Random CCCL pick: cub/test/test_device_scan_warpspeed_shifted_output.cu
(40 lines, full read — minimal reproducer for CCCL issue #8838)
CCCL bug: InclusiveScan with out+1 (shifted output pointer) caused
illegal memory access in lookahead scan warpspeed path. Root cause:
uninitialized memory before the output offset was read by the kernel.
Our V2 attention has analogous shifted outputs:
tmp_output[seq_idx, :, :num_partitions, :] — only first num_partitions
written, rest is max_num_partitions-sized buffer with garbage.
Change: torch.empty → torch.zeros for tmp_output and exp_sums,
torch.empty_like → torch.full(fill_value=-inf) for max_logits.
This is defensive: paged_attention_v2_pytorch.py already initializes
these in its body, but if any code path skips that (early return,
exception), the caller's buffers are now safe by construction.
Cost: one extra memset per decode step. For max_num_seqs=1:
tmp_output: 1×24×200×256×2B = 2.4MB memset (negligible vs matmul)
exp_sums+max_logits: 1×24×200×4B = 19KB each
Base file modified: qwen3_6_scripts/paged_attn.py (deployed via patch_ops.sh)
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