linux-kernel - Re: [RFC v4] wait: prevent waiter starvation in __wait_on_bit

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Date:	Fri, 23 Jan 2009 14:30:50 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Matthew Wilcox <matthew@....cx>,
	Chuck Lever <cel@...i.umich.edu>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC v4] wait: prevent waiter starvation in __wait_on_bit_lock

On 01/23, Oleg Nesterov wrote:
>
> It is no that I think this new helper is really needed for this
> particular case, personally I agree with the patch you sent.
>
> But if we have other places with the similar problem, then perhaps
> it is better to introduce the special finish_wait_exclusive() or
> whatever.

To clarify, I suggest something like this.

	int finish_wait_exclusive(wait_queue_head_t *q, wait_queue_t *wait,
					int ret, int state, void *key)
	{
		unsigned long flags;

		__set_current_state(TASK_RUNNING);

		if (ret || !list_empty_careful(&wait->task_list)) {
			spin_lock_irqsave(&q->lock, flags);
			if (list_empty(&wait->task_list))
				 __wake_up_common(q, state, 1, key);
			else
				list_del_init(&wait->task_list);
			spin_unlock_irqrestore(&q->lock, flags);
		}

		return ret;
	}

Now, __wait_on_bit_lock() becomes:

	int __sched
	__wait_on_bit_lock(wait_queue_head_t *wq, struct wait_bit_queue *q,
				int (*action)(void *), unsigned mode)
	{
		int ret = 0;

		do {
			prepare_to_wait_exclusive(wq, &q->wait, mode);
			if (test_bit(q->key.bit_nr, q->key.flags) &&
			   (ret = (*action)(q->key.flags))
				break;
		} while (test_and_set_bit(q->key.bit_nr, q->key.flags));

		return finish_wait_exclusive(wq, &q->wait, ret, mode, &q->key);
	}

And __wait_event_interruptible_exclusive:

	#define __wait_event_interruptible_exclusive(wq, condition, ret)	\
	do {									\
		DEFINE_WAIT(__wait);						\
										\
		for (;;) {							\
			prepare_to_wait_exclusive(&wq, &__wait,			\
						TASK_INTERRUPTIBLE);		\
			if (condition)						\
				break;						\
			if (!signal_pending(current)) {				\
				schedule();					\
				continue;					\
			}							\
			ret = -ERESTARTSYS;					\
			break;							\
		}								\
		finish_wait_exclusive(&wq, &__wait,				\
					ret, TASK_INTERRUPTIBLE, NULL);		\
	} while (0)

But I can't convince myself this is what we really want. So I am not
sending the patch. And yes, we have to check ret twice.

Oleg.

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