include/linux/rcuwait.h

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

#include <linux/rcupdate.h>

/*
 * rcuwait provides a way of blocking and waking up a single
 * task in an rcu-safe manner.
 *
 * The only time @task is non-nil is when a user is blocked (or
 * checking if it needs to) on a condition, and reset as soon as we
 * know that the condition has succeeded and are awoken.
 */
struct rcuwait {
	struct task_struct __rcu *task;
};

#define __RCUWAIT_INITIALIZER(name)		\
	{ .task = NULL, }

static inline void rcuwait_init(struct rcuwait *w)
{
	w->task = NULL;
}

extern void rcuwait_wake_up(struct rcuwait *w);

/*
 * The caller is responsible for locking around rcuwait_wait_event(),
 * such that writes to @task are properly serialized.
 */
#define rcuwait_wait_event(w, condition)				\
({									\
	rcu_assign_pointer((w)->task, current);				\
	for (;;) {							\
		/*							\
		 * Implicit barrier (A) pairs with (B) in		\
		 * rcuwait_wake_up().					\
		 */							\
		set_current_state(TASK_UNINTERRUPTIBLE);		\
		if (condition)						\
			break;						\
									\
		schedule();						\
	}								\
									\
	WRITE_ONCE((w)->task, NULL);					\
	__set_current_state(TASK_RUNNING);				\
})

#endif /* _LINUX_RCUWAIT_H_ */