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Date: Wed, 11 Jun 2014 19:07:05 +0200
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)
Message-ID: <20140611170705.GA26816@redhat.com>
References: <20140609142956.3d79e9d1@gandalf.local.home> <CA+55aFyO8bb7fS3AaQzfMq7geERQ1JpSdcRZ9Fbs5-tZP0oRXg@mail.gmail.com> <20140609154114.20585056@gandalf.local.home> <alpine.DEB.2.10.1406101051210.3830@nanos> <20140610165704.GA3110@redhat.com> <alpine.DEB.2.10.1406101959420.5170@nanos> <20140610141306.04a4bcf3@gandalf.local.home> <alpine.DEB.2.10.1406102203430.5170@nanos> <alpine.DEB.2.10.1406102210580.5170@nanos> <20140611155251.GA4581@linux.vnet.ibm.com>
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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.