When Python is built and linked with tcmalloc, using ProcessPoolExecutor may deadlock. Here is a reproducible example:

$ cat t.py
from concurrent import futures
import sys
def work(iteration, item):
  sys.stdout.write(f'working: iteration={iteration}, item={item}\n')
  sys.stdout.flush()
for i in range(0, 10000):
    with futures.ProcessPoolExecutor(max_workers=2) as executor:
        executor.submit(work, i, 1)
        executor.submit(work, i, 2)

$ python t.py
working: iteration=0, item=1
working: iteration=0, item=2
working: iteration=1, item=1
working: iteration=1, item=2
...
working: iteration=3631, item=1
working: iteration=3631, item=2
<hang here>

The child process fails to finish. It's more likely to reproduce when the system is busy.

With some bisect search internally, this commit https://github.com/python/cpython/commit/1ac6e379297cc1cf8acf6c1b011fccc7b3da2cbe "triggered" the deadlock threshold with tcmalloc.