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 /* Helpers used from arbitrary contexts.
   9  * Hard irqs are blocked, be cautious.
  10  */
  11 bool __do_once_start(bool *done, unsigned long *flags);
  12 void __do_once_done(bool *done, struct static_key_true *once_key,
  13                     unsigned long *flags, struct module *mod);
  14
  15 /* Variant for process contexts only. */
  16 bool __do_once_slow_start(bool *done);
  17 void __do_once_slow_done(bool *done, struct static_key_true *once_key,
  18                          struct module *mod);
  19
  20 /* Call a function exactly once. The idea of DO_ONCE() is to perform
  21  * a function call such as initialization of random seeds, etc, only
  22  * once, where DO_ONCE() can live in the fast-path. After @func has
  23  * been called with the passed arguments, the static key will patch
  24  * out the condition into a nop. DO_ONCE() guarantees type safety of
  25  * arguments!
  26  *
  27  * Not that the following is not equivalent ...
  28  *
  29  *   DO_ONCE(func, arg);
  30  *   DO_ONCE(func, arg);
  31  *
  32  * ... to this version:
  33  *
  34  *   void foo(void)
  35  *   {
  36  *     DO_ONCE(func, arg);
  37  *   }
  38  *
  39  *   foo();
  40  *   foo();
  41  *
  42  * In case the one-time invocation could be triggered from multiple
  43  * places, then a common helper function must be defined, so that only
  44  * a single static key will be placed there!
  45  */
  46 #define DO_ONCE(func, ...)                                                   \
  47         ({                                                                   \
  48                 bool ___ret = false;                                         \
  49                 static bool ___done = false;                                 \
  50                 static DEFINE_STATIC_KEY_TRUE(___once_key);                  \
  51                 if (static_branch_unlikely(&___once_key)) {                  \
  52                         unsigned long ___flags;                              \
  53                         ___ret = __do_once_start(&___done, &___flags);       \
  54                         if (unlikely(___ret)) {                              \
  55                                 func(__VA_ARGS__);                           \
  56                                 __do_once_done(&___done, &___once_key,       \
  57                                                &___flags, THIS_MODULE);      \
  58                         }                                                    \
  59                 }                                                            \
  60                 ___ret;                                                      \
  61         })
  62
  63 /* Variant of DO_ONCE() for process/sleepable contexts. */
  64 #define DO_ONCE_SLOW(func, ...)                                              \
  65         ({                                                                   \
  66                 bool ___ret = false;                                         \
  67                 static bool __section(.data.once) ___done = false;           \
  68                 static DEFINE_STATIC_KEY_TRUE(___once_key);                  \
  69                 if (static_branch_unlikely(&___once_key)) {                  \
  70                         ___ret = __do_once_slow_start(&___done);             \
  71                         if (unlikely(___ret)) {                              \
  72                                 func(__VA_ARGS__);                           \
  73                                 __do_once_slow_done(&___done, &___once_key,  \
  74                                                     THIS_MODULE);            \
  75                         }                                                    \
  76                 }                                                            \
  77                 ___ret;                                                      \
  78         })
  79
  80 #define get_random_once(buf, nbytes)                                         \
  81         DO_ONCE(get_random_bytes, (buf), (nbytes))
  82 #define get_random_once_wait(buf, nbytes)                                    \
  83         DO_ONCE(get_random_bytes_wait, (buf), (nbytes))                      \
  84
  85 #define get_random_slow_once(buf, nbytes)                                    \
  86         DO_ONCE_SLOW(get_random_bytes, (buf), (nbytes))
  87
  88 #endif /* _LINUX_ONCE_H */