rcutorture: Suppress forward-progress complaints during early boot

Some larger systems can take in excess of 50 seconds to complete their
early boot initcalls prior to spawing init.  This does not in any way
help the forward-progress judgments of built-in rcutorture (when
rcutorture is built as a module, the insmod or modprobe command normally
cannot happen until some time after boot completes).  This commit
therefore suppresses such complaints until about the time that init
is spawned.

This also includes a fix to a stupid error located by kbuild test robot.

[ paulmck: Apply kbuild test robot feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
[ paulmck: Fix to nohz_full slow-expediting recovery logic, per bpetkov. ]
[ paulmck: Restrict splat to CONFIG_PREEMPT_RT=y kernels and simplify. ]
Tested-by: Borislav Petkov <bp@alien8.de>

include/linux/rcutiny.h

@@ -83,6 +83,7 @@ void rcu_scheduler_starting(void);
 static inline void rcu_scheduler_starting(void) { }
 #endif /* #else #ifndef CONFIG_SRCU */
 static inline void rcu_end_inkernel_boot(void) { }
+static inline bool rcu_inkernel_boot_has_ended(void) { return true; }
 static inline bool rcu_is_watching(void) { return true; }
 static inline void rcu_momentary_dyntick_idle(void) { }
 static inline void kfree_rcu_scheduler_running(void) { }

include/linux/rcutree.h

@@ -54,6 +54,7 @@ void exit_rcu(void);
 void rcu_scheduler_starting(void);
 extern int rcu_scheduler_active __read_mostly;
 void rcu_end_inkernel_boot(void);
+bool rcu_inkernel_boot_has_ended(void);
 bool rcu_is_watching(void);
 #ifndef CONFIG_PREEMPTION
 void rcu_all_qs(void);

kernel/rcu/rcutorture.c

@@ -1067,7 +1067,8 @@ rcu_torture_writer(void *arg)
 		if (stutter_wait("rcu_torture_writer") &&
 		    !READ_ONCE(rcu_fwd_cb_nodelay) &&
 		    !cur_ops->slow_gps &&
-		    !torture_must_stop())
+		    !torture_must_stop() &&
+		    rcu_inkernel_boot_has_ended())
 			for (i = 0; i < ARRAY_SIZE(rcu_tortures); i++)
 				if (list_empty(&rcu_tortures[i].rtort_free) &&
 				    rcu_access_pointer(rcu_torture_current) !=

kernel/rcu/tree_exp.h

@@ -485,6 +485,7 @@ static bool synchronize_rcu_expedited_wait_once(long tlimit)
 static void synchronize_rcu_expedited_wait(void)
 {
 	int cpu;
+	unsigned long j;
 	unsigned long jiffies_stall;
 	unsigned long jiffies_start;
 	unsigned long mask;
@@ -496,7 +497,7 @@ static void synchronize_rcu_expedited_wait(void)
 	trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait"));
 	jiffies_stall = rcu_jiffies_till_stall_check();
 	jiffies_start = jiffies;
-	if (IS_ENABLED(CONFIG_NO_HZ_FULL)) {
+	if (tick_nohz_full_enabled() && rcu_inkernel_boot_has_ended()) {
 		if (synchronize_rcu_expedited_wait_once(1))
 			return;
 		rcu_for_each_leaf_node(rnp) {
@@ -508,6 +509,10 @@ static void synchronize_rcu_expedited_wait(void)
 				tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
 			}
 		}
+		j = READ_ONCE(jiffies_till_first_fqs);
+		if (synchronize_rcu_expedited_wait_once(j + HZ))
+			return;
+		WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT));
 	}
 
 	for (;;) {

kernel/rcu/update.c

@@ -183,6 +183,8 @@ void rcu_unexpedite_gp(void)
 }
 EXPORT_SYMBOL_GPL(rcu_unexpedite_gp);
 
+static bool rcu_boot_ended __read_mostly;
+
 /*
  * Inform RCU of the end of the in-kernel boot sequence.
  */
@@ -191,8 +193,18 @@ void rcu_end_inkernel_boot(void)
 	rcu_unexpedite_gp();
 	if (rcu_normal_after_boot)
 		WRITE_ONCE(rcu_normal, 1);
+	rcu_boot_ended = 1;
 }
 
+/*
+ * Let rcutorture know when it is OK to turn it up to eleven.
+ */
+bool rcu_inkernel_boot_has_ended(void)
+{
+	return rcu_boot_ended;
+}
+EXPORT_SYMBOL_GPL(rcu_inkernel_boot_has_ended);
+
 #endif /* #ifndef CONFIG_TINY_RCU */
 
 /*