[PATCH v2 4/9] tracing/osnoise: kthreads: Verify CPU after migrate_disable()

[Crystal Wood <crwood@redhat.com>]

Pass osn vars as the kthread arg, and make sure it matches where ve're
runnning in case of (e.g.) hotplug issues.

Signed-off-by: Crystal Wood <crwood@redhat.com>
---
v2:
 - Do it for osnoise too, not just timerlat
 - I haven't been able to reproduce this in a while, and I'm not seeing the
   hole in the hotplug sync that would allow it, but I also don't see what
   would have allowed it in v1 (maybe it was in some pre-v1 iteration? or
   maybe just a tiny race window).

   So we can maybe drop this patch, though it still feels better to explicitly
   splat if problems happen rather than rely on this_cpu_osn_var() blowing up
   due to the affinity being set to multiple cpus.
---
 kernel/trace/trace_osnoise.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index 17cd5ea61e41..51e9b8837d20 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;
@@ -1724,6 +1725,7 @@ static inline int osnoise_migration_pending(void)
  */
 static int osnoise_main(void *data)
 {
+	struct osnoise_variables *osn = data;
 	unsigned long flags;
 
 	/*
@@ -1734,6 +1736,11 @@ static int osnoise_main(void *data)
 	 * flag.
 	 */
 	migrate_disable();
+	if (WARN_ON_ONCE(osn->cpu != smp_processor_id())) {
+		migrate_enable();
+		return 1;
+	}
+
 	raw_spin_lock_irqsave(&current->pi_lock, flags);
 	current->flags &= ~(PF_NO_SETAFFINITY);
 	raw_spin_unlock_irqrestore(&current->pi_lock, flags);
@@ -1924,8 +1931,8 @@ 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;
@@ -1945,6 +1952,13 @@ static int timerlat_main(void *data)
 	 * flag.
 	 */
 	migrate_disable();
+	if (WARN_ON_ONCE(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);
@@ -2076,7 +2090,7 @@ static int start_kthread(unsigned int cpu)
 		snprintf(comm, 24, "osnoise/%d", cpu);
 	}
 
-	kthread = kthread_create_on_cpu(main, NULL, cpu, comm);
+	kthread = kthread_create_on_cpu(main, cpu_osn_var(cpu), cpu, comm);
 
 	if (IS_ERR(kthread)) {
 		pr_err(BANNER "could not start sampling thread\n");
@@ -3253,6 +3267,7 @@ __init static int init_osnoise_tracer(void)
 		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