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Date: Fri, 13 Jun 2014 17:08:30 +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: <20140613150830.GB31794@redhat.com>
References: <20140611170705.GA26816@redhat.com> <20140611171734.GA27457@redhat.com> <20140611172958.GF4581@linux.vnet.ibm.com> <20140611175934.GA28912@redhat.com> <20140611195613.GM4581@linux.vnet.ibm.com> <20140612172844.GA15795@redhat.com> <20140612203518.GZ4581@linux.vnet.ibm.com> <alpine.DEB.2.10.1406122337060.5170@nanos> <20140612222748.GF4581@linux.vnet.ibm.com> <20140612231944.GA30683@linux.vnet.ibm.com>
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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.

Oleg.