mtd: call external _get and _put in right order

MTD provider provides mtd_info object to mtd subsystem.
With kref patch the mtd_info object can be alive after
provider released mtd device.
Fix calling order in _get and _put functions to allow
mtd provider to safely alloc and release mtd object.

Execute:
1) call external _get
2) get_module
3) add internal kref
in the get function and opposite order in the put one.

The _put_device callback should be the last in put
as the master struct memory may be freed in this callback.

Signed-off-by: Alexander Usyskin <[email protected]>
Signed-off-by: Miquel Raynal <[email protected]>
Link: https://lore.kernel.org/linux-mtd/[email protected]

Author: ausyskin

drivers/mtd/mtdcore.c

@@ -1229,21 +1229,20 @@ int __get_mtd_device(struct mtd_info *mtd)
 	struct mtd_info *master = mtd_get_master(mtd);
 	int err;
 
-	if (!try_module_get(master->owner))
-		return -ENODEV;
-
-	kref_get(&mtd->refcnt);
-
 	if (master->_get_device) {
 		err = master->_get_device(mtd);
-
-		if (err) {
-			kref_put(&mtd->refcnt, mtd_device_release);
-			module_put(master->owner);
+		if (err)
 			return err;
-		}
 	}
 
+	if (!try_module_get(master->owner)) {
+		if (master->_put_device)
+			master->_put_device(master);
+		return -ENODEV;
+	}
+
+	kref_get(&mtd->refcnt);
+
 	while (mtd->parent) {
 		if (IS_ENABLED(CONFIG_MTD_PARTITIONED_MASTER) || mtd->parent != master)
 			kref_get(&mtd->parent->refcnt);
@@ -1340,13 +1339,14 @@ void __put_mtd_device(struct mtd_info *mtd)
 		mtd = parent;
 	}
 
-	if (master->_put_device)
-		master->_put_device(master);
+	if (IS_ENABLED(CONFIG_MTD_PARTITIONED_MASTER))
+		kref_put(&master->refcnt, mtd_device_release);
 
 	module_put(master->owner);
 
-	if (IS_ENABLED(CONFIG_MTD_PARTITIONED_MASTER))
-		kref_put(&master->refcnt, mtd_device_release);
+	/* must be the last as master can be freed in the _put_device */
+	if (master->_put_device)
+		master->_put_device(master);
 }
 EXPORT_SYMBOL_GPL(__put_mtd_device);