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TOMOYO Linux Cross Reference
Linux/include/linux/once.h

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  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 */
 61 

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