Raciness with page table shadows being swapped out

[Nicolai Hähnle]

Hi all,

I just sent out two patches that hopefully make the kernel module more 
robust in the face of page table shadows being swapped out.

However, even with those patches, I can still fairly reliably reproduce 
crashes with a backtrace of the shape

amdgpu_cs_ioctl
  -> amdgpu_vm_update_page_directory
  -> amdgpu_ttm_bind
  -> amdgpu_gtt_mgr_alloc

The plausible reason for these crashes is that nothing seems to prevent 
the shadow BOs from being moved between the calls to amdgpu_cs_validate 
in amdgpu_cs_parser_bos and the calls to amdgpu_ttm_bind.

The attached patch has fixed these crashes for me so far, but it's very 
heavy-handed: it collects all page table shadows and the page directory 
shadow and adds them all to the reservations for the callers of 
amdgpu_vm_update_page_directory.

I feel like there should be a better way. In part, I wonder why the 
shadows are needed in the first place. I vaguely recall the discussions 
about GPU reset and such, but I don't remember why the page tables can't 
just be rebuilt in some other way.

Cheers,
Nicolai
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