[PATCH v4 tip/core/rcu 30/38] rcu-tasks: Handle the running-offline idle-task special case

[paulmck]

From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>

The idle task corresponding to an offline CPU can appear to be running
while that CPU is offline.  This commit therefore adds checks for this
situation, treating it as a quiescent state.  Because the tasklist scan
and the holdout-list scan now exclude CPU-hotplug operations, readers
on the CPU-hotplug paths are still waited for.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/tasks.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index e8c03bd..1888a49 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -818,16 +818,20 @@ static bool trc_inspect_reader(struct task_struct *t, void *arg)
 {
 	int cpu = task_cpu(t);
 	bool in_qs = false;
+	bool ofl = cpu_is_offline(cpu);
 
 	if (task_curr(t)) {
+		WARN_ON_ONCE(ofl & !is_idle_task(t));
+
 		// If no chance of heavyweight readers, do it the hard way.
-		if (!IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
+		if (!ofl && !IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
 			return false;
 
 		// If heavyweight readers are enabled on the remote task,
 		// we can inspect its state despite its currently running.
 		// However, we cannot safely change its state.
-		if (!rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
+		if (!ofl && // Check for "running" idle tasks on offline CPUs.
+		    !rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
 			return false; // No quiescent state, do it the hard way.
 		in_qs = true;
 	} else {
-- 
2.9.5