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@xxxxxxx>
> > 日期: 2020年9月2日 星期三 09:07
> > 收件人: "amd-gfx@xxxxxxxxxxxxxxxxxxxxx" <amd-gfx@xxxxxxxxxxxxxxxxxxxxx>, "dri-devel@xxxxxxxxxxxxxxxxxxxxx" <dri-devel@xxxxxxxxxxxxxxxxxxxxx>
> > 抄送: "Deucher, Alexander" <Alexander.Deucher@xxxxxxx>, Daniel Vetter <daniel@xxxxxxxx>, "Pan, Xinhui" <Xinhui.Pan@xxxxxxx>, "Tuikov, Luben" <Luben.Tuikov@xxxxxxx>
> > 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
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