workqueue: better define locking rules around worker creation / destruction

When a manager creates or destroys workers, the operations are always
done with the manager_mutex held; however, initial worker creation or
worker destruction during pool release don't grab the mutex.  They are
still correct as initial worker creation doesn't require
synchronization and grabbing manager_arb provides enough exclusion for
pool release path.

Still, let's make everyone follow the same rules for consistency and
such that lockdep annotations can be added.

Update create_and_start_worker() and put_unbound_pool() to grab
manager_mutex around thread creation and destruction respectively and
add lockdep assertions to create_worker() and destroy_worker().

This patch doesn't introduce any visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -1715,6 +1715,8 @@ static struct worker *create_worker(struct worker_pool *pool)
 	struct worker *worker = NULL;
 	int id = -1;
 
+	lockdep_assert_held(&pool->manager_mutex);
+
 	spin_lock_irq(&pool->lock);
 	while (ida_get_new(&pool->worker_ida, &id)) {
 		spin_unlock_irq(&pool->lock);
@@ -1796,12 +1798,14 @@ static void start_worker(struct worker *worker)
  * create_and_start_worker - create and start a worker for a pool
  * @pool: the target pool
  *
- * Create and start a new worker for @pool.
+ * Grab the managership of @pool and create and start a new worker for it.
  */
 static int create_and_start_worker(struct worker_pool *pool)
 {
 	struct worker *worker;
 
+	mutex_lock(&pool->manager_mutex);
+
 	worker = create_worker(pool);
 	if (worker) {
 		spin_lock_irq(&pool->lock);
@@ -1809,6 +1813,8 @@ static int create_and_start_worker(struct worker_pool *pool)
 		spin_unlock_irq(&pool->lock);
 	}
 
+	mutex_unlock(&pool->manager_mutex);
+
 	return worker ? 0 : -ENOMEM;
 }
 
@@ -1826,6 +1832,9 @@ static void destroy_worker(struct worker *worker)
 	struct worker_pool *pool = worker->pool;
 	int id = worker->id;
 
+	lockdep_assert_held(&pool->manager_mutex);
+	lockdep_assert_held(&pool->lock);
+
 	/* sanity check frenzy */
 	if (WARN_ON(worker->current_work) ||
 	    WARN_ON(!list_empty(&worker->scheduled)))
@@ -3531,6 +3540,7 @@ static void put_unbound_pool(struct worker_pool *pool)
 	 * manager_mutex.
 	 */
 	mutex_lock(&pool->manager_arb);
+	mutex_lock(&pool->manager_mutex);
 	spin_lock_irq(&pool->lock);
 
 	while ((worker = first_worker(pool)))
@@ -3538,6 +3548,7 @@ static void put_unbound_pool(struct worker_pool *pool)
 	WARN_ON(pool->nr_workers || pool->nr_idle);
 
 	spin_unlock_irq(&pool->lock);
+	mutex_unlock(&pool->manager_mutex);
 	mutex_unlock(&pool->manager_arb);
 
 	/* shut down the timers */