From: Lisa Du <cldu@marvell.com>
To: "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: A race condition between debugfs and seq_file operation
Thread-Topic: A race condition between debugfs and seq_file operation
Thread-Index: AQHQoaOGA0gdqb5zJkWNmEry1J5dTw==
Date: Mon, 8 Jun 2015 04:28:10 +0000
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Hi, All
Recently I met one race condition related to debugfs.

Take an example from ion.c in kernel3.14:
static int ion_debug_client_open(struct inode *inode, struct file *file)
{
	return single_open(file, ion_debug_client_show, inode->i_private);
}

static const struct file_operations debug_client_fops = {
	.open = ion_debug_client_open,
	.read = seq_read,
	.llseek = seq_lseek,
	.release = single_release,
};
client->debug_root = debugfs_create_file(client->display_name, 0664,
					dev->clients_debug_root,
					client, &debug_client_fops);

I find during I read the debugfs node, driver can do debugfs_remove_recursive(dentry);
Is it expected?  

In this case, when do the seq_file read, it grabs the seq_file->lock;
While in debugfs_remove_recursive(), it graps "parent->d_inode->i_mutex".
So there seems no protection between the ion_debug_client_show() and debugfs_remove_recursive().

Please let me know if I didn't describe the issue clear.
Would you help to comment if there's method to avoid such issue?

Best Regards.
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