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

[Shawn Landden]

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)
>
>> 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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