Re: [PATCH 2.6.23-rc6 Resending] NETWORKING : Edge Triggered EPOLLOUT events get missed for TCP sockets

Davide Libenzi
2007-09-20T17:43:18Z

On Wed, 19 Sep 2007, Nagendra Tomar wrote:

> The tcp_check_space() function calls tcp_new_space() only if the
> SOCK_NOSPACE bit is set in the socket flags. This is causing Edge Triggered
> EPOLLOUT events to be missed for TCP sockets, as the ep_poll_callback()
> is not called from the wakeup routine.
>
> The SOCK_NOSPACE bit indicates the user's intent to perform writes
> on that socket (set in tcp_sendmsg and tcp_poll). I believe the idea
> behind the SOCK_NOSPACE check is to optimize away the tcp_new_space call
> in cases when user is not interested in writing to the socket. These two
> take care of all possible scenarios in which a user can convey his intent
> to write on that socket.
>
> Case 1: tcp_sendmsg detects lack of sndbuf space
> Case 2: tcp_poll returns not writable
>
> This is fine if we do not deal with epoll's Edge Triggered events (EPOLLET).
> With ET events we can have a scenario where the SOCK_NOSPACE bit is not set,
> as the user has neither done a sendmsg nor a poll/epoll call that returned
> with the POLLOUT condition not set.

Looking back at it, I think the current TCP code is right, once you look
at the "event" to be a output buffer full->with_space transition.
If you drop an fd inside epoll with EPOLLOUT|EPOLLET and you get an event
(free space on the output buffer), if you do not consume it (say a
tcp_sendmsg that re-fill the buffer), you can't see other OUT event
anymore since they happen on the full->with_space transition.
Yes, I know, the read size (EPOLLIN) works differently and you get an
event for every packet you receive. And yes, I do not like asymmetric
things. But that does not make the EPOLLOUT|EPOLLET wrong IMO.

- Davide