Commit c90aeb94 authored by Hannes Frederic Sowa's avatar Hannes Frederic Sowa Committed by David S. Miller

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

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: default avatarHannes Frederic Sowa <>
Signed-off-by: default avatarDaniel Borkmann <>
Acked-by: default avatarAlexei Starovoitov <>
Signed-off-by: default avatarDavid S. Miller <>
parent 46234253
......@@ -4,21 +4,54 @@
#include <linux/types.h>
#include <linux/jump_label.h>
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 = \
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 */
......@@ -3,52 +3,60 @@
#include <linux/once.h>
#include <linux/random.h>
struct __random_once_work {
struct once_work {
struct work_struct work;
struct static_key *key;
static void __random_once_deferred(struct work_struct *w)
static void once_deferred(struct work_struct *w)
struct __random_once_work *work;
struct once_work *work;
work = container_of(w, struct __random_once_work, work);
work = container_of(w, struct once_work, work);
static void __random_once_disable_jump(struct static_key *key)
static void once_disable_jump(struct static_key *key)
struct __random_once_work *w;
struct once_work *w;
w = kmalloc(sizeof(*w), GFP_ATOMIC);
if (!w)
INIT_WORK(&w->work, __random_once_deferred);
INIT_WORK(&w->work, once_deferred);
w->key = key;
bool __get_random_once(void *buf, int nbytes, bool *done,
struct static_key *once_key)
static DEFINE_SPINLOCK(lock);
unsigned long flags;
static DEFINE_SPINLOCK(once_lock);
spin_lock_irqsave(&lock, flags);
bool __do_once_start(bool *done, unsigned long *flags)
spin_lock_irqsave(&once_lock, *flags);
if (*done) {
spin_unlock_irqrestore(&lock, flags);
spin_unlock_irqrestore(&once_lock, *flags);
/* Keep sparse happy by restoring an even lock count on
* this lock. In case we return here, we don't call into
* __do_once_done but return early in the DO_ONCE() macro.
return false;
get_random_bytes(buf, nbytes);
*done = true;
spin_unlock_irqrestore(&lock, flags);
return true;
void __do_once_done(bool *done, struct static_key *once_key,
unsigned long *flags)
*done = true;
spin_unlock_irqrestore(&once_lock, *flags);
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