stop_machine: Provide RCU quiescent state in multi_cpu_stop()

When multi_cpu_stop() loops waiting for other tasks, it can trigger an RCU
CPU stall warning.  This can be misleading because what is instead needed
is information on whatever task is blocking multi_cpu_stop().  This commit
therefore inserts an RCU quiescent state into the multi_cpu_stop()
function's waitloop.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

include/linux/rcutree.h

@@ -37,6 +37,7 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func);
 
 void rcu_barrier(void);
 bool rcu_eqs_special_set(int cpu);
+void rcu_momentary_dyntick_idle(void);
 unsigned long get_state_synchronize_rcu(void);
 void cond_synchronize_rcu(unsigned long oldstate);
 

kernel/rcu/tree.c

@@ -364,7 +364,7 @@ bool rcu_eqs_special_set(int cpu)
  *
  * The caller must have disabled interrupts and must not be idle.
  */
-static void __maybe_unused rcu_momentary_dyntick_idle(void)
+void rcu_momentary_dyntick_idle(void)
 {
 	int special;
 

kernel/stop_machine.c

@@ -233,6 +233,7 @@ static int multi_cpu_stop(void *data)
 			 */
 			touch_nmi_watchdog();
 		}
+		rcu_momentary_dyntick_idle();
 	} while (curstate != MULTI_STOP_EXIT);
 
 	local_irq_restore(flags);