[PATCH] stop_machine() vs. synchronous IPI send deadlock

This fixes deadlock of stop_machine() vs.  synchronous IPI send.  The
problem is that stop_machine() disables interrupts before disabling
preemption on other CPUs.  So if another CPU is preempted and then calls
something like flush_tlb_all() it will deadlock with CPU doing
stop_machine() and which can't process IPI due to disabled IRQs.

I changed stop_machine() to do the same things exactly as it does on other
CPUs, i.e.  it should disable preemption first on _all_ CPUs including
itself and only after that disable IRQs.
Signed-off-by: NKirill Korotaev <dev@sw.ru>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: "Andrey Savochkin" <saw@sawoct.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

kernel/stop_machine.c

@@ -119,13 +119,12 @@ static int stop_machine(void)
 		return ret;
 	}
 
-	/* Don't schedule us away at this point, please. */
-	local_irq_disable();
-
 	/* Now they are all started, make them hold the CPUs, ready. */
+	preempt_disable();
 	stopmachine_set_state(STOPMACHINE_PREPARE);
 
 	/* Make them disable irqs. */
+	local_irq_disable();
 	stopmachine_set_state(STOPMACHINE_DISABLE_IRQ);
 
 	return 0;
@@ -135,6 +134,7 @@ static void restart_machine(void)
 {
 	stopmachine_set_state(STOPMACHINE_EXIT);
 	local_irq_enable();
+	preempt_enable_no_resched();
 }
 
 struct stop_machine_data