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

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Date:	Wed, 27 Aug 2008 20:37:48 +0200
From:	Oliver Neukum <oliver@...kum.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Greg KH <greg@...ah.com>, 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

Am Mittwoch 27 August 2008 20:28:22 schrieb Alan Stern:
> 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.

And this rule depends on sharing the USB major or not. This needs
a big fat mention in Documentation.

	Regards
		Oliver

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