Re: [patch] sys_epoll ...

[Davide Libenzi]

On Sun, 20 Oct 2002, Andrew Morton wrote:

> Well I'm assuming that you don't want to sleep if, say,
> ep->eventcnt is non-zero.  The code is currently (simplified):
>
> 	add_wait_queue(...);
> 	if (ep->eventcnt)
> 		break;
> 	/* window here */
> 	set_current_state(TASK_INTERRUPTIBLE);
> 	schedule();
>
> If another CPU increments eventcnt and sends this task a wakeup in that
> window, it is missed and we still sleep.  The conventional fix for that
> is:
>
> 	add_wait_queue(...);
> 	set_current_state(TASK_INTERRUPTIBLE);
> 	if (ep->eventcnt)
> 		break;
> 	/* harmless window here */
> 	schedule();
>
> So if someone delivers a wakeup in the "harmless window" then this task
> still calls schedule(), but the wakeup has turned the state from
> TASK_INTERRUPTIBLE into TASK_RUNNING, so the schedule() doesn't actually
> take this task off the runqueue.  This task will zoom straight through the
> schedule() and will then loop back and notice the incremented ep->eventcnt.
>
> So it is important that the waker increment eventcnt _before_ delivering
> the wake_up, too.

It's true ... but the window is pretty small there :) Anyway it makes the
code more correct and I changed it. I have the new patch with your
suggestions ready and I will post as sonn as it'll pass a few tests.

- Davide