New: dccp_ipv4 has >1 usage count and cannot be removed

(switched to email. Please respond via emailed reply-to-all, not via the bugzilla web interface).

| > http://bugzilla.kernel.org/show_bug.cgi?id=13692
| > 
| >            Summary: dccp_ipv4 has >1 usage count and cannot be removed
<snip>
| > 
| > Upon loading of dccp_ipv4 (either through autoload or explicitly, with
| > modprobe) the usage count of the module is immediately set to 2 and the module
| > can not be unloaded.
| > 
| > Usage count seems to be correctly incremented/decremented during operation, but
| > never gets lower than 2.
| > 

I am not sure that this should be treated as a bug. DCCP has a control socket which causes extra reference counts. The above issue, however, is harmless, since dccp_ipv4 can be removed with "rmmod -f", as shown in the trace below.

On the contrary, this provides protection against accidental removal. The module can safely be removed when all timewait sockets have been released. Someone wishing to unload the module should be aware of this possibility and ensured, via 'ss -dane', that no more sockets are used.

In a very early version of DCCP (~ 2005 .. 2006) there used to be an "unload hack" kernel config option:

config IP_DCCP_UNLOAD_HACK
depends on IP_DCCP=m && IP_DCCP_CCID3=m
bool "DCCP control sock unload hack"
---help---
Enable this to be able to unload the dccp module when the it has only one refcount held, the control sock one. Just execute "rmmod dccp_ccid3 dccp"

Just say N.
==> The last sentence also seems to capture the mailing list consensus;
the question whether to reinstate the unload hack came up several times but was not met with widespread approval.

In summary, if a user knows what s/he is doing, it is possible to safely unload dccp_ipv{4,6} when DCCP is compiled as a module.

Originally the unload option was there mainly to allow work on the modules, there was even one suggestion to completely disable module unloading.

I believe that the current state provides some protection against accidental/unwanted removal by unsuspecting users. Arnaldo, what do you think?

Here is the trace (and it worked like this for the past 4 years):
~ $ lsmod
Module Size Used by

~ $ modprobe -v dccp_ipv4
insmod /lib/modules/2.6.31-rc1/kernel/net/dccp/dccp.ko insmod /lib/modules/2.6.31-rc1/kernel/net/dccp/dccp_ipv4.ko

~ $ lsmod

  Module                  Size  Used by
  dccp_ipv4              14856  2 
  dccp                  102984  1 dccp_ipv4

~ $ rmmod -f dccp_ipv4
~ $ lsmod
Module Size Used by
dccp 102984 0
~ $ rmmod dccp
~ $ lsmod
Module Size Used by