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

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

vllm/v1/attention/backends/flashinfer.py

@@ -237,6 +237,8 @@ class FlashInferMetadataBuilder(AttentionMetadataBuilder[FlashInferMetadata]):
                                                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",
@@ -361,12 +363,18 @@ class FlashInferMetadataBuilder(AttentionMetadataBuilder[FlashInferMetadata]):
             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,