Re: RCU idle CPU detection is broken in linux-next

[Michael Wang]

On 09/20/2012 01:06 AM, Paul E. McKenney wrote:
> On Wed, Sep 19, 2012 at 06:35:36PM +0200, Sasha Levin wrote:
>> On 09/19/2012 05:39 PM, Paul E. McKenney wrote:
>>> On Wed, Sep 12, 2012 at 07:56:48PM +0200, Sasha Levin wrote:
>>>>> Hi Paul,
>>>>>
>>>>> While fuzzing using trinity inside a KVM tools guest, I've managed to trigger
>>>>> "RCU used illegally from idle CPU!" warnings several times.
>>>>>
>>>>> There are a bunch of traces which seem to pop exactly at the same time and from
>>>>> different places around the kernel. Here are several of them:
>>> Hello, Sasha,
>>>
>>> OK, interesting.  Could you please try reproducing with the diagnostic
>>> patch shown below?
>>
>> Sure - here are the results (btw, it reproduces very easily):
>>
>> [ 13.525119] ================================================
>> [ 13.527165] [ BUG: lock held when returning to user space! ]
>> [ 13.528752] 3.6.0-rc6-next-20120918-sasha-00002-g190c311-dirty #362 Tainted: GW
>> [ 13.531314] ------------------------------------------------
>> [ 13.532918] init/1 is leaving the kernel with locks still held!
>> [ 13.534574] 1 lock held by init/1:
>> [ 13.535533] #0: (rcu_idle){.+.+..}, at: [<ffffffff811c36d0>]
>> rcu_eqs_enter_common+0x1a0/0x9a0
>>
>> I'm basically seeing lots of the above, so I can't even get to the point where I
>> get the previous lockdep warnings.
> 
> OK, that diagnostic patch was unhelpful.  Back to the drawing board...

May be we could first make sure the cpu_idle() behave properly?

Since according to the log, rcu think cpu is idle while current pid
is not 0, that could happen if things broken in cpu_idle() which
is very dependent on platform.

So check it when idle thread was switched out may could be the first
step? some thing like below.

Regards,
Michael Wang

diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
index b6baf37..f8c7354 100644
--- a/kernel/sched/idle_task.c
+++ b/kernel/sched/idle_task.c
@@ -43,6 +43,7 @@ dequeue_task_idle(struct rq *rq, struct task_struct *p, int flags)
 
 static void put_prev_task_idle(struct rq *rq, struct task_struct *prev)
 {
+       WARN_ON(rcu_is_cpu_idle());
 }
 
 static void task_tick_idle(struct rq *rq, struct task_struct *curr, int queued)


> 
> 							Thanx, Paul
> 
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