linux-kernel - Re: [PATCH] Driver core: fix race in dev_driver

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Message-Id: <200912042055.55565.oliver@neukum.org>
Date:	Fri, 4 Dec 2009 20:55:55 +0100
From:	Oliver Neukum <oliver@...kum.org>
To:	Greg KH <gregkh@...e.de>
Cc:	Alan Stern <stern@...land.harvard.edu>, stable@...nel.org,
	Rickard Bellini <rickard.bellini@...csson.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	Torgny Johansson <torgny.johansson@...csson.com>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Driver core: fix race in dev_driver_string

Am Freitag, 4. Dezember 2009 17:50:40 schrieb Greg KH:
> On Fri, Dec 04, 2009 at 05:16:19PM +0100, Oliver Neukum wrote:
> > Am Freitag, 4. Dezember 2009 17:06:57 schrieb Alan Stern:
> > > Oliver:
> > > 
> > > We don't have to worry about the device structure being deallocated 
> > > while the routine is running.  If that happens it's a bug in the 
> > > caller: improper refcounting.
> >
> > 
> > That raises two points
> > 
> > 1. am I supposed to get a reference just so that I can use dev_err?
> 
> No, you should already have a reference on the device when doing the
> call, right?

No, why? Consider this:

int write(...)
{
	...
	mutex_lock(&instance->lock);
	if (instance->disconnected) {
		dev_dbg(instance->dev,"writing to disconnected device"); 
		rv = -ENODEV;
	} else {
		res = usb_submit_urb(...);
		rv = res < 0 ? -EIO : count;
	}
	mutex_unlock(&instance->lock);
	return rv;
}

void disconnect(...)
{
	...
	mutex_lock(&instance->lock);
	instance->disconnected = 1;
	usb_kill_urb(...);
	usb_kill_urb(...);
	mutex_unlock(&instance->lock);
}

This would be perfectly valid code without any references taken save
for the pesky dev_dbg()

	Regards
		Oliver
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