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Date: Wed, 11 Jun 2014 10:27:55 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Clark Williams <williams@redhat.com>
Subject: Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand
	unprotected when accessed by /proc)
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On Wed, Jun 11, 2014 at 07:07:05PM +0200, Oleg Nesterov wrote:
> On 06/11, Paul E. McKenney wrote:
> >
> > @@ -1202,10 +1204,14 @@ 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;
> > +	init_completion(&rnp->boost_completion);
> 
> can't rcu_init_one() do this? but this is minor,

It could, but I would have to define yet another init-time function under
CONFIG_RCU_BOOST and not.  Yeah, lazy...

> >  	raw_spin_unlock_irqrestore(&rnp->lock, flags);
> >  	rt_mutex_lock(&mtx);  /* Side effect: boosts task t's priority. */
> >  	rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */
> >
> > +	/* Wait until boostee is done accessing mtx before reinitializing. */
> > +	wait_for_completion(&rnp->boost_completion);
> > +
> 
> I must have missed something, I dont understand why we need ->boost_completion.

Because rt_mutex_init_proxy_locked() stomps on mtx periodically.
Which might happen to be work at the moment, but doesn't seem like a
particularly good thing.

> What if you simply move that rt_mutex into rcu_node ?
> 
> Or. Given that rcu_boost_kthread() never exits, it can declare this mutex
> on stack and pass the pointer to rcu_boost() ?

It is true that moving mtx to either the rcu_node structure or to
rcu_boost_kthread()'s stack frame would preserve type safety, but not
initialization safety.

Or maybe I am being too paranoid?

							Thanx, Paul