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

Oleg Nesterov
2014-06-11T17:08:30Z

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,

> 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() ?

Oleg.