linux-kernel - Re: [PATCH] USB: add USB test and measurement class driver

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Date:	Wed, 27 Aug 2008 14:28:22 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Greg KH <greg@...ah.com>
cc:	Oliver Neukum <oliver@...kum.org>, <linux-usb@...r.kernel.org>,
	Stefan Kopp <stefan_kopp@...lent.com>,
	Marcel Janssen <korgull@...e.nl>,
	Felipe Balbi <me@...ipebalbi.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] USB: add USB test and measurement class driver - round
 2

On Wed, 27 Aug 2008, Greg KH wrote:

> Here's an updated version of the usbtmc driver, with all of the
> different issues that have been raised, hopefully addressed.

This is an example of what I was discussing with Oliver.  In all 
likelihood you simply don't need usbtmc_mutex, and using it will cause 
a lockdep violation.

That's why so many of the other USB class drivers don't have an 
analogous static mutex.

> +static int usbtmc_open(struct inode *inode, struct file *filp)
> +{
> +	struct usb_interface *intf;
> +	struct usbtmc_device_data *data;
> +	int retval = -ENODEV;
> +
> +	mutex_lock(&usbtmc_mutex);

You must never acquire a lock in your open method if it will be held
by your disconnect method while unregistering the minor.

> +static int usbtmc_probe(struct usb_interface *intf,
> +			const struct usb_device_id *id)
> +{
...
> +	retcode = usb_register_dev(intf, &usbtmc_class);
> +	if (retcode) {
> +		dev_err(&intf->dev, "Not able to get a minor"
> +			" (base %u, slice default): %d\n", USBTMC_MINOR_BASE,
> +			retcode);
> +		goto error_register;
> +	}
> +	dev_dbg(&intf->dev, "Using minor number %d\n", intf->minor);

You do call usb_register_dev() during probe...

> +
> +	return 0;
> +
> +error_register:
> +	sysfs_remove_group(&intf->dev.kobj, &capability_attr_grp);
> +	sysfs_remove_group(&intf->dev.kobj, &data_attr_grp);
> +	kref_put(&data->kref, usbtmc_delete);
> +	return retcode;
> +}
> +
> +static void usbtmc_disconnect(struct usb_interface *intf)
> +{
> +	struct usbtmc_device_data *data;
> +
> +	dev_dbg(&intf->dev, "usbtmc_disconnect called\n");
> +
> +	mutex_lock(&usbtmc_mutex);
> +	data = usb_get_intfdata(intf);
> +	sysfs_remove_group(&intf->dev.kobj, &capability_attr_grp);
> +	sysfs_remove_group(&intf->dev.kobj, &data_attr_grp);
> +	kref_put(&data->kref, usbtmc_delete);
> +	mutex_unlock(&usbtmc_mutex);
> +}

But you don't call usb_unregister_dev() during disconnect.  An 
oversight?

Alan Stern

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