INSTALL.Linux: Add some troubleshooting instructions for module loading.

Suggested-by: kk yap <yapkke@stanford.edu>

INSTALL.Linux

@@ -186,6 +186,41 @@ Prerequisites section, follow the procedure below to build.
    To verify that the modules have been loaded, run "/sbin/lsmod" and
    check that openvswitch_mod is listed.
 
+   If the "insmod" operation fails, look at the last few kernel log
+   messages (e.g. with "dmesg | tail"):
+
+      - The message "openvswitch_mod: exports duplicate symbol
+        br_should_route_hook (owned by bridge)" means that the bridge
+        module is loaded.  Run "/sbin/rmmod bridge" to remove it.
+
+        If "/sbin/rmmod bridge" fails with "ERROR: Module bridge does
+        not exist in /proc/modules", then the bridge is compiled into
+        the kernel, rather than as a module.  Open vSwitch does not
+        support this configuration (see "Build Requirements", above).
+
+      - The message "openvswitch_mod: exports duplicate symbol
+        dp_ioctl_hook (owned by ofdatapath)" means that the ofdatapath
+        module from the OpenFlow reference implementation is loaded.
+        Run "/sbin/rmmod ofdatapath" to remove it.  (You might have to
+        delete any existing datapaths beforehand, using the "dpctl"
+        program included with the OpenFlow reference implementation.
+        "ovs-dpctl" will not work.)
+
+      - Otherwise, the most likely problem is that Open vSwitch was
+        built for a kernel different from the one into which you are
+        trying to load it.  Run "modinfo" on openvswitch_mod.ko and on
+        a module built for the running kernel, e.g.:
+
+           % /sbin/modinfo openvswitch_mod.ko
+           % /sbin/modinfo /lib/modules/`uname -r`/kernel/net/bridge/bridge.ko
+
+        Compare the "vermagic" lines output by the two commands.  If
+        they differ, then Open vSwitch was built for the wrong kernel.
+
+      - If you decide to report a bug or ask a question related to
+        module loading, please include the output from the "dmesg" and
+        "modinfo" commands mentioned above.
+
 7. Initialize the configuration database using ovsdb-tool, e.g.:
 
       % ovsdb-tool create /usr/local/etc/ovs-vswitchd.conf.db vswitchd/vswitch.ovsschema