[Model Runner V2] Don't use UVA buffer for prefill_len (#29713)

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

vllm/v1/worker/gpu/model_runner.py

@@ -410,6 +410,8 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
                 cu_num_new_blocks[i].append(x + len(block_ids))
                 new_block_ids[i].extend(block_ids)
             overwrite.append(True)
+        if scheduler_output.scheduled_new_reqs:
+            self.req_states.prefill_len.copy_to_gpu()
 
         # Add new blocks for the existing requests.
         cached_reqs = scheduler_output.scheduled_cached_reqs

vllm/v1/worker/gpu/states.py

@@ -117,7 +117,10 @@ class RequestState:
         self.prefill_token_ids = UvaBuffer(
             self.max_num_reqs, self.max_model_len, dtype=torch.int32
         )
-        self.prefill_len = UvaBuffer(self.max_num_reqs, dtype=torch.int32)
+        # NOTE(woosuk): We don't use UVA for prefill_len because its GPU view
+        # can be used outside of update_states and prepare_inputs.
+        # Without async barrier, using UVA can cause race conditions.
+        self.prefill_len = self._make_buffer(self.max_num_reqs, dtype=torch.int32)
         # Number of computed tokens.
         self.num_computed_prefill_tokens = np.zeros(self.max_num_reqs, dtype=np.int32)
         self.num_computed_tokens = torch.zeros(