drm/amdgpu: workaround for TLB seq race

[Limonciello, Mario]

On 11/2/2022 09:58, Christian König wrote:
> It can happen that we query the sequence value before the callback
> had a chance to run.
> 
> Work around that by grabbing the fence lock and releasing it again.
> Should be replaced by hw handling soon.
> 
> Signed-off-by: Christian König <christian.koenig at amd.com>
> Acked-by: Alex Deucher <alexander.deucher at amd.com>

You probably also want to add these tags:

Fixes: 5255e146c99a6 ("drm/amdgpu: rework TLB flushing")
Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2113

> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
> 
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h
> index 9ecb7f663e19..e51a46c9582b 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.h
> @@ -485,6 +485,21 @@ void amdgpu_debugfs_vm_bo_info(struct amdgpu_vm *vm, struct seq_file *m);
>    */
>   static inline uint64_t amdgpu_vm_tlb_seq(struct amdgpu_vm *vm)
>   {
> +	unsigned long flags;
> +	spinlock_t *lock;
> +
> +	/*
> +	 * Work around to stop racing between the fence signaling and handling
> +	 * the cb. The lock is static after initially setting it up, just make
> +	 * sure that the dma_fence structure isn't freed up.
> +	 */
> +	rcu_read_lock();
> +	lock = vm->last_tlb_flush->lock;
> +	rcu_read_unlock();
> +
> +	spin_lock_irqsave(lock, flags);
> +	spin_unlock_irqrestore(lock, flags);
> +
>   	return atomic64_read(&vm->tlb_seq);
>   }
>