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
   7 bool __do_once_start(bool *done, unsigned long *flags);
   8 void __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 */