kthread: Fix task state in kthread worker if being frozen

When analyzing a kernel waring message, Peter pointed out that there is a race
condition when the kworker is being frozen and falls into try_to_freeze() with
TASK_INTERRUPTIBLE, which could trigger a might_sleep() warning in try_to_freeze().
Although the root cause is not related to freeze()[1], it is still worthy to fix
this issue ahead.

One possible race scenario:

        CPU 0                                           CPU 1
        -----                                           -----

        // kthread_worker_fn
        set_current_state(TASK_INTERRUPTIBLE);
                                                       suspend_freeze_processes()
                                                         freeze_processes
                                                           static_branch_inc(&freezer_active);
                                                         freeze_kernel_threads
                                                           pm_nosig_freezing = true;
        if (work) { //false
          __set_current_state(TASK_RUNNING);

        } else if (!freezing(current)) //false, been frozen

                      freezing():
                      if (static_branch_unlikely(&freezer_active))
                        if (pm_nosig_freezing)
                          return true;
          schedule()
	}

        // state is still TASK_INTERRUPTIBLE
        try_to_freeze()
          might_sleep() <--- warning

Fix this by explicitly set the TASK_RUNNING before entering
try_to_freeze().

Fixes: b56c0d89 ("kthread: implement kthread_worker")
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/lkml/Zs2ZoAcUsZMX2B%2FI@chenyu5-mobl2/ [1]

kernel/kthread.c

@@ -845,8 +845,16 @@ int kthread_worker_fn(void *worker_ptr)
 		 * event only cares about the address.
 		 */
 		trace_sched_kthread_work_execute_end(work, func);
-	} else if (!freezing(current))
+	} else if (!freezing(current)) {
 		schedule();
+	} else {
+		/*
+		 * Handle the case where the current remains
+		 * TASK_INTERRUPTIBLE. try_to_freeze() expects
+		 * the current to be TASK_RUNNING.
+		 */
+		__set_current_state(TASK_RUNNING);
+	}
 
 	try_to_freeze();
 	cond_resched();