md: fix 'delete_mutex' deadlock

Commit 3ce94ce5 ("md: fix duplicate filename for rdev") introduce a
new lock 'delete_mutex', and trigger a new deadlock:

t1: remove rdev			t2: sysfs writer

rdev_attr_store			rdev_attr_store
 mddev_lock
 state_store
 md_kick_rdev_from_array
  lock delete_mutex
  list_add mddev->deleting
  unlock delete_mutex
 mddev_unlock
				 mddev_lock
				 ...
  lock delete_mutex
  kobject_del
  // wait for sysfs writers to be done
				 mddev_unlock
				 lock delete_mutex
				 // wait for delete_mutex, deadlock

'delete_mutex' is used to protect the list 'mddev->deleting', turns out
that this list can be protected by 'reconfig_mutex' directly, and this
lock can be removed.

Fix this problem by removing the lock, and use 'reconfig_mutex' to
protect the list. mddev_unlock() will move this list to a local list to
be handled after 'reconfig_mutex' is dropped.

Fixes: 3ce94ce5 ("md: fix duplicate filename for rdev")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230621142933.1395629-1-yukuai1@huaweicloud.com

drivers/md/md.c

@@ -643,7 +643,6 @@ void mddev_init(struct mddev *mddev)
 {
 	mutex_init(&mddev->open_mutex);
 	mutex_init(&mddev->reconfig_mutex);
-	mutex_init(&mddev->delete_mutex);
 	mutex_init(&mddev->bitmap_info.mutex);
 	INIT_LIST_HEAD(&mddev->disks);
 	INIT_LIST_HEAD(&mddev->all_mddevs);
@@ -749,26 +748,15 @@ static void mddev_free(struct mddev *mddev)
 
 static const struct attribute_group md_redundancy_group;
 
-static void md_free_rdev(struct mddev *mddev)
+void mddev_unlock(struct mddev *mddev)
 {
 	struct md_rdev *rdev;
 	struct md_rdev *tmp;
+	LIST_HEAD(delete);
 
-	mutex_lock(&mddev->delete_mutex);
-	if (list_empty(&mddev->deleting))
-		goto out;
+	if (!list_empty(&mddev->deleting))
+		list_splice_init(&mddev->deleting, &delete);
 
-	list_for_each_entry_safe(rdev, tmp, &mddev->deleting, same_set) {
-		list_del_init(&rdev->same_set);
-		kobject_del(&rdev->kobj);
-		export_rdev(rdev, mddev);
-	}
-out:
-	mutex_unlock(&mddev->delete_mutex);
-}
-
-void mddev_unlock(struct mddev *mddev)
-{
 	if (mddev->to_remove) {
 		/* These cannot be removed under reconfig_mutex as
 		 * an access to the files will try to take reconfig_mutex
@@ -808,7 +796,11 @@ void mddev_unlock(struct mddev *mddev)
 	} else
 		mutex_unlock(&mddev->reconfig_mutex);
 
-	md_free_rdev(mddev);
+	list_for_each_entry_safe(rdev, tmp, &delete, same_set) {
+		list_del_init(&rdev->same_set);
+		kobject_del(&rdev->kobj);
+		export_rdev(rdev, mddev);
+	}
 
 	md_wakeup_thread(mddev->thread);
 	wake_up(&mddev->sb_wait);
@@ -2488,9 +2480,7 @@ static void md_kick_rdev_from_array(struct md_rdev *rdev)
 	 * reconfig_mutex is held, hence it can't be called under
 	 * reconfig_mutex and it's delayed to mddev_unlock().
 	 */
-	mutex_lock(&mddev->delete_mutex);
 	list_add(&rdev->same_set, &mddev->deleting);
-	mutex_unlock(&mddev->delete_mutex);
 }
 
 static void export_array(struct mddev *mddev)

drivers/md/md.h

@@ -531,11 +531,9 @@ struct mddev {
 
 	/*
 	 * Temporarily store rdev that will be finally removed when
-	 * reconfig_mutex is unlocked.
+	 * reconfig_mutex is unlocked, protected by reconfig_mutex.
 	 */
 	struct list_head		deleting;
-	/* Protect the deleting list */
-	struct mutex			delete_mutex;
 
 	bool	has_superblocks:1;
 	bool	fail_last_dev:1;