rcu: Track laziness during boot and suspend

Boot and suspend/resume should not be slowed down in kernels built with
CONFIG_RCU_LAZY=y.  In particular, suspend can sometimes fail in such
kernels.

This commit therefore adds rcu_async_hurry(), rcu_async_relax(), and
rcu_async_should_hurry() functions that track whether or not either
a boot or a suspend/resume operation is in progress.  This will
enable a later commit to refrain from laziness during those times.

Export rcu_async_should_hurry(), rcu_async_hurry(), and rcu_async_relax()
for later use by rcutorture.

[ paulmck: Apply feedback from Steve Rostedt. ]

Fixes: 3cb278e73b ("rcu: Make call_rcu() lazy to save power")
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/rcu.h

@@ -448,14 +448,20 @@ do {									\
 /* Tiny RCU doesn't expedite, as its purpose in life is instead to be tiny. */
 static inline bool rcu_gp_is_normal(void) { return true; }
 static inline bool rcu_gp_is_expedited(void) { return false; }
+static inline bool rcu_async_should_hurry(void) { return false; }
 static inline void rcu_expedite_gp(void) { }
 static inline void rcu_unexpedite_gp(void) { }
+static inline void rcu_async_hurry(void) { }
+static inline void rcu_async_relax(void) { }
 static inline void rcu_request_urgent_qs_task(struct task_struct *t) { }
 #else /* #ifdef CONFIG_TINY_RCU */
 bool rcu_gp_is_normal(void);     /* Internal RCU use. */
 bool rcu_gp_is_expedited(void);  /* Internal RCU use. */
+bool rcu_async_should_hurry(void);  /* Internal RCU use. */
 void rcu_expedite_gp(void);
 void rcu_unexpedite_gp(void);
+void rcu_async_hurry(void);
+void rcu_async_relax(void);
 void rcupdate_announce_bootup_oddness(void);
 #ifdef CONFIG_TASKS_RCU_GENERIC
 void show_rcu_tasks_gp_kthreads(void);

kernel/rcu/tree.c

@@ -4414,11 +4414,13 @@ static int rcu_pm_notify(struct notifier_block *self,
 	switch (action) {
 	case PM_HIBERNATION_PREPARE:
 	case PM_SUSPEND_PREPARE:
+		rcu_async_hurry();
 		rcu_expedite_gp();
 		break;
 	case PM_POST_HIBERNATION:
 	case PM_POST_SUSPEND:
 		rcu_unexpedite_gp();
+		rcu_async_relax();
 		break;
 	default:
 		break;

kernel/rcu/update.c

@@ -144,8 +144,45 @@ bool rcu_gp_is_normal(void)
 }
 EXPORT_SYMBOL_GPL(rcu_gp_is_normal);
 
-static atomic_t rcu_expedited_nesting = ATOMIC_INIT(1);
+static atomic_t rcu_async_hurry_nesting = ATOMIC_INIT(1);
+/*
+ * Should call_rcu() callbacks be processed with urgency or are
+ * they OK being executed with arbitrary delays?
+ */
+bool rcu_async_should_hurry(void)
+{
+	return !IS_ENABLED(CONFIG_RCU_LAZY) ||
+	       atomic_read(&rcu_async_hurry_nesting);
+}
+EXPORT_SYMBOL_GPL(rcu_async_should_hurry);
 
+/**
+ * rcu_async_hurry - Make future async RCU callbacks not lazy.
+ *
+ * After a call to this function, future calls to call_rcu()
+ * will be processed in a timely fashion.
+ */
+void rcu_async_hurry(void)
+{
+	if (IS_ENABLED(CONFIG_RCU_LAZY))
+		atomic_inc(&rcu_async_hurry_nesting);
+}
+EXPORT_SYMBOL_GPL(rcu_async_hurry);
+
+/**
+ * rcu_async_relax - Make future async RCU callbacks lazy.
+ *
+ * After a call to this function, future calls to call_rcu()
+ * will be processed in a lazy fashion.
+ */
+void rcu_async_relax(void)
+{
+	if (IS_ENABLED(CONFIG_RCU_LAZY))
+		atomic_dec(&rcu_async_hurry_nesting);
+}
+EXPORT_SYMBOL_GPL(rcu_async_relax);
+
+static atomic_t rcu_expedited_nesting = ATOMIC_INIT(1);
 /*
  * Should normal grace-period primitives be expedited?  Intended for
  * use within RCU.  Note that this function takes the rcu_expedited
@@ -195,6 +232,7 @@ static bool rcu_boot_ended __read_mostly;
 void rcu_end_inkernel_boot(void)
 {
 	rcu_unexpedite_gp();
+	rcu_async_relax();
 	if (rcu_normal_after_boot)
 		WRITE_ONCE(rcu_normal, 1);
 	rcu_boot_ended = true;