Re: [PATCH 0/8] rcu: Ensure rcu read site is deadlock-immunity

[Lai Jiangshan]

On 08/08/2013 03:29 AM, Carsten Emde wrote:
> Hi Paul,
> 
>>> Although all articles declare that rcu read site is deadlock-immunity.
>>> It is not true for rcu-preempt, it will be deadlock if rcu read site
>>> overlaps with scheduler lock.
>>
>> The real rule is that if the scheduler does its outermost rcu_read_unlock()
>> with one of those locks held, it has to have avoided enabling preemption
>> through the entire RCU read-side critical section.
>>
>> That said, avoiding the need for this rule would be a good thing.
>>
>> How did you test this?  The rcutorture tests will not exercise this.
>> (Intentionally so, given that it can deadlock!)
>>
>>> ec433f0c, 10f39bb1 and 016a8d5b just partially solve it. But rcu read site
>>> is still not deadlock-immunity. And the problem described in 016a8d5b
>>> is still existed(rcu_read_unlock_special() calls wake_up).
>>>
>>> The problem is fixed in patch5.
>>
>> This is going to require some serious review and testing.  One requirement
>> is that RCU priority boosting not persist significantly beyond the
>> re-enabling of interrupts associated with the irq-disabled lock.  To do
>> otherwise breaks RCU priority boosting.  At first glance, the added
>> set_need_resched() might handle this, but that is part of the review
>> and testing required.
>>
>> Steven, would you and Carsten be willing to try this and see if it
>> helps with the issues you are seeing in -rt?  (My guess is "no", since
>> a deadlock would block forever rather than waking up after a couple
>> thousand seconds, but worth a try.)
> Your guess was correct, applying this patch doesn't heal the NO_HZ_FULL+PREEMPT_RT_FULL 3.10.4 based system; it still is hanging at -> synchronize_rcu -> wait_rcu_gp.
> 
>     -Carsten.
> 

I didn't find the problem you reported, could you give me a url?

Thanx,
Lai
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