From: "Christian König" <christian.koenig@amd.com>
To: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>,
Jason Ekstrand <jason@jlekstrand.net>
Cc: "Daniel Stone" <daniels@collabora.com>,
"Daniel Vetter" <daniel.vetter@ffwll.ch>,
"James Jones" <jajones@nvidia.com>,
"Chenbo Feng" <fengc@google.com>,
LKML <linux-kernel@vger.kernel.org>,
"Greg Hackmann" <ghackmann@google.com>,
linaro-mm-sig@lists.linaro.org,
"Kristian Høgsberg" <hoegsberg@google.com>,
"Maling list - DRI developers" <dri-devel@lists.freedesktop.org>,
"Jesse Hall" <jessehall@google.com>,
"Dave Airlie" <airlied@redhat.com>,
linux-media@vger.kernel.org
Subject: Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files
Date: Thu, 27 Feb 2020 09:28:28 +0100 [thread overview]
Message-ID: <810a26e7-4294-a615-b7ee-18148ac70641@amd.com> (raw)
In-Reply-To: <CAP+8YyEUz29fXDW5kO_0ZG6c849=TuFWCK8ynT3LuM+Tn+rMzw@mail.gmail.com>
Am 26.02.20 um 17:46 schrieb Bas Nieuwenhuizen:
> On Wed, Feb 26, 2020 at 4:29 PM Jason Ekstrand <jason@jlekstrand.net> wrote:
>> On Wed, Feb 26, 2020 at 4:05 AM Daniel Vetter <daniel@ffwll.ch> wrote:
>>> On Wed, Feb 26, 2020 at 10:16:05AM +0100, Christian König wrote:
>>> [SNIP]
>>>> Just imagine that you access some DMA-buf with a shader and that operation
>>>> is presented as a fence on the DMA-bufs reservation object. And now you can
>>>> go ahead and replace that fence and free up the memory.
>>>>
>>>> Tricking the Linux kernel into allocating page tables in that freed memory
>>>> is trivial and that's basically it you can overwrite page tables with your
>>>> shader and gain access to all of system memory :)
>>>>
>>>> What we could do is to always make sure that the added fences will complete
>>>> later than the already existing ones, but that is also rather tricky to get
>>>> right. I wouldn't do that if we don't have a rather big use case for this.
>> Right. I thought about that but I'm still learning how dma_resv
>> works. It'd be easy enough to make a fence array that contains both
>> the old fence and the new fence and replace the old fence with that.
>> What I don't know is the proper way to replace the exclusive fence
>> safely. Some sort of atomic_cpxchg loop, perhaps? I presume there's
>> some way of doing it properly because DRM drivers are doing it all the
>> time.
First of all you need to grab the lock of the dma_resv object or you
can't replace the exclusive nor the shared ones.
This way you don't need to do a atomic_cmpxchg or anything else and
still guarantee correct ordering.
> I think for an exclusive fence you may need to create a fence array
> that includes the existing exclusive and shared fences in the dma_resv
> combined with the added fence.
Yes, that at least gives us the correct synchronization.
> However, I'm not sure what the best way is to do garbage collection on
> that so that we don't get an impossibly list of fence arrays.
Exactly yes. That's also the reason why the dma_fence_chain container I
came up with for the sync timeline stuff has such a rather sophisticated
garbage collection.
When some of the included fences signal you need to free up the
array/chain and make sure that the memory for the container can be reused.
> (Note
> the dma_resv has a lock that needs to be taken before adding an
> exclusive fence, might be useful). Some code that does a thing like
> this is __dma_resv_make_exclusive in
> drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
Wanted to move that into dma_resv.c for quite a while since there are
quite a few other cases where we need this.
Regards,
Christian.
> The other piece of the puzzle is that on the submit path this would
> need something to ignore implicit fences. And there semantically the
> question comes up whether it is safe for a driver to ignore exclusive
> fences from another driver. (and then we have amdgpu which has its own
> rules on exclusiveness of its shared fences based on the context. e.g.
> the current option to ignore implicit fences for a buffer still syncs
> on exclusive fences on the buffer).
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next prev parent reply other threads:[~2020-02-27 8:28 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-25 23:58 [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files Jason Ekstrand
2020-02-26 9:16 ` Christian König
2020-02-26 10:05 ` Daniel Vetter
2020-02-26 15:28 ` Jason Ekstrand
2020-02-26 16:46 ` Bas Nieuwenhuizen
2020-02-27 8:28 ` Christian König [this message]
2020-03-03 19:10 ` Jason Ekstrand
2020-03-04 8:34 ` Christian König
2020-03-04 16:27 ` Jason Ekstrand
2020-03-04 16:41 ` Jason Ekstrand
2020-03-05 13:06 ` Christian König
2020-03-05 15:54 ` Jason Ekstrand
2020-03-09 16:21 ` Christian König
2020-03-11 3:43 ` Jason Ekstrand
2020-02-26 18:09 ` [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files (v2) Jason Ekstrand
2020-03-03 19:03 ` [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files (v3) Jason Ekstrand
2020-03-03 19:05 ` Jason Ekstrand
2020-03-11 3:43 ` [PATCH 1/3] dma-buf: add dma_fence_array_for_each (v2) Jason Ekstrand
2020-03-11 3:43 ` [PATCH 2/3] dma-buf: add dma_resv_get_singleton (v2) Jason Ekstrand
2020-03-11 3:43 ` [PATCH 3/3] RFC: dma-buf: Add an API for importing and exporting sync files (v4) Jason Ekstrand
2020-03-11 13:18 ` Christian König
2020-03-12 15:57 ` Jason Ekstrand
2020-03-13 10:33 ` Christian König
2020-03-17 21:21 ` [PATCH 3/3] RFC: dma-buf: Add an API for importing and exporting sync files (v5) Jason Ekstrand
2020-09-30 9:39 ` Michel Dänzer
2020-09-30 9:55 ` Daniel Vetter
2021-03-15 21:11 ` Jason Ekstrand
2021-03-15 21:30 ` Daniel Vetter
2021-03-15 21:04 ` [PATCH 0/3] dma-buf: Add an API for exporting sync files (v6) Jason Ekstrand
2021-03-15 21:04 ` [PATCH 1/3] dma-buf: add dma_fence_array_for_each (v2) Jason Ekstrand
2021-03-15 21:04 ` [PATCH 2/3] dma-buf: add dma_resv_get_singleton (v2) Jason Ekstrand
2021-03-15 21:04 ` [PATCH 3/3] dma-buf: Add an API for exporting sync files (v6) Jason Ekstrand
2021-03-15 23:10 ` Jason Ekstrand
2021-03-16 8:51 ` Michel Dänzer
2021-03-16 14:35 ` Jason Ekstrand
2021-03-16 0:10 ` kernel test robot
2021-03-16 2:37 ` kernel test robot
2021-03-16 0:15 ` [PATCH 0/3] " Jason Ekstrand
2021-03-16 4:53 ` [PATCH 0/3] dma-buf: Add an API for exporting sync files (v7) Jason Ekstrand
2021-03-16 4:53 ` [PATCH 1/3] dma-buf: add dma_fence_array_for_each (v2) Jason Ekstrand
2021-03-16 4:53 ` [PATCH 2/3] dma-buf: add dma_resv_get_singleton_rcu (v3) Jason Ekstrand
2021-03-16 4:53 ` [PATCH 3/3] dma-buf: Add an API for exporting sync files (v7) Jason Ekstrand
2021-03-16 8:06 ` kernel test robot
2021-03-17 22:19 ` [PATCH 0/3] dma-buf: Add an API for exporting sync files (v8) Jason Ekstrand
2021-03-17 22:19 ` [PATCH 1/3] dma-buf: add dma_fence_array_for_each (v2) Jason Ekstrand
2021-03-18 9:38 ` Christian König
2021-03-18 13:13 ` Daniel Vetter
2021-03-19 1:40 ` Jason Ekstrand
2021-03-17 22:19 ` [PATCH 2/3] dma-buf: add dma_resv_get_singleton_rcu (v3) Jason Ekstrand
2021-03-17 22:19 ` [PATCH 3/3] dma-buf: Add an API for exporting sync files (v8) Jason Ekstrand
2021-03-23 17:57 ` Jason Ekstrand
2021-03-23 19:06 ` Simon Ser
2021-03-23 19:34 ` Jason Ekstrand
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