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 <[e-mail]>
Acked-by: Cyrill Gorcunov <[e-mail]>
Reviewed-by: Pekka Enberg <[e-mail]>
Cc: Yinghai Lu <[e-mail]>
Cc: [e-mail]
LKML-Reference: <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>

arch/x86/kernel/apic/apic.c

···
 			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;
···
 	}
 #endif
 	perf_events_lapic_init();
-
-	preempt_disable();
 
 	/*
 	 * Double-check whether this APIC is really registered.
···
 	 * 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;
···
 		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
 }