sctp: send the next probe immediately once the last one is acked

These is no need to wait for 'interval' period for the next probe
if the last probe is already acked in search state. The 'interval'
period waiting should be only for probe failure timeout and the
current pmtu check when it's in search complete state.

This change will shorten the probe time a lot in search state, and
also fix the document accordingly.
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.rst

@@ -2835,10 +2835,14 @@ encap_port - INTEGER
 	Default: 0
 
 plpmtud_probe_interval - INTEGER
-        The time interval (in milliseconds) for sending PLPMTUD probe chunks.
-        These chunks are sent at the specified interval with a variable size
-        to probe the mtu of a given path between 2 endpoints. PLPMTUD will
-        be disabled when 0 is set, and other values for it must be >= 5000.
+        The time interval (in milliseconds) for the PLPMTUD probe timer,
+        which is configured to expire after this period to receive an
+        acknowledgment to a probe packet. This is also the time interval
+        between the probes for the current pmtu when the probe search
+        is done.
+
+        PLPMTUD will be disabled when 0 is set, and other values for it
+        must be >= 5000.
 
 	Default: 0
 

net/sctp/sm_statefuns.c

@@ -1275,7 +1275,10 @@ enum sctp_disposition sctp_sf_backbeat_8_3(struct net *net,
 			return SCTP_DISPOSITION_DISCARD;
 
 		sctp_transport_pl_recv(link);
-		return SCTP_DISPOSITION_CONSUME;
+		if (link->pl.state == SCTP_PL_COMPLETE)
+			return SCTP_DISPOSITION_CONSUME;
+
+		return sctp_sf_send_probe(net, ep, asoc, type, link, commands);
 	}
 
 	max_interval = link->hbinterval + link->rto;