rxrpc: Add service upgrade support for client connections

Make it possible for a client to use AuriStor's service upgrade facility.

The client does this by adding an RXRPC_UPGRADE_SERVICE control message to
the first sendmsg() of a call.  This takes no parameters.

When recvmsg() starts returning data from the call, the service ID field in
the returned msg_name will reflect the result of the upgrade attempt.  If
the upgrade was ignored, srx_service will match what was set in the
sendmsg(); if the upgrade happened the srx_service will be altered to
indicate the service the server upgraded to.

Note that:

 (1) The choice of upgrade service is up to the server

 (2) Further client calls to the same server that would share a connection
     are blocked if an upgrade probe is in progress.

 (3) This should only be used to probe the service.  Clients should then
     use the returned service ID in all subsequent communications with that
     server (and not set the upgrade).  Note that the kernel will not
     retain this information should the connection expire from its cache.

 (4) If a server that supports upgrading is replaced by one that doesn't,
     whilst a connection is live, and if the replacement is running, say,
     OpenAFS 1.6.4 or older or an older IBM AFS, then the replacement
     server will not respond to packets sent to the upgraded connection.

     At this point, calls will time out and the server must be reprobed.

Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/Documentation/networking/rxrpc.txt b/Documentation/networking/rxrpc.txt
index 2a16627..18078e6 100644
--- a/Documentation/networking/rxrpc.txt
+++ b/Documentation/networking/rxrpc.txt
@@ -325,6 +325,8 @@ calls, to invoke certain actions and to report certain conditions.  These are:
 	RXRPC_LOCAL_ERROR	-rt error num	Local error encountered
 	RXRPC_NEW_CALL		-r- n/a		New call received
 	RXRPC_ACCEPT		s-- n/a		Accept new call
+	RXRPC_EXCLUSIVE_CALL	s-- n/a		Make an exclusive client call
+	RXRPC_UPGRADE_SERVICE	s-- n/a		Client call can be upgraded
 
 	(SRT = usable in Sendmsg / delivered by Recvmsg / Terminal message)
 
@@ -387,6 +389,23 @@ calls, to invoke certain actions and to report certain conditions.  These are:
      return error ENODATA.  If the user ID is already in use by another call,
      then error EBADSLT will be returned.
 
+ (*) RXRPC_EXCLUSIVE_CALL
+
+     This is used to indicate that a client call should be made on a one-off
+     connection.  The connection is discarded once the call has terminated.
+
+ (*) RXRPC_UPGRADE_SERVICE
+
+     This is used to make a client call to probe if the specified service ID
+     may be upgraded by the server.  The caller must check msg_name returned to
+     recvmsg() for the service ID actually in use.  The operation probed must
+     be one that takes the same arguments in both services.
+
+     Once this has been used to establish the upgrade capability (or lack
+     thereof) of the server, the service ID returned should be used for all
+     future communication to that server and RXRPC_UPGRADE_SERVICE should no
+     longer be set.
+
 
 ==============
 SOCKET OPTIONS
@@ -566,6 +585,17 @@ A client would issue an operation by:
      buffer instead, and MSG_EOR will be flagged to indicate the end of that
      call.
 
+A client may ask for a service ID it knows and ask that this be upgraded to a
+better service if one is available by supplying RXRPC_UPGRADE_SERVICE on the
+first sendmsg() of a call.  The client should then check srx_service in the
+msg_name filled in by recvmsg() when collecting the result.  srx_service will
+hold the same value as given to sendmsg() if the upgrade request was ignored by
+the service - otherwise it will be altered to indicate the service ID the
+server upgraded to.  Note that the upgraded service ID is chosen by the server.
+The caller has to wait until it sees the service ID in the reply before sending
+any more calls (further calls to the same destination will be blocked until the
+probe is concluded).
+
 
 ====================
 EXAMPLE SERVER USAGE