CONFIG_PROVE_RCU breaks proprietary modules (rcu_lock_map)

[Tilman Schmidt <tilman@imap.cc>]

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I know there's not much sympathy for troubles with proprietary modules
here, but the following might save some trouble to kernel developers as
well as users forced to resort to proprietary drivers:

Setting the kernel configuration option PROVE_RCU creates a reference to
the GPL-exported global variable rcu_lock_map even in modules that do
not use RCU. In the case of non-GPL modules, this leads to a build
failure because of the license incompatibility.
(I did not find out where that reference comes from.)

So if building a proprietary module fails with the message:

FATAL: modpost: GPL-incompatible module <module> uses GPL-only symbol
'rcu_lock_map'

try rebuilding your kernel with the PROVE_RCU option disabled.

Hope it helps someone.

Tilman
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