[PATCH RT 7/9] rcu: Update rcuwait

[Daniel Wagner <wagi@monom.org>]

v4.19.255-rt114-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------


This is an all in one commit backporting updates for rcuwait:
 - 03f4b48edae7 ("rcuwait: Annotate task_struct with __rcu")
 - 191a43be61d6 ("rcuwait: Introduce rcuwait_active()")
 - 5c21f7b322cb ("rcuwait: Introduce prepare_to and finish_rcuwait")
 - 80fbaf1c3f29 ("rcuwait: Add @state argument to rcuwait_wait_event()")
 - 9d9a6ebfea32 ("rcuwait: Let rcuwait_wake_up() return whether or not a task was awoken")
 - 58d4292bd037 ("rcu: Uninline multi-use function: finish_rcuwait()")

Signed-off-by: Daniel Wagner <wagi@monom.org>
---
 include/linux/rcuwait.h       | 42 +++++++++++++++++++++++++++--------
 kernel/exit.c                 |  7 ++++--
 kernel/locking/percpu-rwsem.c |  2 +-
 kernel/rcu/update.c           |  8 +++++++
 4 files changed, 47 insertions(+), 12 deletions(-)

diff --git a/include/linux/rcuwait.h b/include/linux/rcuwait.h
index 90bfa3279a01..4fe9ecd56aac 100644
--- a/include/linux/rcuwait.h
+++ b/include/linux/rcuwait.h
@@ -3,6 +3,7 @@
 #define _LINUX_RCUWAIT_H_
 
 #include <linux/rcupdate.h>
+#include <linux/sched/signal.h>
 
 /*
  * rcuwait provides a way of blocking and waking up a single
@@ -18,7 +19,7 @@
  * awoken.
  */
 struct rcuwait {
-	struct task_struct *task;
+	struct task_struct __rcu *task;
 };
 
 #define __RCUWAIT_INITIALIZER(name)		\
@@ -29,14 +30,33 @@ static inline void rcuwait_init(struct rcuwait *w)
 	w->task = NULL;
 }
 
-extern void rcuwait_wake_up(struct rcuwait *w);
+extern int rcuwait_wake_up(struct rcuwait *w);
+
+/*
+ * 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);
+}
 
 /*
  * The caller is responsible for locking around rcuwait_wait_event(),
- * such that writes to @task are properly serialized.
+ * and [prepare_to/finish]_rcuwait() such that writes to @task are
+ * properly serialized.
  */
-#define rcuwait_wait_event(w, condition)				\
+
+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;							\
 	/*								\
 	 * Complain if we are called after do_exit()/exit_notify(),     \
 	 * as we cannot rely on the rcu critical region for the		\
@@ -44,21 +64,25 @@ extern void rcuwait_wake_up(struct rcuwait *w);
 	 */                                                             \
 	WARN_ON(current->exit_state);                                   \
 									\
-	rcu_assign_pointer((w)->task, current);				\
+	prepare_to_rcuwait(w);						\
 	for (;;) {							\
 		/*							\
 		 * Implicit barrier (A) pairs with (B) in		\
 		 * rcuwait_wake_up().					\
 		 */							\
-		set_current_state(TASK_UNINTERRUPTIBLE);		\
+		set_current_state(state);				\
 		if (condition)						\
 			break;						\
 									\
+		if (signal_pending_state(state, current)) {		\
+			__ret = -EINTR;					\
+			break;						\
+		}							\
+									\
 		schedule();						\
 	}								\
-									\
-	WRITE_ONCE((w)->task, NULL);					\
-	__set_current_state(TASK_RUNNING);				\
+	finish_rcuwait(w);						\
+	__ret;								\
 })
 
 #endif /* _LINUX_RCUWAIT_H_ */
diff --git a/kernel/exit.c b/kernel/exit.c
index 2a414fc71b87..cf68896a94fa 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -291,8 +291,9 @@ struct task_struct *task_rcu_dereference(struct task_struct **ptask)
 	return task;
 }
 
-void rcuwait_wake_up(struct rcuwait *w)
+int rcuwait_wake_up(struct rcuwait *w)
 {
+	int ret = 0;
 	struct task_struct *task;
 
 	rcu_read_lock();
@@ -316,8 +317,10 @@ void rcuwait_wake_up(struct rcuwait *w)
 	 */
 	task = rcu_dereference(w->task);
 	if (task)
-		wake_up_process(task);
+		ret = wake_up_process(task);
 	rcu_read_unlock();
+
+	return ret;
 }
 
 /*
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 883cf1b92d90..41787e80dbde 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -159,7 +159,7 @@ void percpu_down_write(struct percpu_rw_semaphore *sem)
 	 */
 
 	/* Wait for all now active readers to complete. */
-	rcuwait_wait_event(&sem->writer, readers_active_check(sem));
+	rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE);
 }
 EXPORT_SYMBOL_GPL(percpu_down_write);
 
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index ed75addd3ccd..4b2ce6bb94a4 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -53,6 +53,7 @@
 #include <linux/rcupdate_wait.h>
 #include <linux/sched/isolation.h>
 #include <linux/kprobes.h>
+#include <linux/rcuwait.h>
 
 #define CREATE_TRACE_POINTS
 
@@ -375,6 +376,13 @@ 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)
 {
-- 
2.38.0