percpu: implement optional weak percpu definitions

/*

 * Base implementations of per-CPU variable declarations and definitions, where

 * the section in which the variable is to be placed is provided by the

 * 'section' argument.  This may be used to affect the parameters governing the

 * 'sec' argument.  This may be used to affect the parameters governing the

 * variable's storage.

 *

 * NOTE!  The sections for the DECLARE and for the DEFINE must match, lest

 * linkage errors occur due the compiler generating the wrong code to access

 * that section.

 */

#define DECLARE_PER_CPU_SECTION(type, name, section)			\

	extern								\

	__attribute__((__section__(PER_CPU_BASE_SECTION section)))	\

	PER_CPU_ATTRIBUTES __typeof__(type) per_cpu__##name

#define DEFINE_PER_CPU_SECTION(type, name, section)			\

	__attribute__((__section__(PER_CPU_BASE_SECTION section)))	\

	PER_CPU_ATTRIBUTES __typeof__(type) per_cpu__##name

#define __PCPU_ATTRS(sec)						\

	__attribute__((section(PER_CPU_BASE_SECTION sec)))		\

	PER_CPU_ATTRIBUTES

#define __PCPU_DUMMY_ATTRS						\

	__attribute__((section(".discard"), unused))

/*

 * s390 and alpha modules require percpu variables to be defined as

 * weak to force the compiler to generate GOT based external

 * references for them.  This is necessary because percpu sections

 * will be located outside of the usually addressable area.

 *

 * This definition puts the following two extra restrictions when

 * defining percpu variables.

 *

 * 1. The symbol must be globally unique, even the static ones.

 * 2. Static percpu variables cannot be defined inside a function.

 *

 * Archs which need weak percpu definitions should define

 * ARCH_NEEDS_WEAK_PER_CPU in asm/percpu.h when necessary.

 *

 * To ensure that the generic code observes the above two

 * restrictions, if CONFIG_DEBUG_FORCE_WEAK_PER_CPU is set weak

 * definition is used for all cases.

 */

#if defined(ARCH_NEEDS_WEAK_PER_CPU) || defined(CONFIG_DEBUG_FORCE_WEAK_PER_CPU)

/*

 * __pcpu_scope_* dummy variable is used to enforce scope.  It

 * receives the static modifier when it's used in front of

 * DEFINE_PER_CPU() and will trigger build failure if

 * DECLARE_PER_CPU() is used for the same variable.

 *

 * __pcpu_unique_* dummy variable is used to enforce symbol uniqueness

 * such that hidden weak symbol collision, which will cause unrelated

 * variables to share the same address, can be detected during build.

 */

#define DECLARE_PER_CPU_SECTION(type, name, sec)			\

	extern __PCPU_DUMMY_ATTRS char __pcpu_scope_##name;		\

	extern __PCPU_ATTRS(sec) __weak __typeof__(type) per_cpu__##name

#define DEFINE_PER_CPU_SECTION(type, name, sec)				\

	__PCPU_DUMMY_ATTRS char __pcpu_scope_##name;			\

	__PCPU_DUMMY_ATTRS char __pcpu_unique_##name;			\

	__PCPU_ATTRS(sec) __weak __typeof__(type) per_cpu__##name

#else

/*

 * Normal declaration and definition macros.

 */

#define DECLARE_PER_CPU_SECTION(type, name, sec)			\

	extern __PCPU_ATTRS(sec) __typeof__(type) per_cpu__##name

#define DEFINE_PER_CPU_SECTION(type, name, sec)				\

	__PCPU_ATTRS(sec) __typeof__(type) per_cpu__##name

#endif

/*

 * Variant on the per-CPU variable declaration/definition theme used for

	  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