percpu: implement optional weak percpu definitions

Some archs (alpha and s390) need to use weak definitions for percpu
variables in modules so that the compiler generates external
references for them.

This patch implements weak percpu definitions which arch can enable by
defining ARCH_NEEDS_WEAK_PER_CPU in arch percpu header file.  This
weak definition adds the following two restrictions on percpu variable
definitions.

  1. percpu symbols must be unique whether static or not
  2. percpu variables can't be defined inside a function

To ensure that these restrictions are observed in generic code, config
option DEBUG_FORCE_WEAK_PER_CPU enables weak percpu definitions for
all cases.

This patch is inspired by Ivan Kokshaysky's alpha percpu patch.

[ Impact: stricter rules for percpu variables, one more debug config option ]

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: David Howells <dhowells@redhat.com>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>

1 files changed, 15 insertions, 0 deletions

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 23067ab1a73c..77e0d8b1b7c5 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -777,6 +777,21 @@ config DEBUG_BLOCK_EXT_DEVT
 
           Say N if you are unsure.
 
+config DEBUG_FORCE_WEAK_PER_CPU
+        bool "Force weak per-cpu definitions"
+        depends on DEBUG_KERNEL
+        help
+          s390 and alpha require percpu variables in modules to be
+          defined weak to work around addressing range issue which
+          puts the following two restrictions on percpu variable
+          definitions.
+
+          1. percpu symbols must be unique whether static or not
+          2. percpu variables can't be defined inside a function
+
+          To ensure that generic code follows the above rules, this
+          option forces all percpu variables to be defined as weak.
+
 config LKDTM
         tristate "Linux Kernel Dump Test Tool Module"
         depends on DEBUG_KERNEL