linux-kernel - [11/80] tcp: select(writefds) dont hang up when a peer close connection

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Message-Id: <20100924162615.849369119@clark.site>
Date:	Fri, 24 Sep 2010 09:23:59 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: [11/80] tcp: select(writefds) dont hang up when a peer close connection

2.6.35-stable review patch.  If anyone has any objections, please let us know.

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


From: KOSAKI Motohiro <kosaki.motohiro@...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@...fujitsu.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>
---
 net/ipv4/tcp.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -453,7 +453,8 @@ 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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