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Subject: Re: INFO: possible circular locking dependency detected
From: Peter Zijlstra <peterz@infradead.org>
To: Ed Tomlinson <edt@aei.ca>
Cc: paulmck@linux.vnet.ibm.com, Steven Rostedt <rostedt@goodmis.org>,
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Date: Fri, 15 Jul 2011 13:39:20 +0200
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On Fri, 2011-07-15 at 13:29 +0200, Peter Zijlstra wrote:
> 
> Paul, what appears to be happening here is that some rcu_read_unlock()
> gets interrupted, possibly before calling rcu_read_unlock_special(),
> possibly not if the interrupt is itself the timer interrupt.
> 
> Supposing ->rcu_read_unlock_special is set before, any wakeup happening
> from an interrupt hitting __rcu_read_unlock():

Hmm, ok not any wakeup from interrupt context because you have that
in_irq() test in there, but if that IRQ doesn't happen to use RCU and
does trigger softirqs and one of that softirq does a wakeup we're still
in the same boat.

> void __rcu_read_unlock(void)
> {
>         struct task_struct *t = current;
>                 
>         barrier();  /* needed if we ever invoke rcu_read_unlock in rcutree.c */
>         --t->rcu_read_lock_nesting;
>         barrier();  /* decrement before load of ->rcu_read_unlock_special */
>         if (t->rcu_read_lock_nesting == 0 &&
>             unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
>                 rcu_read_unlock_special(t);
> #ifdef CONFIG_PROVE_LOCKING
>         WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0);
> #endif /* #ifdef CONFIG_PROVE_LOCKING */
> }
> 
> After --t->rcu_read_lock_nesting, but before calling
> rcu_read_unlock_special(), will trigger this lock inversion.
> 
> The alternative case, ->rcu_read_unlock_special is not set yet, it can
> be set if the interrupt hitting in that same spot above, is the timer
> interrupt, and the wakeup happens either from the softirq ran from the
> hard IRQ tail, or as I suspect here happens, the wakeup of ksoftirqd/#.
> 
>