linux-kernel - Re: INFO: possible circular locking dependency detected

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Date:	Thu, 14 Jul 2011 13:02:09 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: INFO: possible circular locking dependency detected

On Thu, 2011-07-14 at 12:58 -0400, Steven Rostedt wrote:

> 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);
> 
> Thus the question is, how did we get rcu_read_unlock_special set here?

Looks like another process could set this with:

static int rcu_boost(struct rcu_node *rnp)
{
	[...]
	t = container_of(tb, struct task_struct, rcu_node_entry);
	rt_mutex_init_proxy_locked(&mtx, t);
	t->rcu_boost_mutex = &mtx;
	t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BOOSTED;

-- Steve


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