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

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Date:	Mon, 6 Aug 2007 09:30:40 +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,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH] pi-futex: set PF_EXITING without taking ->pi_lock


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

> Question: should we make spinlock_t barrier-safe?
> 
> Suppose that the task "p" does
> 
> 	current->state = TASK_INTERRUPIBLE;
> 	mb();
> 
> 	if (CONDITION)
> 		break;
> 
> 	schedule();
> 
> and another CPU does
> 
> 	CONDITION = 1;
> 	try_to_wake_up(p);
> 
> 
> This is commonly used, but not correct _in theory_. If wake_up() happens
> when p->array != NULL, we have
> 
> 	CONDITION = 1;			// [1]
> 	spin_lock(rq->lock);
> 	task->state = TASK_RUNNING;	// [2]
> 
> and we can miss an event. Because in theory [1] may leak into the critical
> section, and could be re-ordered with [2].
> 
> Another problem is that try_to_wake_up() first checks task->state and does
> nothing if it is TASK_RUNNING, so we need a full mb(), not just wmb().
> 
> Should we change spin_lock(), or introduce smp_mb_before_spinlock(), or I
> missed something?
> 
> NOTE: I do not pretend to know what kind of barrier spin_lock() provides
> in practice, but according to the documentation lock() is only a one-way
> barrier.

i think your worry is legitimate.

spin_lock() provides a full barrier on most platforms (certainly so on 
x86). But ... ia64 might have it as a one-way barrier?

	Ingo
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