Re: [RFC v4] wait: prevent waiter starvation in __wait_on_bit_lock

From: Oleg Nesterov
Date: Fri Jan 23 2009 - 08:33:37 EST

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;


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);
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))
} 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, \
if (condition) \
break; \
if (!signal_pending(current)) { \
schedule(); \
continue; \
} \
break; \
} \
finish_wait_exclusive(&wq, &__wait, \
} 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.


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