linux-kernel - Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand unprotected when accessed by /proc)

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Date:	Wed, 11 Jun 2014 10:27:55 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Clark Williams <williams@...hat.com>
Subject: Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand
	unprotected when accessed by /proc)

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

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