[PATCH 2/4] tracing/osnoise: timerlat_main: Disable migration before per-cpu access

[Crystal Wood <crwood@redhat.com>]

Avoid a preemptible-context splat if we get migrated (e.g. from hotplug
activity) before reaching this_cpu_osn_var(), and verify that we're
on the correct CPU after migrate_disable().

Signed-off-by: Crystal Wood <crwood@redhat.com>
---
 kernel/trace/trace_osnoise.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index e2e1ef3f5a61..a53a07e59197 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c
@@ -236,6 +236,7 @@ struct osnoise_variables {
 	struct mutex		lock; /* covers kthread and fdd changes */
 	struct task_struct	*kthread;
 	struct fd_data		*fdd;
+	int			cpu;
 
 	struct_group(zero,
 		bool			sampling;
@@ -1912,8 +1913,8 @@ static int wait_next_period(struct timerlat_variables *tlat)
  */
 static int timerlat_main(void *data)
 {
-	struct osnoise_variables *osn_var = this_cpu_osn_var();
-	struct timerlat_variables *tlat = this_cpu_tmr_var();
+	struct osnoise_variables *osn_var = data;
+	struct timerlat_variables *tlat;
 	struct timerlat_sample s;
 	struct sched_param sp;
 	unsigned long flags;
@@ -1933,6 +1934,13 @@ static int timerlat_main(void *data)
 	 * flag.
 	 */
 	migrate_disable();
+	if (osn_var->cpu != smp_processor_id()) {
+		migrate_enable();
+		return 1;
+	}
+
+	tlat = this_cpu_tmr_var();
+
 	raw_spin_lock_irqsave(&current->pi_lock, flags);
 	current->flags &= ~(PF_NO_SETAFFINITY);
 	raw_spin_unlock_irqrestore(&current->pi_lock, flags);
@@ -2061,7 +2069,7 @@ static int start_kthread(unsigned int cpu)
 		snprintf(comm, 24, "osnoise/%d", cpu);
 	}
 
-	kthread = kthread_run_on_cpu(main, NULL, cpu, comm);
+	kthread = kthread_run_on_cpu(main, cpu_osn_var(cpu), cpu, comm);
 
 	if (IS_ERR(kthread)) {
 		pr_err(BANNER "could not start sampling thread\n");
@@ -3207,8 +3215,12 @@ __init static int init_osnoise_tracer(void)
 
 	mutex_init(&interface_lock);
 
-	for_each_online_cpu(cpu)
-		mutex_init(&cpu_osn_var(cpu)->lock);
+	for_each_online_cpu(cpu) {
+		struct osnoise_variables *osn = cpu_osn_var(cpu);
+
+		mutex_init(&osn->lock);
+		osn->cpu = cpu;
+	}
 
 	cpumask_copy(&osnoise_cpumask, cpu_all_mask);
 
-- 
2.54.0