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

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Message-ID: <20140611170705.GA26816@redhat.com>
Date:	Wed, 11 Jun 2014 19:07:05 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 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.

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