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; where it is forbidden to use
 * after exit_notify(). task_struct is not properly rcu protected,
 * unless dealing with rcu-aware lists, ie: find_task_by_*().
 *
 * Alternatively we have task_rcu_dereference(), but the return
 * semantics have different implications which would break the
 * wakeup side. 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 *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)				\
({									\
	/*								\
	 * Complain if we are called after do_exit()/exit_notify(),     \
	 * as we cannot rely on the rcu critical region for the		\
	 * wakeup side.							\
	 */                                                             \
	WARN_ON(current->exit_state);                                   \
									\
	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_ */