select-hang.patch

commit d84ba638e4ba3c40023ff997aa5e8d3ed002af36
Author: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Date:   Tue Aug 24 16:05:48 2010 +0000

    tcp: select(writefds) don't hang up when a peer close connection
    
    This issue come from ruby language community. Below test program
    hang up when only run on Linux.
    
        % uname -mrsv
        Linux 2.6.26-2-486 #1 Sat Dec 26 08:37:39 UTC 2009 i686
        % ruby -rsocket -ve '
        BasicSocket.do_not_reverse_lookup = true
        serv = TCPServer.open("127.0.0.1", 0)
        s1 = TCPSocket.open("127.0.0.1", serv.addr[1])
        s2 = serv.accept
        s2.close
        s1.write("a") rescue p $!
        s1.write("a") rescue p $!
        Thread.new {
          s1.write("a")
        }.join'
        ruby 1.9.3dev (2010-07-06 trunk 28554) [i686-linux]
        #<Errno::EPIPE: Broken pipe>
        [Hang Here]
    
    FreeBSD, Solaris, Mac doesn't. because Ruby's write() method call
    select() internally. and tcp_poll has a bug.
    
    SUS defined 'ready for writing' of select() as following.
    
    |  A descriptor shall be considered ready for writing when a call to
    |  function with O_NONBLOCK clear would not block, whether or not th
    |  would transfer data successfully.
    
    That said, EPIPE situation is clearly one of 'ready for writing'.
    
    We don't have read-side issue because tcp_poll() already has read si
    shutdown care.
    
    |        if (sk->sk_shutdown & RCV_SHUTDOWN)
    |                mask |= POLLIN | POLLRDNORM | POLLRDHUP;
    
    So, Let's insert same logic in write side.
    
    - reference url
      http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/31065
      http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/31068
    
    Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index e2add5f..3fb1428 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -451,7 +451,8 @@ unsigned int tcp_poll(struct file *file, struct sock
                                if (sk_stream_wspace(sk) >= sk_stream_mi
                                        mask |= POLLOUT | POLLWRNORM;
                        }
-               }
+               } else
+                       mask |= POLLOUT | POLLWRNORM;
 
                if (tp->urg_data & TCP_URG_VALID)
                        mask |= POLLPRI;