From: Davide Libenzi <davidel@xmailserver.org>
To: Jamie Lokier <jamie@shareable.org>
Cc: "David S. Miller" <davem@redhat.com>,
e0206@foo21.com,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
kuznet@ms2.inr.ac.ru
Subject: Re: [Patch][RFC] epoll and half closed TCP connections
Date: Sun, 13 Jul 2003 16:03:51 -0700 (PDT) [thread overview]
Message-ID: <Pine.LNX.4.55.0307131542000.15022@bigblue.dev.mcafeelabs.com> (raw)
In-Reply-To: <20030713191559.GA20573@mail.jlokier.co.uk>
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
next prev parent reply other threads:[~2003-07-13 22:57 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-07-12 18:16 [Patch][RFC] epoll and half closed TCP connections Eric Varsanyi
2003-07-12 19:44 ` Jamie Lokier
2003-07-12 20:51 ` Eric Varsanyi
2003-07-12 20:48 ` Davide Libenzi
2003-07-12 21:19 ` Eric Varsanyi
2003-07-12 21:20 ` Davide Libenzi
2003-07-12 21:41 ` Davide Libenzi
2003-07-12 23:11 ` Eric Varsanyi
2003-07-12 23:55 ` Davide Libenzi
2003-07-13 1:05 ` Eric Varsanyi
2003-07-13 20:32 ` David Schwartz
2003-07-13 21:10 ` Jamie Lokier
2003-07-13 23:05 ` David Schwartz
2003-07-13 23:09 ` Davide Libenzi
2003-07-14 8:14 ` Alan Cox
2003-07-14 15:03 ` Davide Libenzi
2003-07-14 1:27 ` Jamie Lokier
2003-07-13 21:14 ` Davide Libenzi
2003-07-13 23:05 ` David Schwartz
2003-07-13 23:11 ` Davide Libenzi
2003-07-13 23:52 ` Entrope
2003-07-14 6:14 ` David Schwartz
2003-07-14 7:20 ` Jamie Lokier
2003-07-14 1:51 ` Jamie Lokier
2003-07-14 6:14 ` David Schwartz
2003-07-15 20:27 ` James Antill
2003-07-16 1:46 ` David Schwartz
2003-07-16 2:09 ` James Antill
2003-07-13 13:12 ` Jamie Lokier
2003-07-13 16:55 ` Davide Libenzi
2003-07-12 20:01 ` Davide Libenzi
2003-07-13 5:24 ` David S. Miller
2003-07-13 14:07 ` Jamie Lokier
2003-07-13 17:00 ` Davide Libenzi
2003-07-13 19:15 ` Jamie Lokier
2003-07-13 23:03 ` Davide Libenzi [this message]
2003-07-14 1:41 ` Jamie Lokier
2003-07-14 2:24 ` POLLRDONCE optimisation for epoll users (was: epoll and half closed TCP connections) Jamie Lokier
2003-07-14 2:37 ` Davide Libenzi
2003-07-14 2:43 ` Davide Libenzi
2003-07-14 2:56 ` Jamie Lokier
2003-07-14 3:02 ` Davide Libenzi
2003-07-14 3:16 ` Jamie Lokier
2003-07-14 3:21 ` Davide Libenzi
2003-07-14 3:42 ` Jamie Lokier
2003-07-14 4:00 ` Davide Libenzi
2003-07-14 5:51 ` Jamie Lokier
2003-07-14 6:24 ` Davide Libenzi
2003-07-14 6:57 ` Jamie Lokier
2003-07-14 3:12 ` Jamie Lokier
2003-07-14 3:17 ` Davide Libenzi
2003-07-14 3:35 ` Jamie Lokier
2003-07-14 3:04 ` Jamie Lokier
2003-07-14 3:12 ` Davide Libenzi
2003-07-14 3:27 ` Jamie Lokier
2003-07-14 17:09 ` [Patch][RFC] epoll and half closed TCP connections kuznet
2003-07-14 17:09 ` Davide Libenzi
2003-07-14 21:45 ` Jamie Lokier
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