drm/amdgpu: fix sync handling in amdgpu_dma_buf_move_notify

In the Linux kernel, the following vulnerability has been resolved:

drm/amdgpu: fix sync handling in amdgpu_dma_buf_move_notify

Invalidating a dmabuf will impact other users of the shared BO.

In the scenario where process A moves the BO, it needs to inform

process B about the move and process B will need to update its

page table.

The commit fixes a synchronisation bug caused by the use of the

ticket: it made amdgpu_vm_handle_moved behave as if updating

the page table immediately was correct but in this case it's not.

An example is the following scenario, with 2 GPUs and glxgears

running on GPU0 and Xorg running on GPU1, on a system where P2P

PCI isn't supported:

glxgears:

export linear buffer from GPU0 and import using GPU1

submit frame rendering to GPU0

submit tiled->linear blit

Xorg:

copy of linear buffer

The sequence of jobs would be:

drm_sched_job_run # GPU0, frame rendering

drm_sched_job_queue # GPU0, blit

drm_sched_job_done # GPU0, frame rendering

drm_sched_job_run # GPU0, blit

move linear buffer for GPU1 access #

amdgpu_dma_buf_move_notify -> update pt # GPU0

It this point the blit job on GPU0 is still running and would

likely produce a page fault.