After https://github.com/python/cpython/pull/18334, a "while condition: pass" in a thread will preclude releasing the GIL, hanging the program with CPU at 100%.

Trivial to reproduce:

"""
#!/usr/bin/env python3.10

import threading
import time

def worker():
    while cont:
        pass

def main():
    global cont
    cont = True
    t = threading.Thread(target=worker)
    t.start()
    time.sleep(1)
    cont = False
    t.join()

if __name__ == '__main__':
    main()
"""

https://github.com/python/cpython/pull/18334 assumes that since all loops will contain a backwards edge, checking for interrupts on JUMP_ABSOLUTE should be sufficient.

However, https://github.com/python/cpython/pull/23743 changed the back edges in while statements. So `worker` compiles to:

  2           0 LOAD_GLOBAL              0 (cont)
              2 POP_JUMP_IF_FALSE        7 (to 14)

  3     >>    4 NOP

  2           6 LOAD_GLOBAL              0 (cont)
              8 POP_JUMP_IF_TRUE         2 (to 4)
             10 LOAD_CONST               0 (None)
             12 RETURN_VALUE
        >>   14 LOAD_CONST               0 (None)
             16 RETURN_VALUE

which has no CALL or JUMP_ABSOLUTE instructions.

New changeset 000e70ad5246732fcbd27cf59268185cbd5ad734 by Mark Shannon in branch 'main':
bpo-44645: Check for interrupts on any potentially backwards edge. (GH-27167)
https://github.com/python/cpython/commit/000e70ad5246732fcbd27cf59268185cbd5ad734

New changeset 0e349ea5541104c76cafc173bfcfef8de872f96f by Mark Shannon in branch '3.10':
[3.10] bpo-44645: Check for interrupts on any potentially backwards edge. (GH-27167) (GH-27183)
https://github.com/python/cpython/commit/0e349ea5541104c76cafc173bfcfef8de872f96f

PR 27167 hangs or fails. in the refleak buildbots, so I am going to proceed to revert

To reproduce the problem:

$./python -m test test_threading -R : -v

...

test_original_excepthook (test.test_threading.ExceptHookTests) ... ok
test_system_exit (test.test_threading.ExceptHookTests) ... ok
test_can_interrupt_tight_loops (test.test_threading.InterruptMainTests) ...

(HANGS here or some time fails like in: 

======================================================================
FAIL: test_can_interrupt_tight_loops (test.test_threading.InterruptMainTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/buildbot/buildarea/pull_request.cstratak-RHEL7-ppc64le.refleak/build/Lib/test/support/threading_helper.py", line 62, in decorator
    return func(*args)
           ^^^^^^^^^^^
  File "/home/buildbot/buildarea/pull_request.cstratak-RHEL7-ppc64le.refleak/build/Lib/test/test_threading.py", line 1630, in test_can_interrupt_tight_loops
    self.assertNotEqual(iterations, 0)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 0 == 0
----------------------------------------------------------------------

https://buildbot.python.org/all/#/builders/411/builds/92

New changeset c90c591e5158ab7b531dcd6e2a5f00bc70ba7637 by Pablo Galindo Salgado in branch 'main':
Revert "bpo-44645: Check for interrupts on any potentially backwards edge. (GH-27167)" (#27194)
https://github.com/python/cpython/commit/c90c591e5158ab7b531dcd6e2a5f00bc70ba7637

New changeset 42a5514cca6b4542f6b492b5652e337f15a89227 by Miss Islington (bot) in branch '3.10':
Revert "bpo-44645: Check for interrupts on any potentially backwards edge. (GH-27167)" (GH-27194) (#27195)
https://github.com/python/cpython/commit/42a5514cca6b4542f6b492b5652e337f15a89227

Can we at least include the fix, until we can come up with a better test?

That test fails on 3.9 as well as 3.10 and main.

With a 3.9 build and the test in #27194:

./python -m test test_threading -R :
0:00:00 load avg: 0.95 Run tests sequentially
0:00:00 load avg: 0.95 [1/1] test_threading
beginning 9 repetitions
123456789
...test test_threading failed -- Traceback (most recent call last):
  File "/home/mark/repos/cpython/Lib/test/test_threading.py", line 1479, in test_can_interrupt_tight_loops
    self.assertNotEqual(iterations, 0)
AssertionError: 0 == 0



I don't know what the underlying issue is.

Can you observe a failure without the -R flag?

New changeset d09c13417890427f829e3df297beb0e27133f8f4 by Mark Shannon in branch 'main':
bpo-44645: Check for interrupts on any potentially backwards edge (GH-27216)
https://github.com/python/cpython/commit/d09c13417890427f829e3df297beb0e27133f8f4

New changeset 37bdd2221ce3607a81d5d7fafc4603d95ca3e8cb by Miss Islington (bot) in branch '3.10':
bpo-44645: Check for interrupts on any potentially backwards edge (GH-27216) (GH-27235)
https://github.com/python/cpython/commit/37bdd2221ce3607a81d5d7fafc4603d95ca3e8cb