[systemd-devel] [PATCH] sd-event: we do not support EPOLLONESHOT correctly

[Shawn Landden]

On Fri, Dec 13, 2013 at 8:39 PM, Shawn Landden <shawn at churchofgit.com> wrote:
> On Fri, Dec 13, 2013 at 8:04 PM, Shawn Landden <shawn at churchofgit.com> wrote:
>> On Fri, Dec 13, 2013 at 7:17 PM, Lennart Poettering
>> <lennart at poettering.net> wrote:
>>> On Fri, 13.12.13 13:08, Shawn Landden (shawn at churchofgit.com) wrote:
>>>
>>>> If this event is not the highest priority, then it will not be
>>>> dispatched when the epoll triggers, and since we will not get any more
>>>> wakeups (due to the way EPOLLONESHOT works) will never be dispatched.
>>>
>>> Hmm, so the way this is supposed to work is that as soon as epoll told
>>> us about some event once, we will set the event source to "pending", and
>>> it stays "pending" until we have dispatched it, regardless if we ever
>>> get the event from epoll again or not. However, we currently will
>>> overwrite the "revents" field should we ever get it again with the new
>>> one. It probably makes more sense to OR it in, so that we don't forget
>>> readability if writability happens at a later point or vice versa, if
>>> you follow what I mean. I'll prep a fix for that.
>> good
>>>
>>> With that fixed is there something else to still fix?
>> Oh I missed how the pending code worked. Because of that I think I can
>> use EPOLLET
>> with sd-event, and will send a Distribute= patch that uses it. (already have it,
>> just wasn't sure if it was only working cause my test setup has near-0 load)
> With either of these, can't we end up in a situation where we have
> events pending
> but are still blocking on epoll_wait()?
>
> shouldn't we do something like
>
> p = have_pending()
>
> epoll_wait(foo, foo, foo, p ? timeout : 0);
>
> do prioq stuff()
> mark_pending()
> dispatch_highest_prio()
>
> to avoid this issue?

NVM, i need to read the code better........
>>>
>>>> Since we only handle one event per epoll_wait() wakeup,
>>>> and we dequeue a ONE_SHOT event when we dispatch it, we
>>>> do not have the problem described in epoll(7) of
>>>> "multiple events can be generated upon receipt of multiple
>>>> chunks of data".
>>>
>>> Not sure I follow here? That part appears to be about EPOLLET?
>> That part explains why EPOLLET cannot be used to implement EPOLLONESHOT.
>> The reason is that even with EPOLLET, epoll_wait can generate multiple
>> events for the
>> same watch, if more than one connection came in.
>>>
>>> Lennart
>>>
>>> --
>>> Lennart Poettering, Red Hat
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