LXR linux/include/linux/once.h

   1#ifndef _LINUX_ONCE_H
   2#define _LINUX_ONCE_H
   3
   4#include <linux/types.h>
   5#include <linux/jump_label.h>
   6
   7bool __do_once_start(bool *done, unsigned long *flags);
   8void __do_once_done(bool *done, struct static_key *once_key,
   9                    unsigned long *flags);
  10
  11/* Call a function exactly once. The idea of DO_ONCE() is to perform
  12 * a function call such as initialization of random seeds, etc, only
  13 * once, where DO_ONCE() can live in the fast-path. After @func has
  14 * been called with the passed arguments, the static key will patch
  15 * out the condition into a nop. DO_ONCE() guarantees type safety of
  16 * arguments!
  17 *
  18 * Not that the following is not equivalent ...
  19 *
  20 *   DO_ONCE(func, arg);
  21 *   DO_ONCE(func, arg);
  22 *
  23 * ... to this version:
  24 *
  25 *   void foo(void)
  26 *   {
  27 *     DO_ONCE(func, arg);
  28 *   }
  29 *
  30 *   foo();
  31 *   foo();
  32 *
  33 * In case the one-time invocation could be triggered from multiple
  34 * places, then a common helper function must be defined, so that only
  35 * a single static key will be placed there!
  36 */
  37#define DO_ONCE(func, ...)                                                   \
  38        ({                                                                   \
  39                bool ___ret = false;                                         \
  40                static bool ___done = false;                                 \
  41                static struct static_key ___once_key = STATIC_KEY_INIT_TRUE; \
  42                if (static_key_true(&___once_key)) {                         \
  43                        unsigned long ___flags;                              \
  44                        ___ret = __do_once_start(&___done, &___flags);       \
  45                        if (unlikely(___ret)) {                              \
  46                                func(__VA_ARGS__);                           \
  47                                __do_once_done(&___done, &___once_key,       \
  48                                               &___flags);                   \
  49                        }                                                    \
  50                }                                                            \
  51                ___ret;                                                      \
  52        })
  53
  54#define get_random_once(buf, nbytes)                                         \
  55        DO_ONCE(get_random_bytes, (buf), (nbytes))
  56
  57#endif /* _LINUX_ONCE_H */
  58