Date: Wed, 7 Aug 2013 12:52:15 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Carsten Emde <C.Emde@osadl.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>, Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/8] rcu: Ensure rcu read site is deadlock-immunity
Message-ID: <20130807195215.GF4306@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
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On Wed, Aug 07, 2013 at 09:29:07PM +0200, 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.

I would rather have been wrong, but thank you for trying it out!

							Thanx, Paul

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