[7/9] rcu: Intrdroduce rcuwait.

diff --git a/include/linux/rcuwait.h b/include/linux/rcuwait.h
new file mode 100644
index 0000000000000..12a9d1ad01ccb
--- /dev/null
+++ b/include/linux/rcuwait.h
@@ -0,0 +1,76 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_RCUWAIT_H_
+#define _LINUX_RCUWAIT_H_
+
+#include <linux/rcupdate.h>
+#include <linux/sched.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;
+}
+
+/*
+ * Note: this provides no serialization and, just as with waitqueues,
+ * requires care to estimate as to whether or not the wait is active.
+ */
+static inline int rcuwait_active(struct rcuwait *w)
+{
+	return !!rcu_access_pointer(w->task);
+}
+
+extern int rcuwait_wake_up(struct rcuwait *w);
+
+/*
+ * The caller is responsible for locking around rcuwait_wait_event(),
+ * and [prepare_to/finish]_rcuwait() such that writes to @task are
+ * properly serialized.
+ */
+
+static inline void prepare_to_rcuwait(struct rcuwait *w)
+{
+	rcu_assign_pointer(w->task, current);
+}
+
+extern void finish_rcuwait(struct rcuwait *w);
+
+#define rcuwait_wait_event(w, condition, state)				\
+({									\
+	int __ret = 0;							\
+	prepare_to_rcuwait(w);						\
+	for (;;) {							\
+		/*							\
+		 * Implicit barrier (A) pairs with (B) in		\
+		 * rcuwait_wake_up().					\
+		 */							\
+		set_current_state(state);				\
+		if (condition)						\
+			break;						\
+									\
+		if (signal_pending_state(state, current)) {		\
+			__ret = -EINTR;					\
+			break;						\
+		}							\
+									\
+		schedule();						\
+	}								\
+	finish_rcuwait(w);						\
+	__ret;								\
+})
+
+#endif /* _LINUX_RCUWAIT_H_ */
diff --git a/kernel/exit.c b/kernel/exit.c
index 89e38e5929a9c..6cfbde6a83629 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -54,6 +54,7 @@ 
 #include <linux/writeback.h>
 #include <linux/shm.h>
 #include <linux/kcov.h>
+#include <linux/rcuwait.h>
 
 #include <asm/uaccess.h>
 #include <asm/unistd.h>
@@ -286,6 +287,35 @@  struct task_struct *try_get_task_struct(struct task_struct **ptask)
 	return task;
 }
 
+int rcuwait_wake_up(struct rcuwait *w)
+{
+	int ret = 0;
+	struct task_struct *task;
+
+	rcu_read_lock();
+
+	/*
+	 * Order condition vs @task, such that everything prior to the load
+	 * of @task is visible. This is the condition as to why the user called
+	 * rcuwait_wake() in the first place. Pairs with set_current_state()
+	 * barrier (A) in rcuwait_wait_event().
+	 *
+	 *    WAIT                WAKE
+	 *    [S] tsk = current   [S] cond = true
+	 *        MB (A)              MB (B)
+	 *    [L] cond            [L] tsk
+	 */
+	smp_mb(); /* (B) */
+
+	task = rcu_dereference(w->task);
+	if (task)
+		ret = wake_up_process(task);
+	rcu_read_unlock();
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(rcuwait_wake_up);
+
 /*
  * Determine if a process group is "orphaned", according to the POSIX
  * definition in 2.2.2.52.  Orphaned process groups are not to be affected
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index ee02e1e1b3e57..c4ffd7ead78e2 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -49,6 +49,7 @@ 
 #include <linux/moduleparam.h>
 #include <linux/kthread.h>
 #include <linux/tick.h>
+#include <linux/rcuwait.h>
 
 #define CREATE_TRACE_POINTS
 
@@ -372,6 +373,13 @@  void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array,
 }
 EXPORT_SYMBOL_GPL(__wait_rcu_gp);
 
+void finish_rcuwait(struct rcuwait *w)
+{
+	rcu_assign_pointer(w->task, NULL);
+	__set_current_state(TASK_RUNNING);
+}
+EXPORT_SYMBOL_GPL(finish_rcuwait);
+
 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
 void init_rcu_head(struct rcu_head *head)
 {