Re: [TCP]: rcvbuf lock when tcp_moderate_rcvbuf enabled

[Dan Aloni <da-x@monatomic.org>]

Hello,

Below, I've forwarded change from 2.6.16 which I think may causes 
problems for applications that use setsockopt with SO_RCVBUF. We are 
using an implementation of an iSCSI target and according to network 
sniffs it seems that during data transfer the receive window 
unjustifyingly shrinks to a very low size (180 bytes). I can guess
that the code below indirectly affects the receive window size, but 
I'm not sure how it the logic works here, a clarification could be
helpful.

It's worth to mention that we have sysctl_tcp_moderate_rcvbuf=1, but 
I don't think it should interfere with applications that request to 
have a fixed receive buffer by the means of setsockopt(). I can also 
tell by experiment that reverting the change below makes the problem 
go away.

--- a97ed5416c28ee14ecab0ac4483c079a0c3e4c1d
+++ e9a54ae7d6903845598db14a8e1cba54026faf1b
@@ -456,7 +456,8 @@ void tcp_rcv_space_adjust(struct sock *s
 
 		tp->rcvq_space.space = space;
 
-		if (sysctl_tcp_moderate_rcvbuf) {
+		if (sysctl_tcp_moderate_rcvbuf &&
+		    !(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
 			int new_clamp = space;
 
 			/* Receive space grows, normalize in order to

-- 
Dan Aloni
da-x@monatomic.org, da-x@colinux.org, da-x@gmx.net, dan@xiv.co.il