Re: [PATCH] rcu_barrier VS cpu_hotplug: Ensure callbacks in dead cpu are migrated to online cpu

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Ingo Molnar wrote:
> * Lai Jiangshan <laijs@cn.fujitsu.com> wrote:
> 
>> [RFC]
>> I don't like this patch, but I thought for some days and I can't
>> thought out a better one.
> 
> Interesting find. Found via code review or via testing? If via 
> testing, what is the symptom of the bug when it hits - did you 
> see CPU hotplug stress-tests hanging? Crashing too perhaps? How 
> frequently did it occur?

I found this bug when I tested the draft version of kfree_rcu(V3).

I noticed kfree_rcu_cpu_notify() is called earlier than
rcu_cpu_notify(). This means rcu_barrier() is called earlier than
RCU callbacks migration, it should lockup as expectation. But actually,
this lockup can not occurred, I tried to explore it, and I found that
rcu_barrier() does not handle cpu_hotplug. It includes two bugs.

kfree_rcu(V3) (V4 is available too, it will be sent soon):
http://lkml.org/lkml/2009/3/6/156

The V1 fix of this bug:
http://lkml.org/lkml/2009/3/7/38

The fix of the other bug: (it changed the scheduler's code too)
http://lkml.org/lkml/2009/3/7/39

Subject: [PATCH] rcu_barrier VS cpu_hotplug: Ensure callbacks in dead cpu are migrated to online cpu (V2)

cpu hotplug may be happened asynchronously, some rcu callbacks are maybe
still in dead cpu, rcu_barrier() also needs to wait for these rcu callbacks
to complete, so we must ensure callbacks in dead cpu are migrated to
online cpu.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index cae8a05..2c7b845 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -122,6 +122,8 @@ static void rcu_barrier_func(void *type)
 	}
 }
 
+static inline void wait_migrated_callbacks(void);
+
 /*
  * Orchestrate the specified type of RCU barrier, waiting for all
  * RCU callbacks of the specified type to complete.
@@ -147,6 +149,7 @@ static void _rcu_barrier(enum rcu_barrier type)
 		complete(&rcu_barrier_completion);
 	wait_for_completion(&rcu_barrier_completion);
 	mutex_unlock(&rcu_barrier_mutex);
+	wait_migrated_callbacks();
 }
 
 /**
@@ -176,9 +179,50 @@ void rcu_barrier_sched(void)
 }
 EXPORT_SYMBOL_GPL(rcu_barrier_sched);
 
+static atomic_t rcu_migrate_type_count = ATOMIC_INIT(0);
+static struct rcu_head rcu_migrate_head[3];
+static DECLARE_WAIT_QUEUE_HEAD(rcu_migrate_wq);
+
+static void rcu_migrate_callback(struct rcu_head *notused)
+{
+	if (atomic_dec_and_test(&rcu_migrate_type_count))
+		wake_up(&rcu_migrate_wq);
+}
+
+static inline void wait_migrated_callbacks(void)
+{
+	wait_event(rcu_migrate_wq, !atomic_read(&rcu_migrate_type_count));
+}
+
+static int __cpuinit rcu_barrier_cpu_hotplug(struct notifier_block *self,
+		unsigned long action, void *hcpu)
+{
+	if (action == CPU_DYING) {
+		/*
+		 * preempt_disable() in on_each_cpu() prevents stop_machine(),
+		 * so when "on_each_cpu(rcu_barrier_func, (void *)type, 1);"
+		 * returns, all online cpus have queued rcu_barrier_func(),
+		 * and the dead cpu(if it exist) queues rcu_migrate_callback()s.
+		 *
+		 * These callbacks ensure _rcu_barrier() waits for all
+		 * RCU callbacks of the specified type to complete.
+		 */
+		atomic_set(&rcu_migrate_type_count, 3);
+		call_rcu_bh(rcu_migrate_head, rcu_migrate_callback);
+		call_rcu_sched(rcu_migrate_head + 1, rcu_migrate_callback);
+		call_rcu(rcu_migrate_head + 2, rcu_migrate_callback);
+	} else if (action == CPU_POST_DEAD) {
+		/* rcu_migrate_head is protected by cpu_add_remove_lock */
+		wait_migrated_callbacks();
+	}
+
+	return NOTIFY_OK;
+}
+
 void __init rcu_init(void)
 {
 	__rcu_init();
+	hotcpu_notifier(rcu_barrier_cpu_hotplug, 0);
 }
 
 void rcu_scheduler_starting(void)