Revert "device core: Remove deprecated create_singlethread_workqueue"

This reverts commit 2c507e4 ("device core: Remove deprecated
create_singlethread_workqueue").

This is a temporary fix for qemu-arm hangs on the following command that
schedule_work() triggers (?) dead-lock that nanosleep on hello->main()
never returns.

$ qemu-system-arm -M versatilepb -m 512M -nographic  -serial null \
-semihosting -kernel rumpobj/tests/hello

This should be fixed in a transparent way in a future.

Signed-off-by: Hajime Tazaki <thehajime@gmail.com>

drivers/base/dd.c

@@ -51,6 +51,7 @@
 static DEFINE_MUTEX(deferred_probe_mutex);
 static LIST_HEAD(deferred_probe_pending_list);
 static LIST_HEAD(deferred_probe_active_list);
+static struct workqueue_struct *deferred_wq;
 static atomic_t deferred_trigger_count = ATOMIC_INIT(0);
 
 /*
@@ -174,7 +175,7 @@ static void driver_deferred_probe_trigger(void)
 	 * Kick the re-probe thread.  It may already be scheduled, but it is
 	 * safe to kick it again.
 	 */
-	schedule_work(&deferred_probe_work);
+	queue_work(deferred_wq, &deferred_probe_work);
 }
 
 /**
@@ -210,10 +211,14 @@ void device_unblock_probing(void)
  */
 static int deferred_probe_initcall(void)
 {
+	deferred_wq = create_singlethread_workqueue("deferwq");
+	if (WARN_ON(!deferred_wq))
+		return -ENOMEM;
+
 	driver_deferred_probe_enable = true;
 	driver_deferred_probe_trigger();
 	/* Sort as many dependencies as possible before exiting initcalls */
-	flush_work(&deferred_probe_work);
+	flush_workqueue(deferred_wq);
 	return 0;
 }
 late_initcall(deferred_probe_initcall);
@@ -477,7 +482,8 @@ int driver_probe_done(void)
 void wait_for_device_probe(void)
 {
 	/* wait for the deferred probe workqueue to finish */
-	flush_work(&deferred_probe_work);
+	if (driver_deferred_probe_enable)
+		flush_workqueue(deferred_wq);
 
 	/* wait for the known devices to complete their probing */
 	wait_event(probe_waitqueue, atomic_read(&probe_count) == 0);