[PD] Fix data race in NixlKVManager for NIXL backend (#28897)

python/sglang/srt/disaggregation/nixl/conn.py

@@ -246,7 +246,7 @@ class NixlKVManager(CommonKVManager):
     ):
         super().__init__(args, disaggregation_mode, server_args, is_mla_backend)
         try:
-            from nixl._api import nixl_agent, nixl_agent_config
+            from nixl._api import nixl_agent, nixl_agent_config, nixl_thread_sync_t
         except ImportError as e:
             raise ImportError(
                 "Please install NIXL by following the instructions at "
@@ -267,7 +267,13 @@ class NixlKVManager(CommonKVManager):
                 "SGLANG_DISAGGREGATION_NIXL_BACKEND_PARAMS must be a JSON object "
                 "with string keys and string values"
             )
-        agent_config = nixl_agent_config(backends=[], num_threads=num_threads)
+        # self.transfer_worker and self._start_bootstrap_thread runs concurrently
+        # so we cannot use sync_mode=None which is thread-unsafe.
+        agent_config = nixl_agent_config(
+            backends=[],
+            num_threads=num_threads,
+            sync_mode=nixl_thread_sync_t.NIXL_THREAD_SYNC_STRICT,
+        )
         self.agent = nixl_agent(str(uuid.uuid4()), agent_config)
         if num_threads > 0:
             # TODO: Remove this once NIXL passes thread parameters from