rcu: Reduce dyntick-idle state space

Both extended-quiescent-state entry and exit first update the nesting
counter and then adjust the dyntick-idle state.  This means that there
are four states: (1) Both nesting and dyntick idle indicate idle,
(2) Nesting indicates idle but dyntick idle does not, (3) Nesting indicates
non-idle and dyntick idle does not, and (4) Both nesting and dyntick
idle indicate non-idle.  This commit simplifies the state space by
eliminating #3, reversing the order of updates on exit from extended
quiescent state.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -928,21 +928,21 @@ void rcu_irq_exit_irqson(void)
  * we really have exited idle, and must do the appropriate accounting.
  * The caller must have disabled interrupts.
  */
-static void rcu_eqs_exit_common(long long oldval, int user)
+static void rcu_eqs_exit_common(long long newval, int user)
 {
 	RCU_TRACE(struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);)
 
 	rcu_dynticks_task_exit();
 	rcu_dynticks_eqs_exit();
 	rcu_cleanup_after_idle();
-	trace_rcu_dyntick(TPS("End"), oldval, rdtp->dynticks_nesting);
+	trace_rcu_dyntick(TPS("End"), rdtp->dynticks_nesting, newval);
 	if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
 	    !user && !is_idle_task(current)) {
 		struct task_struct *idle __maybe_unused =
 			idle_task(smp_processor_id());
 
 		trace_rcu_dyntick(TPS("Error on exit: not idle task"),
-				  oldval, rdtp->dynticks_nesting);
+				  rdtp->dynticks_nesting, newval);
 		rcu_ftrace_dump(DUMP_ORIG);
 		WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
 			  current->pid, current->comm,
@@ -967,8 +967,8 @@ static void rcu_eqs_exit(bool user)
 		rdtp->dynticks_nesting += DYNTICK_TASK_NEST_VALUE;
 	} else {
 		__this_cpu_inc(disable_rcu_irq_enter);
+		rcu_eqs_exit_common(DYNTICK_TASK_EXIT_IDLE, user);
 		rdtp->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
-		rcu_eqs_exit_common(oldval, user);
 		__this_cpu_dec(disable_rcu_irq_enter);
 	}
 }
@@ -1037,7 +1037,7 @@ void rcu_user_exit(void)
 void rcu_irq_enter(void)
 {
 	struct rcu_dynticks *rdtp;
-	long long oldval;
+	long long newval;
 
 	lockdep_assert_irqs_disabled();
 	rdtp = this_cpu_ptr(&rcu_dynticks);
@@ -1046,14 +1046,13 @@ void rcu_irq_enter(void)
 	if (rdtp->dynticks_nmi_nesting)
 		return;
 
-	oldval = rdtp->dynticks_nesting;
-	rdtp->dynticks_nesting++;
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
-		     rdtp->dynticks_nesting == 0);
-	if (oldval)
-		trace_rcu_dyntick(TPS("++="), oldval, rdtp->dynticks_nesting);
+	newval = rdtp->dynticks_nesting + 1;
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && newval == 0);
+	if (rdtp->dynticks_nesting)
+		trace_rcu_dyntick(TPS("++="), rdtp->dynticks_nesting, newval);
 	else
-		rcu_eqs_exit_common(oldval, true);
+		rcu_eqs_exit_common(newval, true);
+	rdtp->dynticks_nesting++;
 }
 
 /*