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

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Date:	Sat, 14 Jun 2014 22:40:58 -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 Fri, Jun 13, 2014 at 05:08:30PM +0200, Oleg Nesterov wrote:
> On 06/12, Paul E. McKenney wrote:
> >
> > @@ -398,11 +399,9 @@ void rcu_read_unlock_special(struct task_struct *t)
> >  #ifdef CONFIG_RCU_BOOST
> >  		if (&t->rcu_node_entry == rnp->boost_tasks)
> >  			rnp->boost_tasks = np;
> > -		/* Snapshot/clear ->rcu_boost_mutex with rcu_node lock held. */
> > -		if (t->rcu_boost_mutex) {
> > -			rbmp = t->rcu_boost_mutex;
> > -			t->rcu_boost_mutex = NULL;
> > -		}
> > +		/* Snapshot/clear ->boost_mutex with rcu_node lock held. */
> > +		if (rt_mutex_owner(&rnp->boost_mtx) == t)
> > +			rbmp = &rnp->boost_mtx;
> 
> The comment above looks confusing after this change ;) We do not clear it,
> and it doesn't explain "with rcu_node lock held".
> 
> And, with or without this change it is not obvious why do we need "rbmp",
> after this patch this becomes even more unobvious.
> 
> This is subjective of course, but perhaps it would be more understandable
> to do
> 
> 	bool xxx;
> 
> 	...
> 
> 	// Check this under rcu_node lock to ensure that unlock below
> 	// can't race with rt_mutex_init_proxy_locked() in progress.
> 	xxx = rt_mutex_owner(&rnp->boost_mtx) == t;
> 
> 	...
> 
> 	// rnp->lock was dropped
> 	if (xxx)
> 		rt_mutex_unlock(&rnp->boost_mtx);
> 
> 
> But this is very minor, I won't insist of course. Mostly I am just trying
> to check my understanding.

No, this is good, and I will update accordingly.

							Thanx, Paul

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