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

From: Oleg Nesterov <oleg@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	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)
Date: Wed, 11 Jun 2014 19:17:34 +0200	[thread overview]
Message-ID: <20140611171734.GA27457@redhat.com> (raw)
In-Reply-To: <20140611170705.GA26816@redhat.com>

On 06/11, Oleg Nesterov wrote:
>
> On 06/11, Paul E. McKenney wrote:
>
> >  	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.
>
> 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() ?

Ah, please ignore, I forgot about init_proxy_locked(). Although perhaps this
can be solved easily.

Oleg.