gh-115184: Fix refleak tracking issues in free-threaded build#115188
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- We did not account for abandoned segments, which could miss blocks in multithreaded runs. - The mimalloc heap traversal needed to call "_mi_page_free_collect" earlier in order to get an accurate count of blocks in use. - `_Py_DecRefSharedDebug` was missing a `_Py_IncRefTotal`, but this was mostly offset by a missing accounting in `_Py_ExplicitMergeRefcount`. - get_num_global_allocated_blocks should pause other threads to ensure that traversing the mimalloc heaps is safe.
colesbury added the 🔨 test-with-refleak-buildbots 
bedevere-bot commented Feb 8, 2024
🤖 New build scheduled with the buildbot fleet by @colesbury for commit 1b17640 🤖 If you want to schedule another build, you need to add the 🔨 test-with-refleak-buildbots label again. |
bedevere-bot removed the 🔨 test-with-refleak-buildbots 
DinoV reviewed Feb 9, 2024
| _Py_AddRefTotal(_PyInterpreterState_GET(), extra); | ||
| #endif | ||
| _Py_atomic_store_uint32_relaxed(&op->ob_ref_local, 0); |
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Do I understand this correctly that we're not racing here because we're either own the owning thread or the world is stopped for GC?
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The possible cases are:
- We are the owning thread and gave up ownership (typical)
- We are the queueing thread and the owning thread has exited (gh-110481: Implement inter-thread queue for biased reference counting #114824)
- We are the queueing thread and have stopped the world (due to OOM) (gh-110481: Implement inter-thread queue for biased reference counting #114824)
The GC could have used this code path, but it uses a different function that avoids atomics (because it's stopped the world).
The reason case (2) is thread-safe is a bit subtle:
- Queueing is done once, so only one thread will attempt it
- Although thread ids may be reused, the merge for (2) happens while holding a lock that would need to be acquired by any new thread that reuses the thread id.
fsc-eriker pushed a commit to fsc-eriker/cpython that referenced this pull request Feb 14, 2024
…ython#115188) Fixes a few issues related to refleak tracking in the free-threaded build: - Count blocks in abandoned segments - Call `_mi_page_free_collect` earlier during heap traversal in order to get an accurate count of blocks in use. - Add missing refcount tracking in `_Py_DecRefSharedDebug` and `_Py_ExplicitMergeRefcount`. - Pause threads in `get_num_global_allocated_blocks` to ensure that traversing the mimalloc heaps is safe.
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_mi_page_free_collectearlier in order to get an accurate count of live blocks._Py_DecRefSharedDebugwas missing a_Py_IncRefTotal, but this was mostly offset by a missing accounting in_Py_ExplicitMergeRefcount.