workqueue: Move kthread_flush_worker() out of alloc_and_link_pwqs()

kthread_flush_worker() can't be called with wq_pool_mutex held.

Prepare for moving wq_pool_mutex and cpu hotplug lock out of
alloc_and_link_pwqs().

Cc: Zqiang <qiang.zhang1211@gmail.com>
Link: https://lore.kernel.org/lkml/20230920060704.24981-1-qiang.zhang1211@gmail.com/
Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: Lai Jiangshan

kernel/workqueue.c

@@ -5467,12 +5467,6 @@ static int alloc_and_link_pwqs(struct workqueue_struct *wq)
 	}
 	cpus_read_unlock();
 
-	/* for unbound pwq, flush the pwq_release_worker ensures that the
-	 * pwq_release_workfn() completes before calling kfree(wq).
-	 */
-	if (ret)
-		kthread_flush_worker(pwq_release_worker);
-
 	return ret;
 
 enomem:
@@ -5705,8 +5699,15 @@ struct workqueue_struct *alloc_workqueue(const char *fmt,
 	return wq;
 
 err_free_node_nr_active:
-	if (wq->flags & WQ_UNBOUND)
+	/*
+	 * Failed alloc_and_link_pwqs() may leave pending pwq->release_work,
+	 * flushing the pwq_release_worker ensures that the pwq_release_workfn()
+	 * completes before calling kfree(wq).
+	 */
+	if (wq->flags & WQ_UNBOUND) {
+		kthread_flush_worker(pwq_release_worker);
 		free_node_nr_active(wq->node_nr_active);
+	}
 err_unreg_lockdep:
 	wq_unregister_lockdep(wq);
 	wq_free_lockdep(wq);