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

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Date:	Wed, 27 Aug 2008 11:36:15 -0700
From:	Greg KH <greg@...ah.com>
To:	Alan Stern <stern@...land.harvard.edu>
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, Aug 27, 2008 at 02:28:22PM -0400, Alan Stern wrote:
> 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.

Ok, then it's just safe to drop this static mutex entirely, right?

> > +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?

Ah, yes, my fault, I converted this from using a private cdev to using
the usb_register_dev() call and forgot about this.  Thanks for catching
it, I'll go fix this.

Oh, it's usb_deregister_dev(), stupid programmers with inconsitant
interfaces :)

thanks,

greg k-h
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