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SubjectRe: [PATCH 2/2] sched/wait: avoid abort_exclusive_wait() in __wait_on_bit_lock()
Date
> > On 08/26, Oleg Nesterov wrote:
> >
> > We do not need anything tricky to avoid the race, we can just call
> > finish_wait() if action() fails. test_and_set_bit() implies mb() so
> > the lockless list_empty_careful() case is fine, we can not miss the
> > condition if we race with unlock_page().
>
> To simplify the review see the code with the patch applied:
>
>
> int __sched
> __wait_on_bit_lock(wait_queue_head_t *wq, struct wait_bit_queue *q,
> wait_bit_action_f *action, unsigned mode)
> {
> int ret = 0;
>
> for (;;) {
> prepare_to_wait_exclusive(wq, &q->wait, mode);
> if (test_bit(q->key.bit_nr, q->key.flags)) {
> ret = action(&q->key, mode);
> /*
> * Ensure that clear_bit() + wake_up() right after
> * test_and_set_bit() below can't see us; it should
> * wake up another exclusive waiter if we fail.
> */
> if (ret)
> finish_wait(wq, &q->wait);
> }
> if (!test_and_set_bit(q->key.bit_nr, q->key.flags)) {
> if (!ret)
> finish_wait(wq, &q->wait);
> return 0;
> } else if (ret) {
> return ret;
> }
> }
> }
>
Can we fold two bit operations into one?

thanks
Hillf
--- linux-4.7/kernel/sched/wait.c Mon Jul 25 03:23:50 2016
+++ b/kernel/sched/wait.c Mon Aug 29 16:17:01 2016
@@ -425,20 +425,17 @@ int __sched
__wait_on_bit_lock(wait_queue_head_t *wq, struct wait_bit_queue *q,
wait_bit_action_f *action, unsigned mode)
{
- do {
- int ret;
+ int ret = 0;

- prepare_to_wait_exclusive(wq, &q->wait, mode);
- if (!test_bit(q->key.bit_nr, q->key.flags))
- continue;
+ prepare_to_wait_exclusive(wq, &q->wait, mode);
+ do {
+ if (!test_and_set_bit(q->key.bit_nr, q->key.flags))
+ break;
ret = action(&q->key, mode);
- if (!ret)
- continue;
- abort_exclusive_wait(wq, &q->wait, mode, &q->key);
- return ret;
- } while (test_and_set_bit(q->key.bit_nr, q->key.flags));
+ } while (!ret);
+
finish_wait(wq, &q->wait);
- return 0;
+ return ret;
}
EXPORT_SYMBOL(__wait_on_bit_lock);

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