psi: Fix race between psi_trigger_create/destroy

Race detected between psi_trigger_destroy/create as shown below, which
cause panic by accessing invalid psi_system->poll_wait->wait_queue_entry
and psi_system->poll_timer->entry->next. Under this modification, the
race window is removed by initialising poll_wait and poll_timer in
group_init which are executed only once at beginning.

  psi_trigger_destroy()                   psi_trigger_create()

  mutex_lock(trigger_lock);
  rcu_assign_pointer(poll_task, NULL);
  mutex_unlock(trigger_lock);
					  mutex_lock(trigger_lock);
					  if (!rcu_access_pointer(group->poll_task)) {
					    timer_setup(poll_timer, poll_timer_fn, 0);
					    rcu_assign_pointer(poll_task, task);
					  }
					  mutex_unlock(trigger_lock);

  synchronize_rcu();
  del_timer_sync(poll_timer); <-- poll_timer has been reinitialized by
                                  psi_trigger_create()

So, trigger_lock/RCU correctly protects destruction of
group->poll_task but misses this race affecting poll_timer and
poll_wait.

Fixes: 461daba0 ("psi: eliminate kthread_worker from psi trigger scheduling mechanism")
Co-developed-by: ziwei.dai <ziwei.dai@unisoc.com>
Signed-off-by: ziwei.dai <ziwei.dai@unisoc.com>
Co-developed-by: ke.wang <ke.wang@unisoc.com>
Signed-off-by: ke.wang <ke.wang@unisoc.com>
Signed-off-by: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lkml.kernel.org/r/1623371374-15664-1-git-send-email-huangzhaoyang@gmail.com

kernel/sched/psi.c

@@ -182,6 +182,8 @@ struct psi_group psi_system = {
 
 static void psi_avgs_work(struct work_struct *work);
 
+static void poll_timer_fn(struct timer_list *t);
+
 static void group_init(struct psi_group *group)
 {
 	int cpu;
@@ -201,6 +203,8 @@ static void group_init(struct psi_group *group)
 	memset(group->polling_total, 0, sizeof(group->polling_total));
 	group->polling_next_update = ULLONG_MAX;
 	group->polling_until = 0;
+	init_waitqueue_head(&group->poll_wait);
+	timer_setup(&group->poll_timer, poll_timer_fn, 0);
 	rcu_assign_pointer(group->poll_task, NULL);
 }
 
@@ -1157,9 +1161,7 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group,
 			return ERR_CAST(task);
 		}
 		atomic_set(&group->poll_wakeup, 0);
-		init_waitqueue_head(&group->poll_wait);
 		wake_up_process(task);
-		timer_setup(&group->poll_timer, poll_timer_fn, 0);
 		rcu_assign_pointer(group->poll_task, task);
 	}
 
@@ -1211,6 +1213,7 @@ static void psi_trigger_destroy(struct kref *ref)
 					group->poll_task,
 					lockdep_is_held(&group->trigger_lock));
 			rcu_assign_pointer(group->poll_task, NULL);
+			del_timer(&group->poll_timer);
 		}
 	}
 
@@ -1223,17 +1226,14 @@ static void psi_trigger_destroy(struct kref *ref)
 	 */
 	synchronize_rcu();
 	/*
-	 * Destroy the kworker after releasing trigger_lock to prevent a
+	 * Stop kthread 'psimon' after releasing trigger_lock to prevent a
 	 * deadlock while waiting for psi_poll_work to acquire trigger_lock
 	 */
 	if (task_to_destroy) {
 		/*
 		 * After the RCU grace period has expired, the worker
 		 * can no longer be found through group->poll_task.
-		 * But it might have been already scheduled before
-		 * that - deschedule it cleanly before destroying it.
 		 */
-		del_timer_sync(&group->poll_timer);
 		kthread_stop(task_to_destroy);
 	}
 	kfree(t);