netdev - Re: FW: ccid2/ccid3 oopses

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Date:	Wed, 9 Jan 2008 12:02:11 -0200
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Gerrit Renker <gerrit@....abdn.ac.uk>, devzero@....de,
	dccp@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: FW:  ccid2/ccid3 oopses

Em Wed, Jan 09, 2008 at 12:28:27PM +0000, Gerrit Renker escreveu:
> Roland, -
> 
> >> apparently, i got crashes when loading/unloading other driver modules just
> >> after ccid2 or ccid3 had been loaded/unloaded _once_ (have not used them at
> >> all, just modprobe module;modprobe -r module) >
> >> 
> <snip>
> >> the easiest way to reproduce is:
> >> 
> >> while true;do modprobe dccp_ccid2/3;modprobe -r dccp_ccid2/3;done
> >> after short time, the kernel oopses (messages below)
> >> 
> >> i`m not sure if this is worth to be filed at kernel bugzilla, so i`m contacting
> >> you personally first.
> >>
> The issue is known: once loaded, the DCCP modules can not be unloaded
> without causing a crash as the one you have observed. This is due to the
> fact that dccp_ipv{4,6} use control sockets which need to be released
> before the module can be unloaded.
> When the control sockets are not released then crashes will always
> result.
> In earlier versions of DCCP there was a kernel option known as "unload hack",
> which conditionally inserted 
> 	sock_release(dccp_v{4,6}_ctl_socket);
> in 
> 	dccp_v{4,6}_exit()
> 
> However, as the name says, it is a hack since there are other issues to 
> be considered:
> 	* sockets in timewait state
> 	* other wait states (e.g. half-open connections)
> 	* memory which has not been released
> 	* module dependencies
> 
> With regard to the latter, I am normally using the Unload Hack and
> release modules in the following order:
> 
> 	dccp_probe => dccp_ccid2 => dccp_ccid3 => dccp_tfrc_lib =>
>         dccp_ipv6  => dccp_ipv4  => dccp_diag  => dccp
> 
> Long story short
>  * the CCID/DCCP modules can currently not safely be unloaded
>  * maybe we should disable module unloading for the mainline kernel
>  * if anyone is interested to use the unload hack, here is the old patch
>    http://www.erg.abdn.ac.uk/users/gerrit/dccp/testing_dccp/Unload_Hack.diff

Gerrit, the control socket isn't attached to any CCID module, so the
CCID modules should be safe to remove, and IIRC they were safe to
unload.

The unload hack was for something else, for the core DCCP modules. We
can't unload because there are refcounts held by the control sock, so
the unload hack would just destroy the control sock and thus the module
refcount would reach zero and it could then be unloaded.

I've been consistently being sidetracked with work (huh :-)) and
couldn't look at this issue, but the CCID modules should be safe to
unload.

- Arnaldo
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