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

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Date:	Wed, 07 Aug 2013 21:29:07 +0200
From:	Carsten Emde <C.Emde@...dl.org>
To:	paulmck@...ux.vnet.ibm.com
CC:	Lai Jiangshan <laijs@...fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/8] rcu: Ensure rcu read site is deadlock-immunity

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.
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