s390/smp: smp_emergency_stop() - move cpumask away from stack

Make "cpumask_t cpumask" a static variable to avoid a potential large
stack frame. Also protect against potential concurrent callers by
introducing a local lock.
Note: smp_emergency_stop() gets only called with irqs and machine
checks disabled, therefore a cpu local deadlock is not possible. For
concurrent callers the first cpu which enters the critical section
wins and will stop all other cpus.
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/smp.c

@@ -434,10 +434,12 @@ void notrace smp_yield_cpu(int cpu)
  */
 void notrace smp_emergency_stop(void)
 {
-	cpumask_t cpumask;
+	static arch_spinlock_t lock = __ARCH_SPIN_LOCK_UNLOCKED;
+	static cpumask_t cpumask;
 	u64 end;
 	int cpu;
 
+	arch_spin_lock(&lock);
 	cpumask_copy(&cpumask, cpu_online_mask);
 	cpumask_clear_cpu(smp_processor_id(), &cpumask);
 
@@ -458,6 +460,7 @@ void notrace smp_emergency_stop(void)
 			break;
 		cpu_relax();
 	}
+	arch_spin_unlock(&lock);
 }
 NOKPROBE_SYMBOL(smp_emergency_stop);