Reorder GIL/ipc mutex lock in resource_handles.cpp

cuda_core/cuda/core/_cpp/resource_handles.cpp

@@ -1524,13 +1524,13 @@ DevicePtrHandle deviceptr_import_ipc(const MemoryPoolHandle& h_pool, const void*
         ExportDataKey key;
         std::memcpy(&key.data, data, sizeof(key.data));
 
+        GILReleaseGuard gil;
         std::lock_guard<std::mutex> lock(ipc_import_mutex);
 
         if (auto h = ipc_ptr_cache.lookup(key)) {
             return h;
         }
 
-        GILReleaseGuard gil;
         CUdeviceptr ptr;
         if (CUDA_SUCCESS != (err = p_cuMemPoolImportPointer(&ptr, *h_pool, data))) {
             return {};
@@ -1545,8 +1545,14 @@ DevicePtrHandle deviceptr_import_ipc(const MemoryPoolHandle& h_pool, const void*
         auto box = std::shared_ptr<DevicePtrBox>(
             new DevicePtrBox{ptr, std::move(ds)},
             [h_pool, key](DevicePtrBox* b) {
-                ipc_ptr_cache.unregister_handle(key);
+                // Release the GIL first (the GIL is the outermost lock), then hold
+                // the mutex across unregister + free. A concurrent import that finds
+                // this entry expired must wait until the mapping is gone; otherwise
+                // it re-imports the same allocation and the first cuMemFreeAsync
+                // unmaps it for both (nvbug 5570902).
                 GILReleaseGuard gil;
+                std::lock_guard<std::mutex> lock(ipc_import_mutex);
+                ipc_ptr_cache.unregister_handle(key);
                 const DeallocationStream& stream = b->deallocation;
                 cleanup_in_context(
                     deallocation_context(stream), "cuMemFreeAsync",

cuda_core/tests/memory_ipc/test_ipc_concurrent_import.py

@@ -0,0 +1,87 @@
+# SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+
+"""Regression test for #2840: concurrent IPC imports must not deadlock.
+
+``deviceptr_import_ipc()`` took ``ipc_import_mutex`` while still holding the
+GIL and released the GIL only inside the critical section. Because destruction
+runs in reverse declaration order, the importing thread reacquired the GIL
+before dropping the mutex, while a second thread blocked on the mutex holding
+the GIL.
+
+Every import here succeeds and nothing is reported, so a hang indicates the
+lock ordering rather than anything on an error path. A deadlocked child is
+detected by the parent's join timeout; the per-directory conftest timeout is
+the final backstop.
+"""
+
+import contextlib
+import multiprocessing as mp
+import threading
+
+import pytest
+from helpers.child_processes import child_timeout_sec, kill_subprocesses
+
+from cuda.core import Buffer, Device
+
+CHILD_TIMEOUT_SEC = child_timeout_sec()
+NBYTES = 64
+THREADS = 4
+
+# these tests spawn new processes and files which fails for very many threads
+pytestmark = pytest.mark.parallel_threads_limit(4)
+
+
+def child_main(queue):
+    device = Device()
+    device.set_current()
+    mr = queue.get()
+    descriptor = queue.get()
+
+    # One descriptor for every thread is enough: the mutex is taken before the
+    # pointer cache is consulted, so a thread that would have been a cache hit
+    # still blocks on the lock while holding the GIL.
+    barrier = threading.Barrier(THREADS)
+
+    def importer():
+        # A current context, so the import succeeds and nothing is reported.
+        Device().set_current()
+        barrier.wait()
+        Buffer.from_ipc_descriptor(mr, descriptor, stream=device.default_stream).close()
+
+    threads = [threading.Thread(target=importer) for _ in range(THREADS)]
+    for thread in threads:
+        thread.start()
+    for thread in threads:
+        thread.join()
+
+    device.sync()
+
+
+class TestIpcConcurrentImport:
+    """Importing one descriptor from several threads at once must not hang."""
+
+    @pytest.fixture(autouse=True)
+    def _set_start_method(self):
+        # Ensure spawn is used for multiprocessing
+        with contextlib.suppress(RuntimeError):
+            mp.set_start_method("spawn", force=True)
+
+    def test_main(self, ipc_device, ipc_memory_resource):
+        ipc_device.set_current()
+        mr = ipc_memory_resource
+
+        stream = ipc_device.default_stream
+        buffer = mr.allocate(NBYTES, stream=stream)
+        stream.sync()
+
+        queue = mp.Queue()
+        process = mp.Process(target=child_main, args=(queue,))
+        process.start()
+        queue.put(mr)
+        queue.put(buffer.ipc_descriptor)
+
+        process.join(timeout=CHILD_TIMEOUT_SEC)
+        survivors = kill_subprocesses(process)
+        assert not survivors, "concurrent importers deadlocked (see #2840)"
+        assert process.exitcode == 0, f"child process failed with exit code {process.exitcode}"