fs: assert on ->i_count in iput_final()

Notably make sure the count is 0 after the return from ->drop_inode(),
provided we are going to drop.

Inspired by suspicious games played by f2fs.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: mjguzik

fs/inode.c

@@ -1879,6 +1879,7 @@ static void iput_final(struct inode *inode)
 	int drop;
 
 	WARN_ON(inode->i_state & I_NEW);
+	VFS_BUG_ON_INODE(atomic_read(&inode->i_count) != 0, inode);
 
 	if (op->drop_inode)
 		drop = op->drop_inode(inode);
@@ -1893,6 +1894,12 @@ static void iput_final(struct inode *inode)
 		return;
 	}
 
+	/*
+	 * Re-check ->i_count in case the ->drop_inode() hooks played games.
+	 * Note we only execute this if the verdict was to drop the inode.
+	 */
+	VFS_BUG_ON_INODE(atomic_read(&inode->i_count) != 0, inode);
+
 	state = inode->i_state;
 	if (!drop) {
 		WRITE_ONCE(inode->i_state, state | I_WILL_FREE);