gh-144513: Skip critical section locking during stop-the-world#144524
gh-144513: Skip critical section locking during stop-the-world#144524colesbury wants to merge 2 commits intopython:mainfrom
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When the interpreter is in a stop-the-world pause, critical sections don't need to acquire locks since no other threads can be running. This avoids a potential deadlock where lock fairness hands off ownership to a thread that has already suspended for stop-the-world.
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A few notes:
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| } | ||
| // If the world is stopped, we don't need to acquire the lock because | ||
| // there are no other threads that could be accessing the object. | ||
| if (tstate->interp->stoptheworld.world_stopped) { |
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Is this safe to read non-atomically (in theory as well as in practice), or is it just that we don't have a way of reading a bool atomically? I see comments about HEAD_LOCK protecting these values but in practice world_stopped is written to without the lock, and read all over the place, without apparent synchronization.
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Yes, it's safe (as long as you have a valid PyThreadState). It's only written to when once all other threads are stopped, so no extra synchronization is needed.
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I should probably make this more clear in the _stoptheworld_state comment.
| // All threads: acquire critical section and hold it long enough to | ||
| // trigger TIME_TO_BE_FAIR_NS, which causes direct handoff on unlock. | ||
| Py_BEGIN_CRITICAL_SECTION(test_data->obj); | ||
| pysleep(10); |
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Add a comment mentioning TIME_TO_BE_FAIR_NS is currently 1ms, and this is 10x that, so the relationship is clear in the future?
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When the interpreter is in a stop-the-world pause, critical sections don't need to acquire locks since no other threads can be running. This avoids a potential deadlock where lock fairness hands off ownership to a thread that has already suspended for stop-the-world.