smb3: do not reserve too many oplock credits

There were cases reported where servers will sometimes return more
credits than requested on oplock break responses, which can lead to
most of the credits being allocated for oplock breaks (instead of
for normal operations like read and write) if number of SMB3 requests
in flight always stays above 0 (the oplock and echo credits are
rebalanced when in flight requests goes down to zero).

If oplock credits gets unexpectedly large (e.g. three is more than it
would ever be expected to be) and in flight requests are greater than
zero, then rebalance the oplock credits and regular credits (go
back to reserving just one oplock credit).
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/smb2ops.c

@@ -109,7 +109,11 @@ smb2_add_credits(struct TCP_Server_Info *server,
 			server->credits--;
 			server->oplock_credits++;
 		}
-	}
+	} else if ((server->in_flight > 0) && (server->oplock_credits > 3) &&
+		   ((optype & CIFS_OP_MASK) == CIFS_OBREAK_OP))
+		/* if now have too many oplock credits, rebalance so don't starve normal ops */
+		change_conf(server);
+
 	scredits = *val;
 	in_flight = server->in_flight;
 	spin_unlock(&server->req_lock);