From c90aeb948222a7b3d3391d232ec4f50fd8322ad3 Mon Sep 17 00:00:00 2001 From: Hannes Frederic Sowa Date: Thu, 8 Oct 2015 01:20:36 +0200 Subject: once: make helper generic for calling functions once Make the get_random_once() helper generic enough, so that functions in general would only be called once, where one user of this is then net_get_random_once(). The only implementation specific call is to get_random_bytes(), all the rest of this *_once() facility would be duplicated among different subsystems otherwise. The new DO_ONCE() helper will be used by prandom() later on, but might also be useful for other scenarios/subsystems as well where a one-time initialization in often-called, possibly fast path code could occur. Signed-off-by: Hannes Frederic Sowa Signed-off-by: Daniel Borkmann Acked-by: Alexei Starovoitov Signed-off-by: David S. Miller --- include/linux/once.h | 61 ++++++++++++++++++++++++++++++++++++++++------------ 1 file changed, 47 insertions(+), 14 deletions(-) (limited to 'include/linux') diff --git a/include/linux/once.h b/include/linux/once.h index 2a83b538dd6a..285f12cb40e6 100644 --- a/include/linux/once.h +++ b/include/linux/once.h @@ -4,21 +4,54 @@ #include #include -bool __get_random_once(void *buf, int nbytes, bool *done, - struct static_key *once_key); +bool __do_once_start(bool *done, unsigned long *flags); +void __do_once_done(bool *done, struct static_key *once_key, + unsigned long *flags); -#define get_random_once(buf, nbytes) \ - ({ \ - bool ___ret = false; \ - static bool ___done = false; \ - static struct static_key ___once_key = \ - STATIC_KEY_INIT_TRUE; \ - if (static_key_true(&___once_key)) \ - ___ret = __get_random_once((buf), \ - (nbytes), \ - &___done, \ - &___once_key); \ - ___ret; \ +/* Call a function exactly once. The idea of DO_ONCE() is to perform + * a function call such as initialization of random seeds, etc, only + * once, where DO_ONCE() can live in the fast-path. After @func has + * been called with the passed arguments, the static key will patch + * out the condition into a nop. DO_ONCE() guarantees type safety of + * arguments! + * + * Not that the following is not equivalent ... + * + * DO_ONCE(func, arg); + * DO_ONCE(func, arg); + * + * ... to this version: + * + * void foo(void) + * { + * DO_ONCE(func, arg); + * } + * + * foo(); + * foo(); + * + * In case the one-time invocation could be triggered from multiple + * places, then a common helper function must be defined, so that only + * a single static key will be placed there! + */ +#define DO_ONCE(func, ...) \ + ({ \ + bool ___ret = false; \ + static bool ___done = false; \ + static struct static_key ___once_key = STATIC_KEY_INIT_TRUE; \ + if (static_key_true(&___once_key)) { \ + unsigned long ___flags; \ + ___ret = __do_once_start(&___done, &___flags); \ + if (unlikely(___ret)) { \ + func(__VA_ARGS__); \ + __do_once_done(&___done, &___once_key, \ + &___flags); \ + } \ + } \ + ___ret; \ }) +#define get_random_once(buf, nbytes) \ + DO_ONCE(get_random_bytes, (buf), (nbytes)) + #endif /* _LINUX_ONCE_H */ -- cgit v1.2.3