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

Paul E. McKenney
2014-06-11T17:30:09Z

On Wed, Jun 11, 2014 at 07:17:34PM +0200, Oleg Nesterov wrote:
> 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.

You beat me to it. ;-)

I was thinking of ->boost_completion as the way to solve it easily, but
what did you have in mind?

Thanx, Paul