_cpu_info(cpu);
 	set_dec(tb_ticks_per_jiffy);
+	rcu_cpu_starting(cpu);
 	preempt_disable();
 	cpu_callin_map[cpu] = 1;