Fix free-threaded deadlock when switching greenlets holding a critical section#519
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ddorian wants to merge 1 commit into
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Fix free-threaded deadlock when switching greenlets holding a critical section#519ddorian wants to merge 1 commit into
ddorian wants to merge 1 commit into
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…l section On free-threaded builds the runtime guards objects with per-object locks taken through Py_BEGIN_CRITICAL_SECTION and tracked, per thread, in tstate->critical_section. Those sections are scoped to the C stack. A greenlet switch swaps C stacks but previously only moved the tstate->critical_section head between greenlets -- it never released the underlying PyMutexes. So when a switch happened while a section was held (for example inside asyncio's Task.__step, which holds one on the running task for the whole step), the locks stayed held on the greenlet we left. The greenlet we switched to then blocked forever trying to take one of those same locks. This is what hung Playwright's synchronous API under free-threading; it reproduces with nothing but asyncio and greenlet (see freethread_switch_deadlock.py). Suspend a thread's critical sections in PythonState::operator<< and resume them in operator>>, the same way the interpreter does across a thread detach and reattach. A brand-new greenlet starts with an empty chain, so this is a no-op until something is actually held.
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I think the flaky test is a flake, correct? https://github.com/python-greenlet/greenlet/actions/runs/28684605712/job/85074798831?pr=519 I can't rerun the tests though. |
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fixes #518