[PATCH 4.4 04/17] tipc: correct error in node fsm

Greg Kroah-Hartman
2017-04-28T08:50:46Z

From: Jon Paul Maloy <jon.maloy@ericsson.com>

commit c4282ca76c5b81ed73ef4c5eb5c07ee397e51642 upstream.

commit 88e8ac7000dc ("tipc: reduce transmission rate of reset messages
when link is down") revealed a flaw in the node FSM, as defined in
the log of commit 66996b6c47ed ("tipc: extend node FSM").

We see the following scenario:
1: Node B receives a RESET message from node A before its link endpoint
is fully up, i.e., the node FSM is in state SELF_UP_PEER_COMING. This
event will not change the node FSM state, but the (distinct) link FSM
will move to state RESETTING.
2: As an effect of the previous event, the local endpoint on B will
declare node A lost, and post the event SELF_DOWN to the its node
FSM. This moves the FSM state to SELF_DOWN_PEER_LEAVING, meaning
that no messages will be accepted from A until it receives another
RESET message that confirms that A's endpoint has been reset. This
is wasteful, since we know this as a fact already from the first
received RESET, but worse is that the link instance's FSM has not
wasted this information, but instead moved on to state ESTABLISHING,
meaning that it repeatedly sends out ACTIVATE messages to the reset
peer A.
3: Node A will receive one of the ACTIVATE messages, move its link FSM
to state ESTABLISHED, and start repeatedly sending out STATE messages
to node B.
4: Node B will consistently drop these messages, since it can only accept
accept a RESET according to its node FSM.
5: After four lost STATE messages node A will reset its link and start
repeatedly sending out RESET messages to B.
6: Because of the reduced send rate for RESET messages, it is very
likely that A will receive an ACTIVATE (which is sent out at a much
higher frequency) before it gets the chance to send a RESET, and A
may hence quickly move back to state ESTABLISHED and continue sending
out STATE messages, which will again be dropped by B.
7: GOTO 5.
8: After having repeated the cycle 5-7 a number of times, node A will
by chance get in between with sending a RESET, and the situation is
resolved.

Unfortunately, we have seen that it may take a substantial amount of
time before this vicious loop is broken, sometimes in the order of
minutes.

We correct this by making a small correction to the node FSM: When a
node in state SELF_UP_PEER_COMING receives a SELF_DOWN event, it now
moves directly back to state SELF_DOWN_PEER_DOWN, instead of as now
SELF_DOWN_PEER_LEAVING. This is logically consistent, since we don't
need to wait for RESET confirmation from of an endpoint that we alread
know has been reset. It also means that node B in the scenario above
will not be dropping incoming STATE messages, and the link can come up
immediately.

Finally, a symmetry comparison reveals that the FSM has a similar
error when receiving the event PEER_DOWN in state PEER_UP_SELF_COMING.
Instead of moving to PERR_DOWN_SELF_LEAVING, it should move directly
to SELF_DOWN_PEER_DOWN. Although we have never seen any negative effect
of this logical error, we choose fix this one, too.

The node FSM looks as follows after those changes:

+----------------------------------------+
| PEER_DOWN_EVT|
| |
+------------------------+----------------+ |
|SELF_DOWN_EVT | | |
| | | |
| +-----------+ +-----------+ |
| |NODE_ | |NODE_ | |
| +----------|FAILINGOVER|<---------|SYNCHING |-----------+ |
| |SELF_ +-----------+ FAILOVER_+-----------+ PEER_ | |
| |DOWN_EVT | A BEGIN_EVT A | DOWN_EVT| |
| | | | | | | |
| | | | | | | |
| | |FAILOVER_ |FAILOVER_ |SYNCH_ |SYNCH_ | |
| | |END_EVT |BEGIN_EVT |BEGIN_EVT|END_EVT | |
| | | | | | | |
| | | | | | | |
| | | +--------------+ | | |
| | +-------->| SELF_UP_ |<-------+ | |
| | +-----------------| PEER_UP |----------------+ | |
| | |SELF_DOWN_EVT +--------------+ PEER_DOWN_EVT| | |
| | | A A | | |
| | | | | | | |
| | | PEER_UP_EVT| |SELF_UP_EVT | | |
| | | | | | | |
V V V | | V V V
+------------+ +-----------+ +-----------+ +------------+
|SELF_DOWN_ | |SELF_UP_ | |PEER_UP_ | |PEER_DOWN |
|PEER_LEAVING| |PEER_COMING| |SELF_COMING| |SELF_LEAVING|
+------------+ +-----------+ +-----------+ +------------+
| | A A | |
| | | | | |
| SELF_ | |SELF_ |PEER_ |PEER_ |
| DOWN_EVT| |UP_EVT |UP_EVT |DOWN_EVT |
| | | | | |
| | | | | |
| | +--------------+ | |
|PEER_DOWN_EVT +--->| SELF_DOWN_ |<---+ SELF_DOWN_EVT|
+------------------->| PEER_DOWN |<--------------------+
+--------------+

Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
net/tipc/node.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/net/tipc/node.c
+++ b/net/tipc/node.c
@@ -728,7 +728,7 @@ static void tipc_node_fsm_evt(struct tip
state = SELF_UP_PEER_UP;
break;
case SELF_LOST_CONTACT_EVT:
- state = SELF_DOWN_PEER_LEAVING;
+ state = SELF_DOWN_PEER_DOWN;
break;
case SELF_ESTABL_CONTACT_EVT:
case PEER_LOST_CONTACT_EVT:
@@ -747,7 +747,7 @@ static void tipc_node_fsm_evt(struct tip
state = SELF_UP_PEER_UP;
break;
case PEER_LOST_CONTACT_EVT:
- state = SELF_LEAVING_PEER_DOWN;
+ state = SELF_DOWN_PEER_DOWN;
break;