LXR linux/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
   8bool __do_once_start(bool *done, unsigned long *flags);
   9void __do_once_done(bool *done, struct static_key_true *once_key,
  10                    unsigned long *flags, struct module *mod);
  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 * Note 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, THIS_MODULE);      \
  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 */
  61