NFS/TCP/IPv6 acting strangely in 4.2

On Fri, Sep 11, 2015 at 05:24:17PM +0100, Russell King - ARM Linux wrote:

On Fri, Sep 11, 2015 at 08:18:43AM -0700, Eric Dumazet wrote:

quoted

On Fri, 2015-09-11 at 16:06 +0100, Russell King - ARM Linux wrote:

quoted

On Fri, Sep 11, 2015 at 03:33:47PM +0100, Russell King - ARM Linux wrote:

quoted

It looks like 0c78789e3a030615c6650fde89546cadf40ec2cc might be relevant
too, but I don't see that solving the multiple _concurrent_ connection
attempts with the same port number - presumably it's somehow trying to
make the same socket repeatedly connect despite a previous connection
being in progress, which would have nothing to do with cleaning up a
previous attempt.

As I suspected, applying the above commit in addition does not solve the
problem, I still see the same behaviour: SYN SYNACK SYN RSTACK, SYN
SYNACK SYN RSTACK, and eventual SYN storms.

I do have this captured as well:

2558   0.834316 armada388 -> n2100 TCP [TCP Port numbers reused] rpasswd?nfs [SYN] Seq=1053655487 Win=28800 Len=0 MSS=1440 SACK_PERM=1 TSval=60001 TSecr=0 WS=128
2559   0.834572 n2100 -> armada388 TCP nfs?rpasswd [SYN, ACK] Seq=3076611574 Ack=1053655488 Win=28560 Len=0 MSS=1440 SACK_PERM=1 TSval=869622246 TSecr=60001 WS=64
2560   0.834666 armada388 -> n2100 TCP [TCP Port numbers reused] rpasswd?nfs [SYN] Seq=1054228544 Win=28800 Len=0 MSS=1440 SACK_PERM=1 TSval=60005 TSecr=0 WS=128
2561   0.834895 n2100 -> armada388 TCP nfs?rpasswd [ACK] Seq=3076611575 Ack=1053655488 Win=28560 Len=0 TSval=869622246 TSecr=60001

The packet at 2561 looks wrong to me - this doesn't follow what I know
would be the standard TCP setup of syn, synack, ack, because that final
ack is in the wrong direction.

This 2561 packet is an ACK packet, because n2100 has a SYN_RECV socket
created by packet 2558.

It receives a SYN packet (2560) that it interprets as a packet slightly
out of sequence (1054228544 being above 1053655487) for this SYN_RECV

The wrong packet is 2560, not 2561

Ok.

Looking deeper at the XPRT sunrpc code, I have to wonder about the
sanity of this:

void xprt_connect(struct rpc_task *task)
{
...
        if (!xprt_connected(xprt)) {
...
                if (test_bit(XPRT_CLOSING, &xprt->state))
                        return;
                if (xprt_test_and_set_connecting(xprt))
                        return;
                xprt->stat.connect_start = jiffies;
                xprt->ops->connect(xprt, task);

That calls into xs_connect(), which schedules a workqueue to do the
connection.  The workqueue will call xs_tcp_setup_socket().

xs_tcp_setup_socket() creates a socket if one didn't exist, otherwise
re-using the previously obtained socket (which'll be why its using the
same socket) and then goes on to call xs_tcp_finish_connecting().

xs_tcp_finish_connecting() calls kernel_connect(), which will return
-EINPROGRESS.  We seem to treat EINPROGRESS as if the connection was
successful:

        case 0:
        case -EINPROGRESS:
        case -EALREADY:
                xprt_unlock_connect(xprt, transport);
                xprt_clear_connecting(xprt);
                return;

and the xprt_clear_connecting() results in this whole path being
re-opened: the socket is not yet connected, so xprt_connected() will
return false, and despite the socket connection still being mid-way
through being connected, we clear the "connecting" status, causing
xprt_test_and_set_connecting() to return false.

That allows us to re-call xprt->ops->connect, re-queue the connect
worker, and re-run the call to kernel_connect() for a socket which is
already mid-way through being connected.

Shouldn't the "connecting" status only be cleared when either the socket
has _finished_ connecting, or when the connection has _failed_ to connect,
and not when it's mid-way through connecting?

I've not been able to prove this: I've set rpc_debug to 129 to log
just xprt and trans RPC facilities, and that's sufficient to change
the timing such that this doesn't happen.

Following that idea, I just tried the patch below, and it seems to work.
I don't know whether it handles all cases after a call to kernel_connect(),
but it stops the multiple connection attempts:

  1   0.000000 armada388 -> n2100 TCP 1009?nfs [SYN] Seq=3794066539 Win=28560 Len=0 MSS=1440 SACK_PERM=1 TSval=15712 TSecr=870317691 WS=128
  2   0.000414 n2100 -> armada388 TCP nfs?1009 [SYN, ACK] Seq=1884476522 Ack=3794066540 Win=28560 Len=0 MSS=1440 SACK_PERM=1 TSval=870318939 TSecr=15712 WS=64
  3   0.000787 armada388 -> n2100 TCP 1009?nfs [ACK] Seq=3794066540 Ack=1884476523 Win=28672 Len=0 TSval=15712 TSecr=870318939
  4   0.001304 armada388 -> n2100 NFS V3 ACCESS Call, FH: 0x905379cc, [Check: RD LU MD XT DL]
  5   0.001566 n2100 -> armada388 TCP nfs?1009 [ACK] Seq=1884476523 Ack=3794066660 Win=28608 Len=0 TSval=870318939 TSecr=15712
  6   0.001640 armada388 -> n2100 NFS V3 ACCESS Call, FH: 0x905379cc, [Check: RD LU MD XT DL]
  7   0.001866 n2100 -> armada388 TCP nfs?1009 [ACK] Seq=1884476523 Ack=3794066780 Win=28608 Len=0 TSval=870318939 TSecr=15712
  8   0.003070 n2100 -> armada388 NFS V3 ACCESS Reply (Call In 4), [Allowed: RD LU MD XT DL]
  9   0.003415 armada388 -> n2100 TCP 1009?nfs [ACK] Seq=3794066780 Ack=1884476647 Win=28672 Len=0 TSval=15712 TSecr=870318939
 10   0.003592 armada388 -> n2100 NFS V3 ACCESS Call, FH: 0xe15fc9c9, [Check: RD LU MD XT DL]
 11   0.004354 n2100 -> armada388 NFS V3 ACCESS Reply (Call In 6), [Allowed: RD LU MD XT DL]
 12   0.004682 armada388 -> n2100 NFS V3 ACCESS Call, FH: 0xe15fc9c9, [Check: RD LU MD XT DL]
 13   0.005365 n2100 -> armada388 NFS V3 ACCESS Reply (Call In 10), [Allowed: RD LU MD XT DL]
 14   0.005701 armada388 -> n2100 NFS V3 GETATTR Call, FH: 0xe15fc9c9
...

 net/sunrpc/xprtsock.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
index ff5b6a2e62c3..c456d6e51c56 100644
--- a/net/sunrpc/xprtsock.c
+++ b/net/sunrpc/xprtsock.c

@@ -1450,6 +1450,7 @@ static void xs_tcp_state_change(struct sock *sk)
 	switch (sk->sk_state) {
 	case TCP_ESTABLISHED:
 		spin_lock(&xprt->transport_lock);
+		xprt_clear_connecting(xprt);
 		if (!xprt_test_and_set_connected(xprt)) {
 			struct sock_xprt *transport = container_of(xprt,
 					struct sock_xprt, xprt);

@@ -1474,12 +1475,14 @@ static void xs_tcp_state_change(struct sock *sk)
 		smp_mb__before_atomic();
 		clear_bit(XPRT_CONNECTED, &xprt->state);
 		clear_bit(XPRT_CLOSE_WAIT, &xprt->state);
+		clear_bit(XPRT_CONNECTING, &xprt->state);		
 		smp_mb__after_atomic();
 		break;
 	case TCP_CLOSE_WAIT:
 		/* The server initiated a shutdown of the socket */
 		xprt->connect_cookie++;
 		clear_bit(XPRT_CONNECTED, &xprt->state);
+		clear_bit(XPRT_CONNECTING, &xprt->state);
 		xs_tcp_force_close(xprt);
 	case TCP_CLOSING:
 		/*

@@ -1493,6 +1496,7 @@ static void xs_tcp_state_change(struct sock *sk)
 		set_bit(XPRT_CLOSING, &xprt->state);
 		smp_mb__before_atomic();
 		clear_bit(XPRT_CONNECTED, &xprt->state);
+		clear_bit(XPRT_CONNECTING, &xprt->state);
 		smp_mb__after_atomic();
 		break;
 	case TCP_CLOSE:

@@ -2237,11 +2241,13 @@ static void xs_tcp_setup_socket(struct work_struct *work)
 		xs_tcp_force_close(xprt);
 		break;
 	case 0:
-	case -EINPROGRESS:
 	case -EALREADY:
 		xprt_unlock_connect(xprt, transport);
 		xprt_clear_connecting(xprt);
 		return;
+	case -EINPROGRESS:
+		xprt_unlock_connect(xprt, transport);
+		return;
 	case -EINVAL:
 		/* Happens, for instance, if the user specified a link
 		 * local IPv6 address without a scope-id.

-- 
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.