Date:	Tue, 30 Nov 2010 00:11:42 +0100 (CET)
From:	Guennadi Liakhovetski <g.liakhovetski@....de>
To:	Greg KH <greg@...ah.com>
cc:	Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH/RFC] core: add a function to safely try to get device
 driver owner

On Mon, 29 Nov 2010, Greg KH wrote:

> On Mon, Nov 29, 2010 at 11:10:50PM +0100, Guennadi Liakhovetski wrote:
> > On Mon, 29 Nov 2010, Greg KH wrote:
> > 
> > > On Mon, Nov 29, 2010 at 09:54:10PM +0100, Guennadi Liakhovetski wrote:
> > > > Hi Jon
> > > > 
> > > > On Mon, 29 Nov 2010, Jonathan Corbet wrote:
> > > > 
> > > > > On Mon, 29 Nov 2010 20:43:28 +0100 (CET)
> > > > > Guennadi Liakhovetski <g.liakhovetski@....de> wrote:
> > > > > 
> > > > > > When two drivers interoperate without an explicit dependency, it is often
> > > > > > required to prevent one of them from being unloaded safely by dereferencing
> > > > > > dev->driver->owner. This patch provides a generic function to do this in a
> > > > > > race-free way.
> > > > > 
> > > > > I must ask: why not, instead, make the dependency explicit?  In
> > > > > particular, this looks like an application for the proposed media
> > > > > controller code, which is meant to model the connections between otherwise
> > > > > independent devices.  The fact that your example comes from V4L2 (which is
> > > > > the current domain of the media controller) also argues that way.
> > > > 
> > > > Sorry, don't see a good way to do this. This function is for a general 
> > > > dependency, where you don't have that driver, we are checking for register 
> > > > with us, so, the only way to get to it is via dev->driver->owner.
> > > 
> > > Wait, what?  The device is already bound to a driver, right, so why
> > > would you care about "locking" the module into memory?  What could this
> > > possibly be used for?
> > 
> > To protect against rmmod -> driver_unregister -> dev->driver = NULL?
> 
> But again, why would some other driver ever care about what some random
> dev->driver would be?

It's not a random one, call it a "companion device."

> 
> > > > And I also don't want to move registering the device into the
> > > > dependant driver and then wait (with a timeout) for a driver to probe
> > > > with it... I just want to verify, whether a driver has attached to
> > > > that device and whether I can lock it down.
> > > 
> > > Who cares if a driver is attached to any device?  And again, why would
> > > you want to "lock it down"?
> > 
> > In my case I have two platform devices: CEU and CSI2. In some cases (with 
> > parallel sensors) CEU operates on its own. With serial (CSI-2) camera 
> > sensors we need the CSI2 driver. So, I want to 
> > try_module_get(csi2_dev->driver->owner) the CSI2 driver from my CEU 
> > driver. This call can Oops if not done safely. Am I missing something? Is 
> > there an easier way to achieve the same?
> 
> Yes, from userspace load the module and then don't worry about it.

Right, but I have to prevent the user-space from unloading it again.

> Don't ever think that poking around in a dev->driver field is safe at
> all, it isn't.  I should just go hide the thing from the rest of the
> kernel to keep this from happening, now that you mention it...

Exactly, that's why I'm proposing it for dd.c;)

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/
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