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[BugFix][FlashInfer] Fix potential race condition for paged_kv_indptr_cpu #23737

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Merged
merged 2 commits into from
Aug 28, 2025
+10 −2
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[BugFix][FlashInfer] Fix potential race condition for paged_kv_indptr_cpu #23737

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[FlashInfer] Fix potential race condition for paged_kv_indptr_cpu
WoosukKwon Aug 27, 2025
63cdb44
merge
WoosukKwon Aug 28, 2025
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12 changes: 10 additions & 2 deletions vllm/v1/attention/backends/flashinfer.py
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Original file line number Diff line number Diff line change
Expand Up @@ -237,6 +237,8 @@ def __init__(self, kv_cache_spec: AttentionSpec, layer_names: list[str],
device="cpu",
pin_memory=pin_memory)
self.paged_kv_indptr_np = self.paged_kv_indptr_cpu.numpy()
self.paged_kv_indptr_buffer = torch.zeros_like(
self.paged_kv_indptr_cpu, pin_memory=pin_memory)
self.paged_kv_indices_cpu = torch.zeros(max_num_pages,
dtype=torch.int32,
device="cpu",
Expand Down Expand Up @@ -361,12 +363,18 @@ def build(self,
dtype=np.int32,
out=self.paged_kv_indptr_np[1:num_reqs + 1],
)
# NOTE(woosuk): Because self.paged_kv_indptr_cpu can be modified
# after this line (e.g., for cuda graphs), we need to copy the data to
# self.paged_kv_indptr_buffer to avoid race condition.
self.paged_kv_indptr_buffer[:num_reqs +
1] = (self.paged_kv_indptr_cpu[:num_reqs +
1])
paged_kv_indptr = self.paged_kv_indptr[:num_reqs + 1]
paged_kv_indptr.copy_(self.paged_kv_indptr_cpu[:num_reqs + 1],
paged_kv_indptr.copy_(self.paged_kv_indptr_buffer[:num_reqs + 1],
non_blocking=True)

# write self.paged_kv_indices inplace
num_actual_pages = num_blocks_np.sum().item()
num_actual_pages = self.paged_kv_indptr_np[num_reqs]
paged_kv_indices = self.paged_kv_indices[:num_actual_pages]
_copy_page_indices_kernel[(num_reqs, )](
paged_kv_indices,
Expand Down