x86/mce/therm_throt: Do not access uninitialized therm_work

It is relatively easy to trigger the following boot splat on an Ice Lake
client platform. The call stack is like:

  kernel BUG at kernel/timer/timer.c:1152!

  Call Trace:
  __queue_delayed_work
  queue_delayed_work_on
  therm_throt_process
  intel_thermal_interrupt
  ...

The reason is that a CPU's thermal interrupt is enabled prior to
executing its hotplug onlining callback which will initialize the
throttling workqueues.

Such a race can lead to therm_throt_process() accessing an uninitialized
therm_work, leading to the above BUG at a very early bootup stage.

Therefore, unmask the thermal interrupt vector only after having setup
the workqueues completely.

 [ bp: Heavily massage commit message and correct comment formatting. ]

Fixes: f6656208 ("x86/mce/therm_throt: Optimize notifications of thermal throttle")
Signed-off-by: Chuansheng Liu <chuansheng.liu@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20200107004116.59353-1-chuansheng.liu@intel.com

arch/x86/kernel/cpu/mce/therm_throt.c

@@ -467,6 +467,7 @@ static int thermal_throttle_online(unsigned int cpu)
 {
 	struct thermal_state *state = &per_cpu(thermal_state, cpu);
 	struct device *dev = get_cpu_device(cpu);
+	u32 l;
 
 	state->package_throttle.level = PACKAGE_LEVEL;
 	state->core_throttle.level = CORE_LEVEL;
@@ -474,6 +475,10 @@ static int thermal_throttle_online(unsigned int cpu)
 	INIT_DELAYED_WORK(&state->package_throttle.therm_work, throttle_active_work);
 	INIT_DELAYED_WORK(&state->core_throttle.therm_work, throttle_active_work);
 
+	/* Unmask the thermal vector after the above workqueues are initialized. */
+	l = apic_read(APIC_LVTTHMR);
+	apic_write(APIC_LVTTHMR, l & ~APIC_LVT_MASKED);
+
 	return thermal_throttle_add_dev(dev, cpu);
 }
 
@@ -722,10 +727,6 @@ void intel_init_thermal(struct cpuinfo_x86 *c)
 	rdmsr(MSR_IA32_MISC_ENABLE, l, h);
 	wrmsr(MSR_IA32_MISC_ENABLE, l | MSR_IA32_MISC_ENABLE_TM1, h);
 
-	/* Unmask the thermal vector: */
-	l = apic_read(APIC_LVTTHMR);
-	apic_write(APIC_LVTTHMR, l & ~APIC_LVT_MASKED);
-
 	pr_info_once("CPU0: Thermal monitoring enabled (%s)\n",
 		      tm2 ? "TM2" : "TM1");