[PATCH 0/3] Use implicit kref infra

Wed Sep 2 06:52:36 UTC 2020

On Tue, Sep 01, 2020 at 11:46:18PM -0400, Luben Tuikov wrote:
> On 2020-09-01 21:42, Pan, Xinhui wrote:
> > If you take a look at the below function, you should not use driver's release to free adev. As dev is embedded in adev.
> 
> Do you mean "look at the function below", using "below" as an adverb?
> "below" is not an adjective.
> 
> I know dev is embedded in adev--I did that patchset.
> 
> > 
> >  809 static void drm_dev_release(struct kref *ref)
> >  810 {
> >  811         struct drm_device *dev = container_of(ref, struct drm_device, ref);
> >  812        
> >  813         if (dev->driver->release)
> >  814                 dev->driver->release(dev);
> >  815 
> >  816         drm_managed_release(dev);
> >  817 
> >  818         kfree(dev->managed.final_kfree);
> >  819 }
> 
> That's simple--this comes from change c6603c740e0e3
> and it should be reverted. Simple as that.
> 
> The version before this change was absolutely correct:
> 
> static void drm_dev_release(struct kref *ref)
> {
> 	if (dev->driver->release)
> 		dev->driver->release(dev);
> 	else
> 		drm_dev_fini(dev);
> }
> 
> Meaning, "the kref is now 0"--> if the driver
> has a release, call it, else use our own.
> But note that nothing can be assumed after this point,
> about the existence of "dev".
> 
> It is exactly because struct drm_device is statically
> embedded into a container, struct amdgpu_device,
> that this change above should be reverted.
> 
> This is very similar to how fops has open/release
> but no close. That is, the "release" is called
> only when the last kref is released, i.e. when
> kref goes from non-zero to zero.
> 
> This uses the kref infrastructure which has been
> around for about 20 years in the Linux kernel.
> 
> I suggest reading the comments
> in drm_dev.c mostly, "DOC: driver instance overview"
> starting at line 240 onwards. This is right above
> drm_put_dev(). There is actually an example of a driver
> in the comment. Also the comment to drm_dev_init().
> 
> Now, take a look at this:
> 
> /**
>  * drm_dev_put - Drop reference of a DRM device
>  * @dev: device to drop reference of or NULL
>  *
>  * This decreases the ref-count of @dev by one. The device is destroyed if the
>  * ref-count drops to zero.
>  */
> void drm_dev_put(struct drm_device *dev)
> {
>         if (dev)
>                 kref_put(&dev->ref, drm_dev_release);
> }
> EXPORT_SYMBOL(drm_dev_put);
> 
> Two things:
> 
> 1. It is us, who kzalloc the amdgpu device, which contains
> the drm_device (you'll see this discussed in the reading
> material I pointed to above). We do this because we're
> probing the PCI device whether we'll work it it or not.
> 
> 2. Using the kref infrastructure, when the ref goes to 0,
> drm_dev_release is called. And here's the KEY:
> Because WE allocated the container, we should free it--after the release
> method is called, DRM cannot assume anything about the drm
> device or the container. The "release" method is final.
> 
> We allocate, we free. And we free only when the ref goes to 0.
> 
> DRM can, in due time, "free" itself of the DRM device and stop
> having knowledge of it--that's fine, but as long as the ref
> is not 0, the amdgpu device and thus the contained DRM device,
> cannot be freed.
> 
> > 
> > You have to make another change something like
> > diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> > index 13068fdf4331..2aabd2b4c63b 100644
> > --- a/drivers/gpu/drm/drm_drv.c
> > +++ b/drivers/gpu/drm/drm_drv.c
> > @@ -815,7 +815,8 @@ static void drm_dev_release(struct kref *ref)
> >  
> >         drm_managed_release(dev);
> >  
> > -       kfree(dev->managed.final_kfree);
> > +       if (dev->driver->final_release)
> > +               dev->driver->final_release(dev);
> >  }
> 
> No. What's this?
> There is no such thing as "final" release, nor is there a "partial" release.
> When the kref goes to 0, the device disappears. Simple.
> If someone is using it, they should kref-get it, and when they're
> done with it, they should kref-put it.
> 
> The whole point is that this is done implicitly, via the kref infrastructure.
> drm_dev_init() which we call in our PCI probe function, sets the kref to 1--all
> as per the documentation I pointed you to above.
> 
> Another point is that we can do some other stuff in the release
> function, notify someone, write some registers, free memory we use
> for that PCI device, etc.
> 
> If the "managed resources" infrastructure wants to stay, it should hook
> itself into drm_dev_fini() and into drm_dev_init() or drm_dev_register().
> It shouldn't have to be so out-of-place like in patch 2/3 of this series,
> where the drmm_add_final_kfree() is smack-dab in the middle of our PCI
> discovery function, surrounded on top and bottom by drm_dev_init()
> and drm_dev_register(). The "managed resources" infra should be non-invasive
> and drivers shouldn't have to change to use it--it should be invisible to them.
> Then our kref would just work.

Unfortunately some part of that drm managed series is stuck still waiting
for review (I guess I need to resubmit), but with that the docs would tell
you to use devm_drm_dev_alloc instead of hand-rolling all this.

Definitely not any of the ideas proposed in this discussion or patches :-)

I'll cc you on that series when I send it out again.

Cheers, Daniel
> 
> > 
> > And in the final_release callback we free the dev. But that is a little complex now. so I prefer still using final_kfree.
> > Of course we can do some cleanup work in the driver's release callback. BUT no kfree.
> 
> No! No final_kfree. It's a hack.
> 
> Read the documentation in drm_drv.c I noted above--it lays out how this happens. Reading is required.
> 
> Regards,
> Luben
> 
> 
> > 
> > -----原始邮件-----
> > 发件人: "Tuikov, Luben" <Luben.Tuikov at amd.com>
> > 日期: 2020年9月2日 星期三 09:07
> > 收件人: "amd-gfx at lists.freedesktop.org" <amd-gfx at lists.freedesktop.org>, "dri-devel at lists.freedesktop.org" <dri-devel at lists.freedesktop.org>
> > 抄送: "Deucher, Alexander" <Alexander.Deucher at amd.com>, Daniel Vetter <daniel at ffwll.ch>, "Pan, Xinhui" <Xinhui.Pan at amd.com>, "Tuikov, Luben" <Luben.Tuikov at amd.com>
> > 主题: [PATCH 0/3] Use implicit kref infra
> > 
> >     Use the implicit kref infrastructure to free the container
> >     struct amdgpu_device, container of struct drm_device.
> >     
> >     First, in drm_dev_register(), do not indiscriminately warn
> >     when a DRM driver hasn't opted for managed.final_kfree,
> >     but instead check if the driver has provided its own
> >     "release" function callback in the DRM driver structure.
> >     If that is the case, no warning.
> >     
> >     Remove drmm_add_final_kfree(). We take care of that, in the
> >     kref "release" callback when all refs are down to 0, via
> >     drm_dev_put(), i.e. the free is implicit.
> >     
> >     Remove superfluous NULL check, since the DRM device to be
> >     suspended always exists, so long as the underlying PCI and
> >     DRM devices exist.
> >     
> >     Luben Tuikov (3):
> >       drm: No warn for drivers who provide release
> >       drm/amdgpu: Remove drmm final free
> >       drm/amdgpu: Remove superfluous NULL check
> >     
> >      drivers/gpu/drm/amd/amdgpu/amdgpu_device.c | 3 ---
> >      drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c    | 2 --
> >      drivers/gpu/drm/drm_drv.c                  | 3 ++-
> >      3 files changed, 2 insertions(+), 6 deletions(-)
> >     
> >     -- 
> >     2.28.0.394.ge197136389
> >     
> >     
> > 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch