linux-kernel - Re: [PATCH 05/13] drm: provide device-refcount

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Date:	Wed, 12 Feb 2014 08:40:55 -0800
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Daniel Vetter <daniel@...ll.ch>
Cc:	David Herrmann <dh.herrmann@...il.com>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	Dave Airlie <airlied@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/13] drm: provide device-refcount

On Wed, Feb 12, 2014 at 05:26:57PM +0100, Daniel Vetter wrote:
> On Wed, Feb 12, 2014 at 3:44 PM, David Herrmann <dh.herrmann@...il.com> wrote:
> >>> +/**
> >>> + * drm_dev_ref - Take reference of a DRM device
> >>> + * @dev: device to take reference of or NULL
> >>> + *
> >>> + * This increases the ref-count of @dev by one. You *must* already own a
> >>> + * reference when calling this. Use drm_dev_unref() to drop this reference
> >>> + * again.
> >>> + *
> >>> + * This function never fails. However, this function does not provide *any*
> >>> + * guarantee whether the device is alive or running. It only provides a
> >>> + * reference to the object and the memory associated with it.
> >>> + */
> >>> +void drm_dev_ref(struct drm_device *dev)
> >>> +{
> >>> +     if (dev)
> >>
> >> This check here (and below in the unref code) look funny. What's the
> >> reason for it? Trying to grab/drop a ref on a NULL pointer sounds like a
> >> pretty serious bug to me. This is in contrast to kfree(NULL) which imo
> >> makes sense - freeing nothing is a legitimate operation imo.
> >
> > I added it mainly to simplify cleanup-code paths. You can then just
> > call unref() and set it to NULL regardless whether you actually hold a
> > reference or not. For ref() I don't really care but I think the
> > NULL-test doesn't hurt either.
> >
> > I copied this behavior from get_device() and put_device(), btw.
> > Similar to these functions, I think a lot more will go wrong if the
> > NULL pointer is not intentional. Imo, ref-counting on a NULL object
> > just means "no object", so it shouldn't do anything.
> 
> My fear with this kind of magic is that someone accidentally exchanges
> the pointer clearing to NULL (or assignement when grabbing a ref) with
> the unref/ref call and then we have a very subtle bug at hand. If we
> don't accept NULL objects the failure will be much more obvious.
> 
> The entire kernel kobject stuff is very consistent about this, but I
> couldn't find a reason for it - all the NULL checks predate git
> history. Greg can you please shed some lights on best practice here
> and whether my fears are justified given your experience with shoddy
> drivers in general?

Yes, the driver core does test for NULL here, as sometimes you are
passing in a "parent" pointer, and don't really care if it is NULL or
not, so just treating it as if you really do have a reference is usually
fine.

But, for a subsystem where you "know" you will not be doing anything as
foolish as that, I'd not allow that :)

So I'd recommend taking those checks out of the drm code.

thanks,

greg k-h
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