softirq: Fix unplug deadlock

PATCH New Peter Zijlstra 1 participant 1 message Sep 30, 2011

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Peter Zijlstra

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2011-09-30 13:59 UTC #0 lore raw

PATCH 1/1 softirq: Fix unplug deadlock

Subject: [RT] softirq: Fix unplug deadlock
From: Peter Zijlstra <[email protected]>
Date: Fri Sep 30 15:52:14 CEST 2011

If ksoftirqd gets woken during hot-unplug, __thread_do_softirq() will call pin_current_cpu() which will block on the held cpu_hotplug.lock. Moving the offline check in __thread_do_softirq() before the pin_current_cpu() call doesn't work, since the wakeup can happen before we mark the cpu offline.

So here we have the ksoftirq thread stuck until hotplug finishes, but then the ksoftirq CPU_DOWN notifier issues kthread_stop() which will wait for the ksoftirq thread to go away -- while holding the hotplug lock.

Sort this by delaying the kthread_stop() until CPU_POST_DEAD, which is outside of the cpu_hotplug.lock, but still serialized by the cpu_add_remove_lock.

Signed-off-by Peter Zijlstra <[email protected]>

---
 kernel/softirq.c |    8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

kernel/softirq.c +2 -6

Index: linux-rt/kernel/softirq.c
===================================================================
@@ -1104,9 +1104,8 @@ static int __cpuinit cpu_callback(struct
 	int hotcpu = (unsigned long)hcpu;
 	struct task_struct *p;
 
-	switch (action) {
+	switch (action & ~CPU_TASKS_FROZEN) {
 	case CPU_UP_PREPARE:
-	case CPU_UP_PREPARE_FROZEN:
 		p = kthread_create_on_node(run_ksoftirqd,
 					   hcpu,
 					   cpu_to_node(hotcpu),

@@ -1119,19 +1118,16 @@ static int __cpuinit cpu_callback(struct
   		per_cpu(ksoftirqd, hotcpu) = p;
  		break;
 	case CPU_ONLINE:
-	case CPU_ONLINE_FROZEN:
 		wake_up_process(per_cpu(ksoftirqd, hotcpu));
 		break;
 #ifdef CONFIG_HOTPLUG_CPU
 	case CPU_UP_CANCELED:
-	case CPU_UP_CANCELED_FROZEN:
 		if (!per_cpu(ksoftirqd, hotcpu))
 			break;
 		/* Unbind so it can run.  Fall thru. */
 		kthread_bind(per_cpu(ksoftirqd, hotcpu),
 			     cpumask_any(cpu_online_mask));
-	case CPU_DEAD:
-	case CPU_DEAD_FROZEN: {
+	case CPU_POST_DEAD: {
 		static const struct sched_param param = {
 			.sched_priority = MAX_RT_PRIO-1
 		};