@vstinner, on Debian 9 I can see the problem as well but wasn't able to debug with the level of details you did. Could you please share the process you followed?

What I found was:

./python -X dev test_lock_sigsegv.py
Parent r_q: <Lock(owner=None)>, <Lock(owner=None)>, <BoundedSemaphore(value=2147483647, maxvalue=2147483647)>
Fatal Python error: Segmentation fault

Current thread 0x00007fab36124480 (most recent call first):
  File "/home/gus/Workspace/cpython/Lib/multiprocessing/synchronize.py", line 170 in __repr__
  File "/home/gus/Workspace/cpython/test_lock_sigsegv.py", line 17 in child
  File "/home/gus/Workspace/cpython/Lib/multiprocessing/process.py", line 99 in run
  File "/home/gus/Workspace/cpython/Lib/multiprocessing/process.py", line 297 in _bootstrap
  File "/home/gus/Workspace/cpython/Lib/multiprocessing/spawn.py", line 130 in _main
  File "/home/gus/Workspace/cpython/Lib/multiprocessing/spawn.py", line 117 in spawn_main
  File "<string>", line 1 in <module>
-11

Using GDB:

(gdb) set follow-fork-mode child
(gdb) run test_lock_sigsegv.py 
Starting program: /home/gus/Workspace/cpython/python test_lock_sigsegv.py
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Parent r_q: <Lock(owner=None)>, <Lock(owner=None)>, <BoundedSemaphore(value=2147483647, maxvalue=2147483647)>
[New process 4941]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
process 4941 is executing new program: /home/gus/Workspace/cpython/python
-11
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[Inferior 2 (process 4941) exited normally]
(gdb) where
No stack.
(gdb) py-bt
Unable to locate python frame
(gdb)