linux/include/linux/once.h
Hannes Frederic Sowa c90aeb9482 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 <hannes@stressinduktion.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
2015-10-08 05:26:36 -07:00

58 lines
1.7 KiB
C

#ifndef _LINUX_ONCE_H
#define _LINUX_ONCE_H
#include <linux/types.h>
#include <linux/jump_label.h>
bool __do_once_start(bool *done, unsigned long *flags);
void __do_once_done(bool *done, struct static_key *once_key,
unsigned long *flags);
/* 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 */