linux/include/linux/percpu-defs.h

#ifndef _LINUX_PERCPU_DEFS_H
#define _LINUX_PERCPU_DEFS_H

/*
 * Determine the real variable name from the name visible in the
 * kernel sources.
 */
#define per_cpu_var(var) per_cpu__##var

/*
 * 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
 * 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 PER_CPU_DEF_ATTRIBUTES			\
	__typeof__(type) per_cpu__##name

/*
 * Variant on the per-CPU variable declaration/definition theme used for
 * ordinary per-CPU variables.
 */
#define DECLARE_PER_CPU(type, name)					\
	DECLARE_PER_CPU_SECTION(type, name, "")

#define DEFINE_PER_CPU(type, name)					\
	DEFINE_PER_CPU_SECTION(type, name, "")

/*
 * Declaration/definition used for per-CPU variables that must come first in
 * the set of variables.
 */
#define DECLARE_PER_CPU_FIRST(type, name)				\
	DECLARE_PER_CPU_SECTION(type, name, PER_CPU_FIRST_SECTION)

#define DEFINE_PER_CPU_FIRST(type, name)				\
	DEFINE_PER_CPU_SECTION(type, name, PER_CPU_FIRST_SECTION)

/*
 * Declaration/definition used for per-CPU variables that must be cacheline
 * aligned under SMP conditions so that, whilst a particular instance of the
 * data corresponds to a particular CPU, inefficiencies due to direct access by
 * other CPUs are reduced by preventing the data from unnecessarily spanning
 * cachelines.
 *
 * An example of this would be statistical data, where each CPU's set of data
 * is updated by that CPU alone, but the data from across all CPUs is collated
 * by a CPU processing a read from a proc file.
 */
#define DECLARE_PER_CPU_SHARED_ALIGNED(type, name)			\
	DECLARE_PER_CPU_SECTION(type, name, PER_CPU_SHARED_ALIGNED_SECTION) \
	____cacheline_aligned_in_smp

#define DEFINE_PER_CPU_SHARED_ALIGNED(type, name)			\
	DEFINE_PER_CPU_SECTION(type, name, PER_CPU_SHARED_ALIGNED_SECTION) \
	____cacheline_aligned_in_smp

/*
 * Declaration/definition used for per-CPU variables that must be page aligned.
 */
#define DECLARE_PER_CPU_PAGE_ALIGNED(type, name)				\
	DECLARE_PER_CPU_SECTION(type, name, ".page_aligned")

#define DEFINE_PER_CPU_PAGE_ALIGNED(type, name)				\
	DEFINE_PER_CPU_SECTION(type, name, ".page_aligned")

/*
 * Intermodule exports for per-CPU variables.
 */
#define EXPORT_PER_CPU_SYMBOL(var) EXPORT_SYMBOL(per_cpu__##var)
#define EXPORT_PER_CPU_SYMBOL_GPL(var) EXPORT_SYMBOL_GPL(per_cpu__##var)


#endif /* _LINUX_PERCPU_DEFS_H */