[Mesa-dev] [PATCH 01/11] drm/amdgpu: Comply with implicit fencing rules

[Daniel Vetter]

On Tue, May 25, 2021 at 5:05 PM Christian König
<ckoenig.leichtzumerken at gmail.com> wrote:
>
> Hi Daniel,
>
> Am 25.05.21 um 15:05 schrieb Daniel Vetter:
> > Hi Christian,
> >
> > On Sat, May 22, 2021 at 10:30:19AM +0200, Christian König wrote:
> >> Am 21.05.21 um 20:31 schrieb Daniel Vetter:
> >> This works by adding the fence of the last eviction DMA operation to BOs
> >> when their backing store is newly allocated. That's what the
> >> ttm_bo_add_move_fence() function you stumbled over is good for: https://elixir.bootlin.com/linux/v5.13-rc2/source/drivers/gpu/drm/ttm/ttm_bo.c#L692
> >>
> >> Now the problem is it is possible that the application is terminated before
> >> it can complete it's command submission. But since resource management only
> >> waits for the shared fences when there are some there is a chance that we
> >> free up memory while it is still in use.
> > Hm where is this code? Would help in my audit that I wanted to do this
> > week? If you look at most other places like
> > drm_gem_fence_array_add_implicit() I mentioned earlier, then we don't
> > treat the shared fences special and always also include the exclusive one.
>
> See amdgpu_gem_object_close():
>
> ...
>          fence = dma_resv_get_excl(bo->tbo.base.resv);
>          if (fence) {
>                  amdgpu_bo_fence(bo, fence, true);
>                  fence = NULL;
>          }
> ...
>
> We explicitly added that because resource management of some other
> driver was going totally bananas without that.
>
> But I'm not sure which one that was. Maybe dig a bit in the git and
> mailing history of that.

Hm I looked and it's

commit 82c416b13cb7d22b96ec0888b296a48dff8a09eb
Author: Christian König <christian.koenig at amd.com>
Date:   Thu Mar 12 12:03:34 2020 +0100

   drm/amdgpu: fix and cleanup amdgpu_gem_object_close v4

That sounded more like amdgpu itself needing this, not another driver?

But looking at amdgpu_vm_bo_update_mapping() it seems to pick the
right fencing mode for gpu pte clearing, so I'm really not sure what
the bug was that you worked around here?The implementation boils down
to amdgpu_sync_resv() which syncs for the exclusive fence, always. And
there's nothing else that I could find in public history at least, no
references to bug reports or anything. I think you need to dig
internally, because as-is I'm not seeing the problem here.

Or am I missing something here?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch