Re: [RFC v3 1/2] epoll: avoid spinlock contention with wfcqueue

From: Arve Hjønnevåg
Date: Fri Mar 22 2013 - 18:18:14 EST

On Fri, Mar 22, 2013 at 3:31 AM, Eric Wong <normalperson@xxxxxxxx> wrote:
> Arve Hjønnevåg <arve@xxxxxxxxxxx> wrote:
>> On Thu, Mar 21, 2013 at 8:24 PM, Eric Wong <normalperson@xxxxxxxx> wrote:
>> >
>> > With EPOLLET and improper usage (not hitting EAGAIN), the event now
>> > has a larger window to be lost (as mentioned in my changelog).
>> >
>> What about the case where EPOLLET is not set? The old code did not
>> drop events in that case.
> Nothing is dropped, if the event wasn't on the ready list before,
> ep_poll_callback may still append the ready list while __put_user
> is running.
> If the event was on the ready list:
> 1) It does not matter for EPOLLONESHOT, it'll get masked out and
> discarded in the next ep_send_events call until ep_modify reenables
> it. Since ep_modify and ep_send_events both take ep->mtx, there's
> no conflict.
> 2) Level Trigger - event stays ready, so nothing is dropped.

At some point the level triggered event has to get cleared. As far as
I can tell, your new code will drop new events that occur between
"revents = ep_item_poll(epi, &pt);" and "epi->state = EP_STATE_IDLE;"
in that case.

Arve Hjønnevåg
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