[PATCH 3/8] drm/amdgpu: overwrite signaled fence in amdgpu_sync

[SRINIVASAN SHANMUGAM]

On 3/7/2025 7:18 PM, Christian König wrote:
> This allows using amdgpu_sync even without peeking into the fences for a
> long time.
>
> Signed-off-by: Christian König <christian.koenig at amd.com>
> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c | 13 +++++++++----
>   1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c
> index 86c17a8946f5..bfe12164d27d 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_sync.c
> @@ -135,11 +135,16 @@ static bool amdgpu_sync_add_later(struct amdgpu_sync *sync, struct dma_fence *f)
>   	struct amdgpu_sync_entry *e;
>   
>   	hash_for_each_possible(sync->fences, e, node, f->context) {
> -		if (unlikely(e->fence->context != f->context))
> -			continue;
> +		if (dma_fence_is_signaled(e->fence)) {
> +			dma_fence_put(e->fence);
> +			e->fence = dma_fence_get(f);
> +			return true;
> +		}
>   
> -		amdgpu_sync_keep_later(&e->fence, f);
> -		return true;
> +		if (likely(e->fence->context == f->context)) {
> +			amdgpu_sync_keep_later(&e->fence, f);
--> The call to amdgpu_sync_keep_later(&e->fence, f); ensures that the 
new fence is tracked for future synchronization., ie., so If the driver 
only replaced the old fence without keeping a reference to the new one 
of the next job or second job for example, it could lead to situations 
where the synchronization state is lost. This could cause race 
conditions where one job might proceed before another job has completed, 
leading to errors. , so this is  " amdgpu_sync_keep_later(&e->fence, 
f);" done, for tracking purpose of next job/second job, if we have 
multiple jobs in gang submissions of same context/client, is my 
understanding is correct here pls?
> +			return true;
> +		}
>   	}
>   	return false;
>   }