include/linux/once.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #ifndef _LINUX_ONCE_H
   3 #define _LINUX_ONCE_H
   4
   5 #include <linux/types.h>
   6 #include <linux/jump_label.h>
   7
   8 bool __do_once_start(bool *done, unsigned long *flags);
   9 void __do_once_done(bool *done, struct static_key_true *once_key,
  10                     unsigned long *flags);
  11
  12 /* Call a function exactly once. The idea of DO_ONCE() is to perform
  13  * a function call such as initialization of random seeds, etc, only
  14  * once, where DO_ONCE() can live in the fast-path. After @func has
  15  * been called with the passed arguments, the static key will patch
  16  * out the condition into a nop. DO_ONCE() guarantees type safety of
  17  * arguments!
  18  *
  19  * Not that the following is not equivalent ...
  20  *
  21  *   DO_ONCE(func, arg);
  22  *   DO_ONCE(func, arg);
  23  *
  24  * ... to this version:
  25  *
  26  *   void foo(void)
  27  *   {
  28  *     DO_ONCE(func, arg);
  29  *   }
  30  *
  31  *   foo();
  32  *   foo();
  33  *
  34  * In case the one-time invocation could be triggered from multiple
  35  * places, then a common helper function must be defined, so that only
  36  * a single static key will be placed there!
  37  */
  38 #define DO_ONCE(func, ...)                                                   \
  39         ({                                                                   \
  40                 bool ___ret = false;                                         \
  41                 static bool ___done = false;                                 \
  42                 static DEFINE_STATIC_KEY_TRUE(___once_key);                  \
  43                 if (static_branch_unlikely(&___once_key)) {                  \
  44                         unsigned long ___flags;                              \
  45                         ___ret = __do_once_start(&___done, &___flags);       \
  46                         if (unlikely(___ret)) {                              \
  47                                 func(__VA_ARGS__);                           \
  48                                 __do_once_done(&___done, &___once_key,       \
  49                                                &___flags);                   \
  50                         }                                                    \
  51                 }                                                            \
  52                 ___ret;                                                      \
  53         })
  54
  55 #define get_random_once(buf, nbytes)                                         \
  56         DO_ONCE(get_random_bytes, (buf), (nbytes))
  57 #define get_random_once_wait(buf, nbytes)                                    \
  58         DO_ONCE(get_random_bytes_wait, (buf), (nbytes))                      \
  59
  60 #endif /* _LINUX_ONCE_H */