linux-kernel - Re: [PATCH] pi-futex: set PF_EXITING without taking ->pi

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Date:	Sat, 21 Jul 2007 14:31:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	Eric Dumazet <dada1@...mosbay.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ulrich Drepper <drepper@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pi-futex: set PF_EXITING without taking ->pi_lock


* Oleg Nesterov <oleg@...sign.ru> wrote:

> It is a bit annoying that do_exit() takes ->pi_lock to set PF_EXITING.
> All we need is to synchronize with lookup_pi_state() which saw this task
> without PF_EXITING under ->pi_lock.
> 
> Change do_exit() to use spin_unlock_wait().
> 
> Signed-off-by: Oleg Nesterov <oleg@...sign.ru>

Acked-by: Ingo Molnar <mingo@...e.hu>

> -	spin_lock_irq(&tsk->pi_lock);
> -	tsk->flags |= PF_EXITING;
> -	spin_unlock_irq(&tsk->pi_lock);
> +	smp_mb();
> +	spin_unlock_wait(&tsk->pi_lock);

hm, isnt spin_unlock_wait() an SMP barrier in itself? (if not then it 
should be.)

	Ingo
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