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Subject: futex code and barriers
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
       Paul E McKenney <paulmck@linux.vnet.ibm.com>,
       Oleg Nesterov <oleg@tv-sign.ru>, Nick Piggin <npiggin@suse.de>
Cc: linux-kernel <linux-kernel@vger.kernel.org>
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Date: Tue, 29 Apr 2008 13:57:16 +0200
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Hi All,

While looking through the futex code I stumbled upon the following bit:

kernel/futex.c:

	/* add_wait_queue is the barrier after __set_current_state. */
	__set_current_state(TASK_INTERRUPTIBLE);
	add_wait_queue(&q.waiters, &wait);


However,

void add_wait_queue(wait_queue_head_t *q, wait_queue_t *wait)
{
	unsigned long flags;

	wait->flags &= ~WQ_FLAG_EXCLUSIVE;
	spin_lock_irqsave(&q->lock, flags);
	__add_wait_queue(q, wait);
	spin_unlock_irqrestore(&q->lock, flags);
}

static inline void __add_wait_queue(wait_queue_head_t *head, wait_queue_t *new)
{
	list_add(&new->task_list, &head->task_list);
}

static inline void list_add(struct list_head *new, struct list_head *head)
{
	__list_add(new, head, head->next);
}

static inline void __list_add(struct list_head *new,
			      struct list_head *prev,
			      struct list_head *next)
{
	next->prev = new;
	new->next = next;
	new->prev = prev;
	prev->next = new;
}

Non of which implies a full barrier.