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once: make helper generic for calling functions once

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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>
This commit is contained in:
Hannes Frederic Sowa 2015-10-08 01:20:36 +02:00 committed by David S. Miller
parent 46234253b9
commit c90aeb9482
2 changed files with 76 additions and 35 deletions
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61
include/linux/once.h
View file

@ -4,21 +4,54 @@
#include <linux/types.h> #include <linux/types.h>
#include <linux/jump_label.h> #include <linux/jump_label.h>
bool __get_random_once(void *buf, int nbytes, bool *done, bool __do_once_start(bool *done, unsigned long *flags);
struct static_key *once_key); void __do_once_done(bool *done, struct static_key *once_key,
unsigned long *flags);
#define get_random_once(buf, nbytes) \ /* Call a function exactly once. The idea of DO_ONCE() is to perform
({ \ * a function call such as initialization of random seeds, etc, only
bool ___ret = false; \ * once, where DO_ONCE() can live in the fast-path. After @func has
static bool ___done = false; \ * been called with the passed arguments, the static key will patch
static struct static_key ___once_key = \ * out the condition into a nop. DO_ONCE() guarantees type safety of
STATIC_KEY_INIT_TRUE; \ * arguments!
if (static_key_true(&___once_key)) \ *
___ret = __get_random_once((buf), \ * Not that the following is not equivalent ...
(nbytes), \ *
&___done, \ * DO_ONCE(func, arg);
&___once_key); \ * DO_ONCE(func, arg);
___ret; \ *
* ... 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 */ #endif /* _LINUX_ONCE_H */

50
lib/once.c
View file

@ -3,52 +3,60 @@
#include <linux/once.h> #include <linux/once.h>
#include <linux/random.h> #include <linux/random.h>
struct __random_once_work { struct once_work {
struct work_struct work; struct work_struct work;
struct static_key *key; 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);
BUG_ON(!static_key_enabled(work->key)); BUG_ON(!static_key_enabled(work->key));
static_key_slow_dec(work->key); static_key_slow_dec(work->key);
kfree(work); kfree(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); w = kmalloc(sizeof(*w), GFP_ATOMIC);
if (!w) if (!w)
return; return;
INIT_WORK(&w->work, __random_once_deferred); INIT_WORK(&w->work, once_deferred);
w->key = key; w->key = key;
schedule_work(&w->work); schedule_work(&w->work);
} }
bool __get_random_once(void *buf, int nbytes, bool *done, static DEFINE_SPINLOCK(once_lock);
struct static_key *once_key)
{
static DEFINE_SPINLOCK(lock);
unsigned long flags;
spin_lock_irqsave(&lock, flags); bool __do_once_start(bool *done, unsigned long *flags)
__acquires(once_lock)
{
spin_lock_irqsave(&once_lock, *flags);
if (*done) { 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.
*/
__acquire(once_lock);
return false; return false;
} }
get_random_bytes(buf, nbytes);
*done = true;
spin_unlock_irqrestore(&lock, flags);
__random_once_disable_jump(once_key);
return true; return true;
} }
EXPORT_SYMBOL(__get_random_once); EXPORT_SYMBOL(__do_once_start);
void __do_once_done(bool *done, struct static_key *once_key,
unsigned long *flags)
__releases(once_lock)
{
*done = true;
spin_unlock_irqrestore(&once_lock, *flags);
once_disable_jump(once_key);
}
EXPORT_SYMBOL(__do_once_done);