DF-1863 amdgpu_ctx_add_fence TOCTOU harness  (N_THREADS=2, ITERS=4000)
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BUGGY  : iters=8000  double_put=1320  live_fences=1352  occupied_slots=32  leaked=1320
FIXED  : iters=8000  double_put=0  live_fences=32  occupied_slots=32  leaked=0
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RESULT: BUGGY variant produced 1320 double dma_fence_put() events and 1320 leaked fences => confirms TOCTOU primitive.
        On the kernel this is a use-after-free / double-free
        of struct dma_fence (drm_sched_fence) in kmalloc.