Messages in this thread | | | Date | Wed, 07 Aug 2013 21:29:07 +0200 | From | Carsten Emde <> | Subject | Re: [PATCH 0/8] rcu: Ensure rcu read site is deadlock-immunity |
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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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