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    Subject[06/68] tcp: select(writefds) dont hang up when a peer close connection
    2.6.32-stable review patch.  If anyone has any objections, please let us know.

    ------------------


    From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

    [ Upstream commit d84ba638e4ba3c40023ff997aa5e8d3ed002af36 ]

    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 an output
    | function with O_NONBLOCK clear would not block, whether or not the function
    | 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 side
    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>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
    ---
    net/ipv4/tcp.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    --- a/net/ipv4/tcp.c
    +++ b/net/ipv4/tcp.c
    @@ -451,7 +451,8 @@ unsigned int tcp_poll(struct file *file,
    if (sk_stream_wspace(sk) >= sk_stream_min_wspace(sk))
    mask |= POLLOUT | POLLWRNORM;
    }
    - }
    + } else
    + mask |= POLLOUT | POLLWRNORM;

    if (tp->urg_data & TCP_URG_VALID)
    mask |= POLLPRI;



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