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

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Date:	Fri, 4 Dec 2009 16:35:33 -0800
From:	Greg KH <gregkh@...e.de>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Oliver Neukum <oliver@...kum.org>, 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

On Fri, Dec 04, 2009 at 06:50:35PM -0500, Alan Stern wrote:
> On Fri, 4 Dec 2009, Dmitry Torokhov wrote:
> 
> > > Maybe reference counting is inelegant; it depends on your point of
> > > view.  Can you think of a more elegant way to make sure that a pointer
> > > isn't stale?
> > 
> > Yes, just say "no" to device_create() and friends.
> 
> device_create() wasn't used in the case Oliver is discussing.

It was implied, as you had a pointer to the device, not the device
itself.

> >  Embed device structure in
> > yours,
> 
> You can't do that when the device structure wasn't created by your 
> driver.

But for USB devices, it is part of the device you are handed.  Same goes
for PCI devices, and most other types of drivers, right?

> >  be mindful of lifetime rules and only use "your" device (i.e device
> > bound to your driver).
> 
> What do you mean by "use"?  In Oliver's case he wasn't using the
> device, he was using the device structure.  (Maybe that's what you
> meant.)

I think that is what is meant here.

> And he wanted to use it at a time when it wasn't bound to his
> driver, because userspace still had an open file reference to it.  
> There isn't really any way around this.

But you still have a valid device, just not maybe a driver bound to it.

> > This way, as long as your refcount your instance you
> > can rest assured the device structure is there as well.
> 
> I rather think that a simple device_get() and device_put() is easier 
> than trying to follow a bunch of rules, especially in cases where they 
> don't apply!  :-)

Like here :)

thanks,

greg k-h
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