rcu-tasks: Disable CPU hotplug across RCU tasks trace scans

This commit disables CPU hotplug across RCU tasks trace scans, which
is a first step towards correctly recognizing idle tasks "running" on
offline CPUs.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -910,16 +910,16 @@ static void rcu_tasks_trace_pregp_step(void)
 {
 	int cpu;
 
-	// Wait for CPU-hotplug paths to complete.
-	cpus_read_lock();
-	cpus_read_unlock();
-
 	// Allow for fast-acting IPIs.
 	atomic_set(&trc_n_readers_need_end, 1);
 
 	// There shouldn't be any old IPIs, but...
 	for_each_possible_cpu(cpu)
 		WARN_ON_ONCE(per_cpu(trc_ipi_to_cpu, cpu));
+
+	// Disable CPU hotplug across the tasklist scan.
+	// This also waits for all readers in CPU-hotplug code paths.
+	cpus_read_lock();
 }
 
 /* Do first-round processing for the specified task. */
@@ -935,6 +935,9 @@ static void rcu_tasks_trace_pertask(struct task_struct *t,
 /* Do intermediate processing between task and holdout scans. */
 static void rcu_tasks_trace_postscan(void)
 {
+	// Re-enable CPU hotplug now that the tasklist scan has completed.
+	cpus_read_unlock();
+
 	// Wait for late-stage exiting tasks to finish exiting.
 	// These might have passed the call to exit_tasks_rcu_finish().
 	synchronize_rcu();
@@ -979,6 +982,9 @@ static void check_all_holdout_tasks_trace(struct list_head *hop,
 {
 	struct task_struct *g, *t;
 
+	// Disable CPU hotplug across the holdout list scan.
+	cpus_read_lock();
+
 	list_for_each_entry_safe(t, g, hop, trc_holdout_list) {
 		// If safe and needed, try to check the current task.
 		if (READ_ONCE(t->trc_ipi_to_cpu) == -1 &&
@@ -991,6 +997,10 @@ static void check_all_holdout_tasks_trace(struct list_head *hop,
 		else if (needreport)
 			show_stalled_task_trace(t, firstreport);
 	}
+
+	// Re-enable CPU hotplug now that the holdout list scan has completed.
+	cpus_read_unlock();
+
 	if (needreport) {
 		if (firstreport)
 			pr_err("INFO: rcu_tasks_trace detected stalls? (Late IPI?)\n");