f2fs: exclude free nids building and allocation

During nid allocation, it needs to exclude building and allocating flow
of free nids, this is because while building free nid cache, there are two
steps: a) load free nids from unused nat entries in NAT pages, b) update
free nid cache by checking nat journal. The two steps should be atomical,
otherwise an used nid can be allocated as free one after a) and before b).

This patch adds missing lock which covers build_free_nids in
unlock_operation and f2fs_balance_fs_bg to avoid that.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.c

@@ -1785,7 +1785,7 @@ static void scan_nat_page(struct f2fs_sb_info *sbi,
 	}
 }
 
-void build_free_nids(struct f2fs_sb_info *sbi)
+void __build_free_nids(struct f2fs_sb_info *sbi)
 {
 	struct f2fs_nm_info *nm_i = NM_I(sbi);
 	struct curseg_info *curseg = CURSEG_I(sbi, CURSEG_HOT_DATA);
@@ -1839,6 +1839,13 @@ void build_free_nids(struct f2fs_sb_info *sbi)
 					nm_i->ra_nid_pages, META_NAT, false);
 }
 
+void build_free_nids(struct f2fs_sb_info *sbi)
+{
+	mutex_lock(&NM_I(sbi)->build_lock);
+	__build_free_nids(sbi);
+	mutex_unlock(&NM_I(sbi)->build_lock);
+}
+
 /*
  * If this function returns success, caller can obtain a new nid
  * from second parameter of this function.
@@ -1875,9 +1882,7 @@ bool alloc_nid(struct f2fs_sb_info *sbi, nid_t *nid)
 	spin_unlock(&nm_i->free_nid_list_lock);
 
 	/* Let's scan nat pages and its caches to get free nids */
-	mutex_lock(&nm_i->build_lock);
 	build_free_nids(sbi);
-	mutex_unlock(&nm_i->build_lock);
 	goto retry;
 }