Messages in this thread | | | Date | Fri, 23 Jan 2009 10:59:04 +0100 | From | Johannes Weiner <> | Subject | Re: [RFC v4] wait: prevent waiter starvation in __wait_on_bit_lock |
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On Thu, Jan 22, 2009 at 09:25:50PM +0100, Oleg Nesterov wrote: > On 01/21, Johannes Weiner wrote: > > > > @@ -187,6 +187,31 @@ __wait_on_bit_lock(wait_queue_head_t *wq, struct wait_bit_queue *q, > > } > > } while (test_and_set_bit(q->key.bit_nr, q->key.flags)); > > finish_wait(wq, &q->wait); > > + if (unlikely(ret)) { > > + /* > > + * Contenders are woken exclusively. If we were woken > > + * by an unlock we have to take the lock ourselves and > > + * wake the next contender on unlock. But the waiting > > + * function failed, we do not take the lock and won't > > + * unlock in the future. Make sure the next contender > > + * does not wait forever on an unlocked bit. > > + * > > + * We can also get here without being woken through > > + * the waitqueue, so there is a small chance of doing a > > + * bogus wake up between an unlock clearing the bit and > > + * the next contender being woken up and setting it again. > > + * > > + * It does no harm, though, the scheduler will ignore it > > + * as the process in question is already running. > > + * > > + * The unlock path clears the bit and then wakes up the > > + * next contender. If the next contender is us, the > > + * barrier makes sure we also see the bit cleared. > > + */ > > + smp_rmb(); > > + if (!test_bit(q->key.bit_nr, q->key.flags))) > > + __wake_up_bit(wq, q->key.flags, q->key.bit_nr); > > I think this is correct, and (unfortunately ;) you are right: > we need rmb() even after finish_wait(). > > And we have to check ret twice, and the false wakeup is still > possible. This is minor, but just for discussion, can't we do > this differently? > > int finish_wait_xxx(wait_queue_head_t *q, wait_queue_t *wait) > { > unsigned long flags; > int woken; > > __set_current_state(TASK_RUNNING); > spin_lock_irqsave(&q->lock, flags); > woken = list_empty(&wait->task_list); > list_del_init(&wait->task_list); > spin_unlock_irqrestore(&q->lock, flags); > > return woken; > }
Hehe, there is only n solutions to this problem. I had thought about that too, even written it down. But I was not sure if taking the spinlock, toggling irqs and (re)storing the flags is better than an untaken branch. ;)
> Now, __wait_on_bit_lock() does: > > if (test_bit(q->key.bit_nr, q->key.flags)) { > if ((ret = (*action)(q->key.flags))) { > if (finish_wait_xxx(...)) > __wake_up_bit(...); > return ret; > } > }
If you don't mind putting a second finish_wait() in there (you still need the one after the loop, right?), we can fix up my version to not check ret twice but do finish_wait() as you describe and then the test_bit() && wake up:
do { if (test_bit()) if ((ret = action())) { finish_wait() smp_rmb() if (!test_bit()) __wake_up_bit() return ret } } } while (test_and_set_bit()) finish_wait() return 0
> Or we can introduce > > int finish_wait_yyy(wait_queue_head_t *q, wait_queue_t *wait, > int mode, void *key) > { > unsigned long flags; > int woken; > > __set_current_state(TASK_RUNNING); > spin_lock_irqsave(&q->lock, flags); > woken = list_empty(&wait->task_list); > if (woken) > __wake_up_common(q, mode, 1, key); > else > list_del_init(&wait->task_list); > spin_unlock_irqrestore(&q->lock, flags); > > return woken; > } > > Perhaps a bit too much for this particular case, but I am thinking > about other cases when we need to abort the exclusive wait. > > For example, don't we have the similar problems with > wait_event_interruptible_exclusive() ?
Yeah, we do IIUC. Then having finish_wait() extended is probably a good idea.
> Oleg.
Hannes
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