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

[Michael Wang <wangyun@linux.vnet.ibm.com>]

On 09/20/2012 03:44 PM, Sasha Levin wrote:
> On 09/20/2012 09:33 AM, Michael Wang wrote:
>> 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)
> 
> Looks like you're on to something, with the small patch above applied:

Thanks for the test.

So, looks like some thing 'illegal' happened in cpu_idle().

Could I know what's the platform we are focus on now?

Another possibility is that the dynaticks is wrong before enter
cpu_idle(), I think we could make a formal patch later for diagnosis.

And we need suggestions on that from Paul.

Regards,
Michael Wang

> 
> [   23.514223] ------------[ cut here ]------------
> [   23.515496] WARNING: at kernel/sched/idle_task.c:46
> put_prev_task_idle+0x1e/0x30()
> [   23.517498] Pid: 0, comm: swapper/0 Tainted: G        W
> 3.6.0-rc6-next-20120919-sasha-00001-gb54aafe-dirty #366
> [   23.520393] Call Trace:
> [   23.521882]  [<ffffffff8115167e>] ? put_prev_task_idle+0x1e/0x30
> [   23.524220]  [<ffffffff81106736>] warn_slowpath_common+0x86/0xb0
> [   23.524220]  [<ffffffff81106825>] warn_slowpath_null+0x15/0x20
> [   23.524220]  [<ffffffff8115167e>] put_prev_task_idle+0x1e/0x30
> [   23.524220]  [<ffffffff839ea61e>] __schedule+0x25e/0x8f0
> [   23.524220]  [<ffffffff81175ebd>] ? tick_nohz_idle_exit+0x18d/0x1c0
> [   23.524220]  [<ffffffff839ead05>] schedule+0x55/0x60
> [   23.524220]  [<ffffffff81078540>] cpu_idle+0x90/0x160
> [   23.524220]  [<ffffffff8383043c>] rest_init+0x130/0x144
> [   23.524220]  [<ffffffff8383030c>] ? csum_partial_copy_generic+0x16c/0x16c
> [   23.524220]  [<ffffffff858acc18>] start_kernel+0x38d/0x39a
> [   23.524220]  [<ffffffff858ac5fe>] ? repair_env_string+0x5e/0x5e
> [   23.524220]  [<ffffffff858ac326>] x86_64_start_reservations+0x101/0x105
> [   23.524220]  [<ffffffff858ac472>] x86_64_start_kernel+0x148/0x157
> [   23.524220] ---[ end trace 2c3061ab727afec2 ]---
>