ARM: SMP: wait for CPU to be marked active

When we bring a CPU online, we should wait for it to become active
before entering the idle thread, so we know that the scheduler and
thread migration is going to work.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/smp.c

@@ -318,9 +318,13 @@ asmlinkage void __cpuinit secondary_start_kernel(void)
 	smp_store_cpu_info(cpu);
 
 	/*
-	 * OK, now it's safe to let the boot CPU continue
+	 * OK, now it's safe to let the boot CPU continue.  Wait for
+	 * the CPU migration code to notice that the CPU is online
+	 * before we continue.
 	 */
 	set_cpu_online(cpu, true);
+	while (!cpu_active(cpu))
+		cpu_relax();
 
 	/*
 	 * OK, it's off to the idle thread for us