gh-121368: Fix seq lock memory ordering in _PyType_Lookup #121388
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colesbury
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topic-free-threading
needs backport to 3.13
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After running all repros I had for several hours I confirm this change solves the issue. |
The `_PySeqLock_EndRead` function needs an acquire fence to ensure that the load of the sequence happens after any loads within the read side critical section. The acquire on the final sequence load isn't sufficent (or necessary).
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Here's the relevant portion of a Rust seqlock implementation for reference: |
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I've added a model for the C11 model check here: https://github.com/colesbury/c11-model-checker/blob/cpython-models/test/seqlock.c (Turns out we need the release fence too) |
DinoV
reviewed
Jul 8, 2024
Python/lock.c
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| if (_Py_atomic_load_uint32_acquire(&seqlock->sequence) == previous) { | ||
| // gh-121368: We need an explicit acquire fence here to ensure that | ||
| // this load of the sequence number is not reordered before any loads | ||
| // withing the read lock. |
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Thanks @colesbury for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13. |
miss-islington
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Jul 8, 2024
…onGH-121388) The `_PySeqLock_EndRead` function needs an acquire fence to ensure that the load of the sequence happens after any loads within the read side critical section. The missing fence can trigger bugs on macOS arm64. Additionally, we need a release fence in `_PySeqLock_LockWrite` to ensure that the sequence update is visible before any modifications to the cache entry. (cherry picked from commit 1d3cf79) Co-authored-by: Sam Gross <[email protected]>
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GH-121505 is a backport of this pull request to the 3.13 branch. |
colesbury
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Jul 8, 2024
…121388) (#121505) The `_PySeqLock_EndRead` function needs an acquire fence to ensure that the load of the sequence happens after any loads within the read side critical section. The missing fence can trigger bugs on macOS arm64. Additionally, we need a release fence in `_PySeqLock_LockWrite` to ensure that the sequence update is visible before any modifications to the cache entry. (cherry picked from commit 1d3cf79) Co-authored-by: Sam Gross <[email protected]>
noahbkim
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Jul 11, 2024
…on#121388) The `_PySeqLock_EndRead` function needs an acquire fence to ensure that the load of the sequence happens after any loads within the read side critical section. The missing fence can trigger bugs on macOS arm64. Additionally, we need a release fence in `_PySeqLock_LockWrite` to ensure that the sequence update is visible before any modifications to the cache entry.
estyxx
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Jul 17, 2024
…on#121388) The `_PySeqLock_EndRead` function needs an acquire fence to ensure that the load of the sequence happens after any loads within the read side critical section. The missing fence can trigger bugs on macOS arm64. Additionally, we need a release fence in `_PySeqLock_LockWrite` to ensure that the sequence update is visible before any modifications to the cache entry.
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The
_PySeqLock_EndReadfunction needs an acquire fence to ensure thatthe load of the sequence happens after any loads within the read side
critical section. The missing fence can trigger bugs on macOS arm64.
Additionally, we need a release fence in
_PySeqLock_LockWritetoensure that the sequence update is visible before any modifications to
the cache entry.
Also fix up the return types of a few seqlock functions.