[3.13] gh-121368: Fix seq lock memory ordering in _PyType_Lookup (GH-121388) #121505
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…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 <colesbury@gmail.com>
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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.
(cherry picked from commit 1d3cf79)
Co-authored-by: Sam Gross colesbury@gmail.com