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gh-135641: Fix flaky test_capi.test_lock_two_threads test case #135642

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Merged
colesbury merged 1 commit into from
Jun 18, 2025
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gh-135641: Fix flaky test_capi.test_lock_two_threads test case #135642

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5 changes: 4 additions & 1 deletion Modules/_testinternalcapi/test_lock.c
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Original file line number Diff line number Diff line change
Expand Up @@ -57,7 +57,10 @@ lock_thread(void *arg)
_Py_atomic_store_int(&test_data->started, 1);

PyMutex_Lock(m);
assert(m->_bits == 1);
// gh-135641: in rare cases the lock may still have `_Py_HAS_PARKED` set
// (m->_bits == 3) due to bucket collisions in the parking lot hash table
// between this mutex and the `test_data.done` event.
Comment on lines +60 to +62

@mpage mpage Jun 17, 2025

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Are there any consequences of this being set incorrectly? I think another solution, though much more invasive, might be to move num_waiters from Bucket onto wait_entry.

@colesbury colesbury Jun 18, 2025 •
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Are there any consequences of this being set incorrectly?

The unlock will just take a slower path. The approximate (conservative) setting of _Py_HAS_PARKED is taken from WebKit, so I'm not too worried about it. It's just easy to forget about when writing these sorts of tests.

I think another solution, though much more invasive, might be to move num_waiters from Bucket onto wait_entry.

You can do this nicely and efficiently if you restructure the buckets into a tree of linked lists, which is something we talked about when merging the original parking lot PR, but hasn't been a priority.

There's currently a potential O(N^2) behavior in parking lot due to a linear scan (where N=threads waiting on locks) that using a balanced tree would fix. That's not something I'd want to backport though.

Here's the equivalent fix in Go for their mutex implementation: golang/go@990124d

assert(m->_bits == 1 || m->_bits == 3);

PyMutex_Unlock(m);
assert(m->_bits == 0);
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