Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754986Ab0K2WdD (ORCPT ); Mon, 29 Nov 2010 17:33:03 -0500 Received: from kroah.org ([198.145.64.141]:49259 "EHLO coco.kroah.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754709Ab0K2Wcn (ORCPT ); Mon, 29 Nov 2010 17:32:43 -0500 Date: Mon, 29 Nov 2010 14:32:09 -0800 From: Greg KH To: Guennadi Liakhovetski Cc: Jonathan Corbet , linux-kernel@vger.kernel.org Subject: Re: [PATCH/RFC] core: add a function to safely try to get device driver owner Message-ID: <20101129223209.GB14663@kroah.com> References: <20101129131737.3dd6eb03@bike.lwn.net> <20101129214128.GA9691@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2954 Lines: 64 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 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? > > > 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. 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... thanks, greg k-h -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/