gluon.data.DataLoader when set num_workers>0, Exceptions always occurred

[CookingANut]

Description

train_iter = gluon.data.DataLoader(
mnist_train.transform_first(transformer),
batch_size,
shuffle=True,
num_workers=4
)

when i set num_workers = 4
seems code can run through, but will appear below exception:

Exception ignored in: <function DataLoader.del at 0x7f1d7fe165e0>
Traceback (most recent call last):
File "/home/daemonh/miniconda3/envs/d2l/lib/python3.8/site-packages/mxnet/gluon/data/dataloader.py", line 720, in del
AttributeError: 'NoneType' object has no attribute 'Pool'
Exception ignored in: <function Pool.del at 0x7f1d7fe84670>
Traceback (most recent call last):
File "/home/daemonh/miniconda3/envs/d2l/lib/python3.8/multiprocessing/pool.py", line 268, in del
File "/home/daemonh/miniconda3/envs/d2l/lib/python3.8/multiprocessing/queues.py", line 362, in put
AttributeError: 'NoneType' object has no attribute 'dumps'

Does it need to focus or just common issue need to ignore, thanks you very much!

Environment

----------Python Info----------
Version : 3.8.13
Compiler : GCC 7.5.0
Build : ('default', 'Mar 28 2022 11:38:47')
Arch : ('64bit', 'ELF')
----------MXNet Info-----------
Version : 1.9.1
Directory : /home/daemonh/miniconda3/envs/d2l/lib/python3.8/site-packages/mxnet
Commit hash file "/home/daemonh/miniconda3/envs/d2l/lib/python3.8/site-packages/mxnet/COMMIT_HASH" not found. Not installed from pre-built package or built from source.
Library : ['/home/daemonh/miniconda3/envs/d2l/lib/python3.8/site-packages/mxnet/libmxnet.so']
Build features:
✔ CUDA
✔ CUDNN
✔ NCCL
✔ CUDA_RTC
✖ TENSORRT
✔ CPU_SSE
✔ CPU_SSE2
✔ CPU_SSE3
✖ CPU_SSE4_1
✖ CPU_SSE4_2
✖ CPU_SSE4A
✖ CPU_AVX
✖ CPU_AVX2
✔ OPENMP
✖ SSE
✖ F16C
✖ JEMALLOC
✔ BLAS_OPEN
✖ BLAS_ATLAS
✖ BLAS_MKL
✖ BLAS_APPLE
✔ LAPACK
✔ MKLDNN
✔ OPENCV
✖ CAFFE
✖ PROFILER
✔ DIST_KVSTORE
✖ CXX14
✖ INT64_TENSOR_SIZE
✔ SIGNAL_HANDLER
✖ DEBUG
✖ TVM_OP
----------System Info----------
Platform : Linux-5.10.102.1-microsoft-standard-WSL2-x86_64-with-glibc2.17
system : Linux
release : 5.10.102.1-microsoft-standard-WSL2
version : #1 SMP Wed Mar 2 00:30:59 UTC 2022

[github-actions]

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[CookingANut]

@apache/mxnet-committers