Re: [PATCH 2/2] sched/wait: avoid abort_exclusive_wait() in __wait_on_bit_lock()

["Hillf Danton" <hillf.zj@alibaba-inc.com>]

> > 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);
 
--