drm/amdgpu: fix dma_fence_wait without reference

We need to grab a reference to the fence we wait for.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c

@@ -541,21 +541,24 @@ int amdgpu_ctx_wait_prev_fence(struct amdgpu_ctx *ctx,
 			       struct drm_sched_entity *entity)
 {
 	struct amdgpu_ctx_entity *centity = to_amdgpu_ctx_entity(entity);
-	unsigned idx = centity->sequence & (amdgpu_sched_jobs - 1);
-	struct dma_fence *other = centity->fences[idx];
+	struct dma_fence *other;
+	unsigned idx;
+	long r;
+
+	spin_lock(&ctx->ring_lock);
+	idx = centity->sequence & (amdgpu_sched_jobs - 1);
+	other = dma_fence_get(centity->fences[idx]);
+	spin_unlock(&ctx->ring_lock);
+
+	if (!other)
+		return 0;
 
-	if (other) {
-		signed long r;
 	r = dma_fence_wait(other, true);
-		if (r < 0) {
-			if (r != -ERESTARTSYS)
+	if (r < 0 && r != -ERESTARTSYS)
 		DRM_ERROR("Error (%ld) waiting for fence!\n", r);
 
+	dma_fence_put(other);
 	return r;
-		}
-	}
-
-	return 0;
 }
 
 void amdgpu_ctx_mgr_init(struct amdgpu_ctx_mgr *mgr)