x86: apic: Cleanup and simplify setup_local_APIC()

setup_local_APIC() is used to setup local APIC early during CPU
initialization and already assumes that preemption is disabled on
entry. However, The function unnecessarily disables and enables
preemption and uses smp_processor_id() multiple times in and out of
the nested preemption disabled section. This gives the wrong
impression that the function might be able to handle being called with
preemption enabled and/or migrated to another processor in the middle.

Make it clear that the function is always called with preemption
disabled, drop the confusing preemption disable block and call
smp_processor_id() once at the beginning of the function.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: brgerst@gmail.com
LKML-Reference: <4D00B3B9.7060702@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/apic/apic.c

@@ -1195,12 +1195,15 @@ static void __cpuinit lapic_setup_esr(void)
 			oldvalue, value);
 }
 
-
 /**
  * setup_local_APIC - setup the local APIC
+ *
+ * Used to setup local APIC while initializing BSP or bringin up APs.
+ * Always called with preemption disabled.
  */
 void __cpuinit setup_local_APIC(void)
 {
+	int cpu = smp_processor_id();
 	unsigned int value, queued;
 	int i, j, acked = 0;
 	unsigned long long tsc = 0, ntsc;
@@ -1225,8 +1228,6 @@ void __cpuinit setup_local_APIC(void)
 #endif
 	perf_events_lapic_init();
 
-	preempt_disable();
-
 	/*
 	 * Double-check whether this APIC is really registered.
 	 * This is meaningless in clustered apic mode, so we skip it.
@@ -1342,21 +1343,19 @@ void __cpuinit setup_local_APIC(void)
 	 * TODO: set up through-local-APIC from through-I/O-APIC? --macro
 	 */
 	value = apic_read(APIC_LVT0) & APIC_LVT_MASKED;
-	if (!smp_processor_id() && (pic_mode || !value)) {
+	if (!cpu && (pic_mode || !value)) {
 		value = APIC_DM_EXTINT;
-		apic_printk(APIC_VERBOSE, "enabled ExtINT on CPU#%d\n",
-				smp_processor_id());
+		apic_printk(APIC_VERBOSE, "enabled ExtINT on CPU#%d\n", cpu);
 	} else {
 		value = APIC_DM_EXTINT | APIC_LVT_MASKED;
-		apic_printk(APIC_VERBOSE, "masked ExtINT on CPU#%d\n",
-				smp_processor_id());
+		apic_printk(APIC_VERBOSE, "masked ExtINT on CPU#%d\n", cpu);
 	}
 	apic_write(APIC_LVT0, value);
 
 	/*
 	 * only the BP should see the LINT1 NMI signal, obviously.
 	 */
-	if (!smp_processor_id())
+	if (!cpu)
 		value = APIC_DM_NMI;
 	else
 		value = APIC_DM_NMI | APIC_LVT_MASKED;
@@ -1364,11 +1363,9 @@ void __cpuinit setup_local_APIC(void)
 		value |= APIC_LVT_LEVEL_TRIGGER;
 	apic_write(APIC_LVT1, value);
 
-	preempt_enable();
-
 #ifdef CONFIG_X86_MCE_INTEL
 	/* Recheck CMCI information after local APIC is up on CPU #0 */
-	if (smp_processor_id() == 0)
+	if (!cpu)
 		cmci_recheck();
 #endif
 }