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[Stephen Parry <1156138@xxxxxxxxxxxxxxxxxx>]

OK, I have spent quite a while bashing my head against this brick wall
now, trying to suss out why the lowlatency kernel build chokes, whilst
the generic does not. I suspect it is heavily buried somewhere either in
the compiler flags or in the deeper headers.

This is what I have found so far: if you compile low latency, as soon as
wl_cfg80211.c includes an invocation either to rtnl_lock or rtnl_unlock
(a fairly essential thing to do), the compile adds a U symbol for the
__rcu_read_lock and __rcu_read_unlock. Remove the all the rtnl_lock and
rtnl_unlock calls and the rcu symbols vanish too. Under generic they are
never there; it relies on whatever module defines rtnetlink to request
those symbols. The rtnetlink.h header is identical tween the two and
there is only minimal #if stuff, none of which seems to relate to low
latency stuff.

any ideas anyone?

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Packages, which is subscribed to bcmwl in Ubuntu.
https://bugs.launchpad.net/bugs/1156138

Title:
  bcmwl-kernel-source fails to build on lowlatency kernel [FATAL:
  modpost: GPL-incompatible module wl.ko uses GPL-only symbol
  '__rcu_read_unlock']

Status in “bcmwl” package in Ubuntu:
  Confirmed

Bug description:
  Ubuntu 13.04

  ProblemType: Package
  DistroRelease: Ubuntu 13.04
  Package: bcmwl-kernel-source 6.20.155.1+bdcom-0ubuntu6
  ProcVersionSignature: Ubuntu 3.8.0-13.22-generic 3.8.3
  Uname: Linux 3.8.0-13-generic i686
  NonfreeKernelModules: wl
  ApportVersion: 2.9.1-0ubuntu1
  Architecture: i386
  DKMSKernelVersion: 3.8.0-12-lowlatency
  Date: Sat Mar 16 23:37:03 2013
  InstallationDate: Installed on 2013-01-03 (72 days ago)
  InstallationMedia: Ubuntu 12.10 "Quantal Quetzal" - Release i386 (20121017.2)
  MarkForUpload: True
  PackageVersion: 6.20.155.1+bdcom-0ubuntu6
  SourcePackage: bcmwl
  Title: bcmwl-kernel-source 6.20.155.1+bdcom-0ubuntu6: bcmwl kernel module failed to build
  UpgradeStatus: Upgraded to raring on 2013-03-12 (4 days ago)

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