timers: fix timer hotplug on -rt

Here we are in the CPU_DEAD notifier, and we must not sleep nor
enable interrupts.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/timer.c

@@ -1610,6 +1610,7 @@ static void __cpuinit migrate_timers(int cpu)
 {
 	struct tvec_base *old_base;
 	struct tvec_base *new_base;
+	unsigned long flags;
 	int i;
 
 	BUG_ON(cpu_online(cpu));
@@ -1619,8 +1620,11 @@ static void __cpuinit migrate_timers(int cpu)
 	 * The caller is globally serialized and nobody else
 	 * takes two locks at once, deadlock is not possible.
 	 */
-	spin_lock_irq(&new_base->lock);
-	spin_lock_nested(&old_base->lock, SINGLE_DEPTH_NESTING);
+	local_irq_save(flags);
+	while (!spin_trylock(&new_base->lock))
+		cpu_relax();
+	while (!spin_trylock(&old_base->lock))
+		cpu_relax();
 
 	BUG_ON(old_base->running_timer);
 
@@ -1634,7 +1638,9 @@ static void __cpuinit migrate_timers(int cpu)
 	}
 
 	spin_unlock(&old_base->lock);
-	spin_unlock_irq(&new_base->lock);
+	spin_unlock(&new_base->lock);
+	local_irq_restore(flags);
+
 	put_cpu_var(tvec_bases);
 }
 #endif /* CONFIG_HOTPLUG_CPU */