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 - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html