stop_machine() vs. synchronous IPI send deadlock

Hello Andrew,

This patch 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.

Kirill

@@ -114,13 +114,12 @@ 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;

Sorry, hunk with corresponding preempt_enable was lost, sending patch again.

This patch 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.

Kirill

@@ -114,13 +114,12 @@ 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;

@@ -130,6 +129,7 @@ static void restart_machine(void)
 {
 	stopmachine_set_state(STOPMACHINE_EXIT);
 	local_irq_enable();
+	preempt_enable_no_resched();
 }
 
 struct stop_machine_data