Re: [Patch][RFC] epoll and half closed TCP connections

["David S. Miller" <davem@redhat.com>]

On Sat, 12 Jul 2003 13:01:21 -0700 (PDT)
Davide Libenzi <davidel@xmailserver.org> wrote:

> 
> [Cc:ing DaveM ]

[Cc:ing Alexey :-) ]

Alexey, they seem to want to add some kind of POLLRDHUP thing,
comments wrt. TCP and elsewhere in the networking?  See below...

> On Sat, 12 Jul 2003, Eric Varsanyi wrote:
> 
> > I'm proposing adding a new POLL event type (POLLRDHUP) as way to solve
> > a new race introduced by having an edge triggered event mechanism
> > (epoll). The problem occurs when a client writes data and then does a
> > write side shutdown(). The server (using epoll) sees only one event for
> > the read data ready and the read EOF condition and has no way to tell
> > that an EOF occurred.
> >
> > -Eric Varsanyi
> >
> > Details
> > -----------
> > 	- remote sends data and does a shutdown
> > 	   [ we see a data bearing packet and FIN from client on the wire ]
> >
> > 	- user mode server gets around to doing accept() and registers
> > 	  for EPOLLIN events (along with HUP and ERR which are forced on)
> >
> > 	- epoll_wait() returns a single EPOLLIN event on the FD representing
> > 	  both the 1/2 shutdown state and data available
> >
> > At this point there is no way the app can tell if there is a half closed
> > connection so it may issue a close() back to the client after writing
> > results. Normally the server would distinguish these events by assuming
> > EOF if it got a read ready indication and the first read returned 0 bytes,
> > or would issue read calls until less data was returned than was asked for.
> >
> > In a level triggered world this all just works because the read ready
> > indication is driven back to the app as long as the socket state is half
> > closed. The event driven epoll mechanism folds these two indications
> > together and thus loses one 'edge'.
> >
> > One would be tempted to issue an extra read() after getting back less than
> > expected, but this is an extra system call on every read event and you get
> > back the same '0' bytes that you get if the buffer is just empty. The only
> > sure bet seems to be CTL_MODding the FD to force a re-poll (which would
> > cost a syscall and hash-lookup in eventpoll for every read event).
> >
> 
> Yes, this is overhead that should be avoided. It is true that you could
> use Level Triggered events, but if you structured your app on edge you
> should be able to solve this w/out overhead.
> 
> 
> 
> > 	2) add a new 1/2 closed event type that a poll routine can return
> >
> > The implementation is trivial, a patch is included below. If this idea sees
> > favor I'll fix the other architectures, ipv6, epoll.h, and make a 'real'
> > patch. I do not believe any drivers deserve to be modified to return this
> > new event.
> 
> This looks good to me. David what do you think ?
> 
> 
> 
> > diff -Naur linux-2.4.20/include/asm-i386/poll.h linux-2.4.20_ev/include/asm-i386/poll.h
> > --- linux-2.4.20/include/asm-i386/poll.h	Thu Jan 23 13:01:28 1997
> > +++ linux-2.4.20_ev/include/asm-i386/poll.h	Sat Jul 12 12:29:11 2003
> > @@ -15,6 +15,7 @@
> >  #define POLLWRNORM	0x0100
> >  #define POLLWRBAND	0x0200
> >  #define POLLMSG		0x0400
> > +#define POLLRDHUP	0x0800
> >
> >  struct pollfd {
> >  	int fd;
> > diff -Naur linux-2.4.20/net/ipv4/tcp.c linux-2.4.20_ev/net/ipv4/tcp.c
> > --- linux-2.4.20/net/ipv4/tcp.c	Tue Jul  8 09:40:42 2003
> > +++ linux-2.4.20_ev/net/ipv4/tcp.c	Sat Jul 12 12:29:56 2003
> > @@ -424,7 +424,7 @@
> >  	if (sk->shutdown == SHUTDOWN_MASK || sk->state == TCP_CLOSE)
> >  		mask |= POLLHUP;
> >  	if (sk->shutdown & RCV_SHUTDOWN)
> > -		mask |= POLLIN | POLLRDNORM;
> > +		mask |= POLLIN | POLLRDNORM | POLLRDHUP;
> >
> >  	/* Connected? */
> >  	if ((1 << sk->state) & ~(TCPF_SYN_SENT|TCPF_SYN_RECV)) {
> >
> 
> 
> 
> - Davide