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

Paul E. McKenney
2014-06-15T05:41:10Z

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