> 2020年9月2日 11:46，Tuikov, Luben <Luben.Tuikov@xxxxxxx> 写道：
>
> 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.
>
that is true.
My understanding of the drm core code is like something below.
struct B {
strcut A
}
we initialize A firstly and initialize B in the end. But destroy B firstly and destory A in the end.
But yes, practice is more complex.
if B has nothing to be destroyed. we can destory A directly, otherwise destroy B firstly.
in this case, we can do something below in our release()
//some cleanup work of B
drm_dev_fini(dev);//destroy A
kfree(adev)
> 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.
I just take an example here. add another release in the end. then no one could touch us. IOW, final_release.
A destroy B by a callback, then A destroy itself. It assumes B just free its own resource.
but that makes trouble if some resource of A is allocated by B.
Because B must take care of these common resource shared between A and B.
yes, that logical is more complex. So I think we can revert drm_dev_release to its previous version.
>
> 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.
I am not taking about kref. what we are discussing is all about the release sequence.
>
> 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.
>
yep, that make sense. But you need more changes to fix this issue. this patchset is insufficient.
>>
>> 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
>>
>>
>>
>
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