[BUG]: cudaLaunchHostFunc doesn't work with CUDA Graph

Is this a duplicate?

• I confirmed there appear to be no duplicate issues for this bug and that I agree to the Code of Conduct

Type of Bug

Runtime Error

Component

cuda.bindings

Describe the bug

Hi team,

I tested cuda.bindings.runtime.cudaLaunchHostFunc, it seemed not working well with CUDA Graph. However, the C++ cudaLaunchHostFunc can work with CUDA Graph.

Similar to https://github.com/cupy/cupy/issues/9274, the host function can be captured by CUDA graph, but when replying CUDA graph, only the first reply can call the host function, while more replays cannot. Is this expected?

Please see the repro steps below. Thanks!

How to Reproduce

import ctypes

import cuda.bindings.runtime as cudart
import torch


class Struct(ctypes.Structure):
    _fields_ = [
        ("a", ctypes.c_int),
    ]


def hostfunc(userData):
    data = Struct.from_address(userData)
    print(f"Hello, World! {data.a}")
    return 0


HostFn_t = ctypes.PYFUNCTYPE(ctypes.c_int, ctypes.c_void_p)


def main():
    data = Struct(a=1)

    # ctypes is managing the pointer value for us
    c_hostfunc = HostFn_t(hostfunc)
    cuda_hostfunc = cudart.cudaHostFn_t(_ptr=ctypes.addressof(c_hostfunc))

    # Run
    stream = torch.cuda.Stream()
    cudart_stream = cudart.cudaStream_t(stream.cuda_stream)

    g = torch.cuda.CUDAGraph()
    with torch.cuda.graph(g, stream=stream):
        (err, ) = cudart.cudaLaunchHostFunc(cudart_stream, cuda_hostfunc,
                                            ctypes.addressof(data))
        assert err == cudart.cudaError_t.cudaSuccess
    torch.cuda.synchronize()
    print("Graph captured", flush=True)

    with torch.cuda.stream(stream):
        for i in range(10):
            g.replay()

    torch.cuda.synchronize()


if __name__ == "__main__":
    main()

Expected behavior

The hostfunc should be called for 10 times. However, only the first time is successful:

Graph captured
Hello, World! 1
Caught signal 11 (Segmentation fault: address not mapped to object at address 0x2e906)
Segmentation fault (core dumped)

Operating System

Ubuntu 24.04.2 LTS

nvidia-smi output

Mon Aug  4 10:10:28 2025       
+-----------------------------------------------------------------------------------------+
| NVIDIA-SMI 575.57.08              Driver Version: 575.57.08      CUDA Version: 12.9     |
|-----------------------------------------+------------------------+----------------------+
| GPU  Name                 Persistence-M | Bus-Id          Disp.A | Volatile Uncorr. ECC |
| Fan  Temp   Perf          Pwr:Usage/Cap |           Memory-Usage | GPU-Util  Compute M. |
|                                         |                        |               MIG M. |
|=========================================+========================+======================|
|   0  NVIDIA H20                     On  |   00000000:D1:00.0 Off |                    0 |
| N/A   37C    P0             73W /  500W |       0MiB /  97871MiB |      0%      Default |
|                                         |                        |             Disabled |
+-----------------------------------------+------------------------+----------------------+
|   1  NVIDIA H20                     On  |   00000000:DF:00.0 Off |                    0 |
| N/A   39C    P0             76W /  500W |       0MiB /  97871MiB |      0%      Default |
|                                         |                        |             Disabled |
+-----------------------------------------+------------------------+----------------------+
                                                                                         
+-----------------------------------------------------------------------------------------+
| Processes:                                                                              |
|  GPU   GI   CI              PID   Type   Process name                        GPU Memory |
|        ID   ID                                                               Usage      |
|=========================================================================================|
|  No running processes found                                                             |
+-----------------------------------------------------------------------------------------+

Inizia eseguendo la riproduzione Python di cuda.bindings.runtime.cudaLaunchHostFunc con CUDA Graph capture e replay ripetuti di torch graph. Ispeziona il binding runtime di cuda.bindings per questo entry point e confronta il suo comportamento con quello di C++ cudaLaunchHostFunc descritto nell’issue. Il lavoro è completato quando la funzione host viene eseguita correttamente per tutti i 10 replay del grafo senza causare un segmentation fault.