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

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Date:	Thu, 12 Dec 2013 12:18:09 +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:
> +/*
> + * Event API
> + */
> +#define __swait_event(wq, condition)					\
> +do {									\
> +	DEFINE_SWAITER(__wait);						\
> +									\
> +	for (;;) {							\
> +		swait_prepare(&wq, &__wait, TASK_UNINTERRUPTIBLE);	\
> +		if (condition)						\
> +			break;						\
> +		schedule();						\
> +	}								\
> +	swait_finish(&wq, &__wait);					\
> +} while (0)
> +
> +#define __swait_event_interruptible(wq, condition, ret)			\
> +do {									\
> +	DEFINE_SWAITER(__wait);						\
> +									\
> +	for (;;) {							\
> +		swait_prepare(&wq, &__wait, TASK_INTERRUPTIBLE);	\
> +		if (condition)						\
> +			break;						\
> +		if (signal_pending(current)) {				\
> +			ret = -ERESTARTSYS;				\
> +			break;						\
> +		}							\
> +		schedule();						\
> +	}								\
> +	swait_finish(&wq, &__wait);					\
> +} while (0)
> +
> +#define __swait_event_interruptible_timeout(wq, condition, ret)		\
> +do {									\
> +	DEFINE_SWAITER(__wait);						\
> +									\
> +	for (;;) {							\
> +		swait_prepare(&wq, &__wait, TASK_INTERRUPTIBLE);	\
> +		if (condition)						\
> +			break;						\
> +		if (signal_pending(current)) {				\
> +			ret = -ERESTARTSYS;				\
> +			break;						\
> +		}							\
> +		ret = schedule_timeout(ret);				\
> +		if (!ret)						\
> +			break;						\
> +	}								\
> +	swait_finish(&wq, &__wait);					\
> +} while (0)

Urgh, please have a look at ___wait_event() we just killed all the
pointless replication for the normal waitqueues, please don't add more
of it.


> +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;
> +}
> +
> +unsigned int
> +__swake_up(struct swait_queue_head *head, unsigned int state, unsigned int num)
> +{
> +	unsigned long flags;
> +	int woken;
> +
> +	if (!swaitqueue_active(head))
> +		return 0;
> +
> +	raw_spin_lock_irqsave(&head->lock, flags);
> +	woken = __swake_up_locked(head, state, num);
> +	raw_spin_unlock_irqrestore(&head->lock, flags);
> +	return woken;
> +}
> +EXPORT_SYMBOL(__swake_up);

Urgh, fail. Do not put unbounded loops in raw_spin_lock.

I think I posted a patch a while back to cure this.


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