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>

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index dd311e93ed0ff0adbd5566a862dab932ba6a38e6..361e17d571910b760dc4a743e6d5e98c82ef0c2b 100644 (file)
--- a/kernel/rcu/tasks.h
+++ b/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");