device core: Remove deprecated create_singlethread_workqueue

The workqueue "deferred_wq" queues a single work item
&deferred_probe_work and hence doesn't require ordering.
It is involved in probing devices and is not being used on a memory
reclaim path. Hence, it has been converted to use system_wq.

System workqueues have been able to handle high level of concurrency
for a long time now and hence it's not required to have a singlethreaded
workqueue just to gain concurrency. Unlike a dedicated per-cpu workqueue
created with create_singlethread_workqueue(), system_wq allows multiple
work items to overlap executions even on the same CPU; however, a
per-cpu workqueue doesn't have any CPU locality or global ordering
guarantee unless the target CPU is explicitly specified and thus the
increase of local concurrency shouldn't make any difference.

The work item has been flushed in driver_probe_done() to ensure that
there are no pending tasks while disconnecting the driver.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/dd.c

@@ -51,7 +51,6 @@
 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);
 
 /*
@@ -175,7 +174,7 @@ 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.
 	 */
-	queue_work(deferred_wq, &deferred_probe_work);
+	schedule_work(&deferred_probe_work);
 }
 
 /**
@@ -211,14 +210,10 @@ 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_workqueue(deferred_wq);
+	flush_work(&deferred_probe_work);
 	return 0;
 }
 late_initcall(deferred_probe_initcall);
@@ -481,8 +476,7 @@ int driver_probe_done(void)
 void wait_for_device_probe(void)
 {
 	/* wait for the deferred probe workqueue to finish */
-	if (driver_deferred_probe_enable)
-		flush_workqueue(deferred_wq);
+	flush_work(&deferred_probe_work);
 
 	/* wait for the known devices to complete their probing */
 	wait_event(probe_waitqueue, atomic_read(&probe_count) == 0);