linux-kernel - Re: [PATCH 1/3] wait-simple: Introduce the simple waitqueue implementation

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Date:	Thu, 12 Dec 2013 12:44:47 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Gortmaker <paul.gortmaker@...driver.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] wait-simple: Introduce the simple waitqueue
 implementation

On Wed, Dec 11, 2013 at 08:06:37PM -0500, Paul Gortmaker wrote:
> +/* Adds w to head->task_list. Must be called with head->lock locked. */
> +static inline void __swait_enqueue(struct swait_queue_head *head,
> +				   struct swaiter *w)
> +{
> +	list_add(&w->node, &head->task_list);
> +	/* We can't let the condition leak before the setting of head */
> +	smp_mb();
> +}

> +unsigned int
> +__swake_up_locked(struct swait_queue_head *head, unsigned int state,
> +		  unsigned int num)
> +{
> +	struct swaiter *curr, *next;
> +	int woken = 0;
> +
> +	list_for_each_entry_safe(curr, next, &head->task_list, node) {
> +		if (wake_up_state(curr->task, state)) {
> +			__swait_dequeue(curr);
> +			/*
> +			 * The waiting task can free the waiter as
> +			 * soon as curr->task = NULL is written,
> +			 * without taking any locks. A memory barrier
> +			 * is required here to prevent the following
> +			 * store to curr->task from getting ahead of
> +			 * the dequeue operation.
> +			 */
> +			smp_wmb();
> +			curr->task = NULL;
> +			if (++woken == num)
> +				break;
> +		}
> +	}
> +	return woken;
> +}

Are these two barriers matched or are they both unmatched and thus
probabyl wrong?

In any case the comments need updating to be more explicit.
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