cpufreq: powernv: Fix hardlockup due to synchronous smp_call in timer interrupt

gpstate_timer_handler() uses synchronous smp_call to set the pstate
on the requested core. This causes the below hard lockup:

  smp_call_function_single+0x110/0x180 (unreliable)
  smp_call_function_any+0x180/0x250
  gpstate_timer_handler+0x1e8/0x580
  call_timer_fn+0x50/0x1c0
  expire_timers+0x138/0x1f0
  run_timer_softirq+0x1e8/0x270
  __do_softirq+0x158/0x3e4
  irq_exit+0xe8/0x120
  timer_interrupt+0x9c/0xe0
  decrementer_common+0x114/0x120
  -- interrupt: 901 at doorbell_global_ipi+0x34/0x50
  LR = arch_send_call_function_ipi_mask+0x120/0x130
  arch_send_call_function_ipi_mask+0x4c/0x130
  smp_call_function_many+0x340/0x450
  pmdp_invalidate+0x98/0xe0
  change_huge_pmd+0xe0/0x270
  change_protection_range+0xb88/0xe40
  mprotect_fixup+0x140/0x340
  SyS_mprotect+0x1b4/0x350
  system_call+0x58/0x6c

One way to avoid this is removing the smp-call. We can ensure that the
timer always runs on one of the policy-cpus. If the timer gets
migrated to a cpu outside the policy then re-queue it back on the
policy->cpus. This way we can get rid of the smp-call which was being
used to set the pstate on the policy->cpus.

Fixes: 7bc54b652f13 ("timers, cpufreq/powernv: Initialize the gpstate timer as pinned")
Cc: stable@vger.kernel.org # v4.8+
Reported-by: Nicholas Piggin <npiggin@gmail.com>
Reported-by: Pridhiviraj Paidipeddi <ppaidipe@linux.vnet.ibm.com>
Signed-off-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
Acked-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

drivers/cpufreq/powernv-cpufreq.c

@@ -679,6 +679,16 @@ void gpstate_timer_handler(struct timer_list *t)
 
 	if (!spin_trylock(&gpstates->gpstate_lock))
 		return;
+	/*
+	 * If the timer has migrated to the different cpu then bring
+	 * it back to one of the policy->cpus
+	 */
+	if (!cpumask_test_cpu(raw_smp_processor_id(), policy->cpus)) {
+		gpstates->timer.expires = jiffies + msecs_to_jiffies(1);
+		add_timer_on(&gpstates->timer, cpumask_first(policy->cpus));
+		spin_unlock(&gpstates->gpstate_lock);
+		return;
+	}
 
 	/*
 	 * If PMCR was last updated was using fast_swtich then
@@ -718,10 +728,8 @@ void gpstate_timer_handler(struct timer_list *t)
 	if (gpstate_idx != gpstates->last_lpstate_idx)
 		queue_gpstate_timer(gpstates);
 
+	set_pstate(&freq_data);
 	spin_unlock(&gpstates->gpstate_lock);
-
-	/* Timer may get migrated to a different cpu on cpu hot unplug */
-	smp_call_function_any(policy->cpus, set_pstate, &freq_data, 1);
 }
 
 /*