rxrpc: Fix life check

The life-checking function, which is used by kAFS to make sure that a call
is still live in the event of a pending signal, only samples the received
packet serial number counter; it doesn't actually provoke a change in the
counter, rather relying on the server to happen to give us a packet in the
time window.

Fix this by adding a function to force a ping to be transmitted.

kAFS then keeps track of whether there's been a stall, and if so, uses the
new function to ping the server, resetting the timeout to allow the reply
to come back.

If there's a stall, a ping and the call is *still* stalled in the same
place after another period, then the call will be aborted.

Fixes: bc5e3a546d ("rxrpc: Use MSG_WAITALL to tell sendmsg() to temporarily ignore signals")
Fixes: f4d15fb6f9 ("rxrpc: Provide functions for allowing cleaner handling of signals")
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/rxrpc.txt

@@ -1056,18 +1056,23 @@ The kernel interface functions are as follows:
 
 	u32 rxrpc_kernel_check_life(struct socket *sock,
 				    struct rxrpc_call *call);
+	void rxrpc_kernel_probe_life(struct socket *sock,
+				     struct rxrpc_call *call);
 
-     This returns a number that is updated when ACKs are received from the peer
-     (notably including PING RESPONSE ACKs which we can elicit by sending PING
-     ACKs to see if the call still exists on the server).  The caller should
-     compare the numbers of two calls to see if the call is still alive after
-     waiting for a suitable interval.
+     The first function returns a number that is updated when ACKs are received
+     from the peer (notably including PING RESPONSE ACKs which we can elicit by
+     sending PING ACKs to see if the call still exists on the server).  The
+     caller should compare the numbers of two calls to see if the call is still
+     alive after waiting for a suitable interval.
 
      This allows the caller to work out if the server is still contactable and
      if the call is still alive on the server whilst waiting for the server to
      process a client operation.
 
-     This function may transmit a PING ACK.
+     The second function causes a ping ACK to be transmitted to try to provoke
+     the peer into responding, which would then cause the value returned by the
+     first function to change.  Note that this must be called in TASK_RUNNING
+     state.
 
  (*) Get reply timestamp.