stop_machine: Fix rcu_momentary_eqs() call in multi_cpu_stop()

The multi_cpu_stop() contains a loop that can initially be executed with
interrupts enabled (in the MULTI_STOP_NONE and MULTI_STOP_PREPARE states).
Interrupts are guaranteed to be once the MULTI_STOP_DISABLE_IRQ state
is reached.  Unfortunately, the rcu_momentary_eqs() function that is
currently invoked on each pass through this loop requires that interrupts
be disabled.

This commit therefore moves this call to rcu_momentary_eqs() to the body
of the "else if (curstate > MULTI_STOP_PREPARE)" portion of the loop, thus
guaranteeing that interrupts will be disabled on each call, as required.

Kudos to 朱恺乾 (Kaiqian) for noting that this had not made it to mainline.

[ paulmck: Update from rcu_momentary_dyntick_idle() to rcu_momentary_eqs(). ]

Link: https://lore.kernel.org/all/[email protected]/

Signed-off-by: Mukesh Ojha <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

Author: Mukesh Ojha

kernel/stop_machine.c

@@ -250,8 +250,8 @@ static int multi_cpu_stop(void *data)
 			 * be detected and reported on their side.
 			 */
 			touch_nmi_watchdog();
+			rcu_momentary_eqs();
 		}
-		rcu_momentary_eqs();
 	} while (curstate != MULTI_STOP_EXIT);
 
 	local_irq_restore(flags);