gh-151518: Avoid STW starvation of attaching threads#152826
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Free-threaded stop-the-world pauses can otherwise starve a thread trying to reattach after it was suspended while detached. A tight manual gc.collect() loop can release and immediately request the next stop-the-world pause, repeatedly parking the detached thread before it can attach and make progress. Add a distinct _Py_THREAD_SUSPENDED_DETACHED state for tstates parked from DETACHED. tstate_wait_attach() marks an attach waiter only after observing that detached-origin suspended state, and park_detached_threads() skips only those active waiters on later stop-the-world passes. The ordinary successful tstate_try_attach() path remains the baseline CAS-only path. Teach the related stop-the-world paths about both suspended states, including start_the_world() and tstate_delete_common(). Keep the new wait flag after the existing hot free-threaded _PyThreadStateImpl fields so their offsets do not move. Add a free-threaded GC regression test that runs a subprocess with a tight gc.collect() worker and verifies the main thread can reattach after sleeping and stop the worker.
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Fixes #151518.
Repeated stop-the-world requests can starve a thread that was detached when
an earlier request parked it.
start_the_world()restores the tstate toDETACHED, but the next requester can park it again before its OS threadcompletes
_PyThreadState_Attach().This change distinguishes tstates parked while detached from those suspended
while attached, and marks a waiter only when
tstate_wait_attach()observesthat detached-origin state. Later STW passes leave only active attach waiters
unparked until they attach. Threads that are merely detached for sleep or I/O
remain immediately parkable, and the normal uncontended attach path remains a
single CAS. The waiter flag reuses existing
_PyThreadStateImpltail padding.The regression test runs a tight
gc.collect()loop in a subprocess andverifies that another thread can return from
time.sleep()and stop thecollector.
Validation:
mainatefcfb1a4e0f4.timed out after 30 seconds in 2/3 runs on unpatched
main; the patchedCI-style debug build passed 5/5 in about 1.2 seconds per run.
(
--with-pydebug --enable-safety --enable-slower-safety --disable-gil):PYTHON_GIL=0 ./python -m test --fast-ci test_free_threading.test_gc test_threadingpassed, 251 tests run and 5 skipped.
--disable-gil): the same focused test commandpassed, 251 tests run and 5 skipped.
_PyThreadStateImplare both 15,792bytes, with all common offsets unchanged;
stw_attach_waitingoverlays__paddingat offset 15,728.make patchcheckpassed.Earlier no-debug performance comparisons on 64-CPU AMD and 28-CPU Intel
systems found the no-thread and detached-sleeper paths roughly flat. In the
adversarial attached/churn cases, lower repeated-collector throughput was
paired with greater worker progress, which is the intended fairness effect.