Release 4.7 include/linux/once.h

#ifndef _LINUX_ONCE_H

#define _LINUX_ONCE_H

#include <linux/types.h>
#include <linux/jump_label.h>

bool __do_once_start(bool *done, unsigned long *flags);
void __do_once_done(bool *done, struct static_key *once_key,
		    unsigned long *flags);

/* Call a function exactly once. The idea of DO_ONCE() is to perform
 * a function call such as initialization of random seeds, etc, only
 * once, where DO_ONCE() can live in the fast-path. After @func has
 * been called with the passed arguments, the static key will patch
 * out the condition into a nop. DO_ONCE() guarantees type safety of
 * arguments!
 *
 * Not that the following is not equivalent ...
 *
 *   DO_ONCE(func, arg);
 *   DO_ONCE(func, arg);
 *
 * ... to this version:
 *
 *   void foo(void)
 *   {
 *     DO_ONCE(func, arg);
 *   }
 *
 *   foo();
 *   foo();
 *
 * In case the one-time invocation could be triggered from multiple
 * places, then a common helper function must be defined, so that only
 * a single static key will be placed there!
 */

#define DO_ONCE(func, ...)						     \
	({                                                                   \
                bool ___ret = false;                                         \
                static bool ___done = false;                                 \
                static struct static_key ___once_key = STATIC_KEY_INIT_TRUE; \
                if (static_key_true(&___once_key)) {                         \
                        unsigned long ___flags;                              \
                        ___ret = __do_once_start(&___done, &___flags);       \
                        if (unlikely(___ret)) {                              \
                                func(__VA_ARGS__);                           \
                                __do_once_done(&___done, &___once_key,       \
                                               &___flags);                   \
                        }                                                    \
                }                                                            \
                ___ret;                                                      \
        })


#define get_random_once(buf, nbytes)					     \
	DO_ONCE(get_random_bytes, (buf), (nbytes))

#endif /* _LINUX_ONCE_H */
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