Messages in this thread | | | Date | Sun, 13 Jul 2003 16:03:51 -0700 (PDT) | From | Davide Libenzi <> | Subject | Re: [Patch][RFC] epoll and half closed TCP connections |
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On Sun, 13 Jul 2003, Jamie Lokier wrote:
> > After ppl reporting the O_RDONLY|O_TRUNC case I'm inclined to expect > > everything from existing apps ;) POLLHUP should be returned to apps > > waiting for POLLOUT while POLLRDHUP to ones for POLLIN. > > Not sure exactly how you're thinking with that last sentence.
Brain farting, delete it ;) This is a nice page about POLLHUP treatment :
http://www.greenend.org.uk/rjk/2001/06/poll.html
> At present, it's impossible for socket code to return POLLHUP only to > apps which are waiting on POLLOUT - because POLLHUP is not maskable in > sys_poll's API. Therefore sockets return POLLHUP only if they are > closed in both directions. > > There is no way for a socket to return a HUP condition for someone who > is waiting only to write, but fortunately that doesn't matter :)
Yes, this could be improved though. If we could only pass our event interest mask to f_op->poll() the function will be able to register it inside the wait queue structure and release only waiters that matches the available condition.
> (*) There aren't that many places which set POLLHUP; they divide into: > sockets, ttys, SCSI-generic and PPP. The latter two are not important > as they don't do half-close. > > __The critical thing with POLL_RDHUP is that it is set if read() would > return EOF after returning data.__ > > If this condition isn't met, than an app which is using POLL_RDHUP to > optimise performance using epoll will hang occasionally. > > Sockets are important: TCP is not the only thing to support > half-closing. If an app is waiting for POLLRDHUP, and it doesn't know > what kind of socket it has been given (e.g. AF_UNIX), the network > stack had better return POLL_RDHUP when there's an EOF pending. > > So we'd better add POLLRDHUP to all the socket types which do > half-closing. For the rest, no change is required as POLLHUP is > non-maskable :) (So apps should always say "if (events & > (POLLHUP|POLLRDHUP)) check_for_eof()"). > > And ttys? They are problematic, because ttys can return EOF _after_ > returning data without closing (and without being hung-up). An epoll > loop which is reading a tty (and isn't programmed specially for a tty) > _must_ receive POLLRDHUP when the EOF is pending, else it may hang. > > In other words, POLLRDHUP is the wrong name: the correct name is > POLLRDEOF.
Please replace 'it may hung' with 'it may hung if it is using the read() return bytes check trick' ;)
- Davide
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