xref: /linux-6.15/include/linux/once.h (revision dbefa1f3)

/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _LINUX_ONCE_H
#define _LINUX_ONCE_H

#include <linux/types.h>
#include <linux/jump_label.h>

/* Helpers used from arbitrary contexts.
 * Hard irqs are blocked, be cautious.
 */
bool __do_once_start(bool *done, unsigned long *flags);
void __do_once_done(bool *done, struct static_key_true *once_key,
		    unsigned long *flags, struct module *mod);

/* Variant for process contexts only. */
bool __do_once_sleepable_start(bool *done);
void __do_once_sleepable_done(bool *done, struct static_key_true *once_key,
			      struct module *mod);

/* 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!
 *
 * Note 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 __section(".data..once") ___done = false;	     \
		static DEFINE_STATIC_KEY_TRUE(___once_key);		     \
		if (static_branch_unlikely(&___once_key)) {		     \
			unsigned long ___flags;				     \
			___ret = __do_once_start(&___done, &___flags);	     \
			if (unlikely(___ret)) {				     \
				func(__VA_ARGS__);			     \
				__do_once_done(&___done, &___once_key,	     \
					       &___flags, THIS_MODULE);	     \
			}						     \
		}							     \
		___ret;							     \
	})

/* Variant of DO_ONCE() for process/sleepable contexts. */
#define DO_ONCE_SLEEPABLE(func, ...)						\
	({									\
		bool ___ret = false;						\
		static bool __section(".data..once") ___done = false;		\
		static DEFINE_STATIC_KEY_TRUE(___once_key);			\
		if (static_branch_unlikely(&___once_key)) {			\
			___ret = __do_once_sleepable_start(&___done);		\
			if (unlikely(___ret)) {					\
				func(__VA_ARGS__);				\
				__do_once_sleepable_done(&___done, &___once_key,\
						    THIS_MODULE);		\
			}							\
		}								\
		___ret;								\
	})

#define get_random_once(buf, nbytes)					     \
	DO_ONCE(get_random_bytes, (buf), (nbytes))

#define get_random_sleepable_once(buf, nbytes)				     \
	DO_ONCE_SLEEPABLE(get_random_bytes, (buf), (nbytes))

#endif /* _LINUX_ONCE_H */