gh-121368: Fix seq lock memory ordering in _PyType_Lookup#121388
Merged
colesbury merged 4 commits intopython:mainfrom Jul 8, 2024
Merged
gh-121368: Fix seq lock memory ordering in _PyType_Lookup#121388colesbury merged 4 commits intopython:mainfrom
colesbury merged 4 commits intopython:mainfrom
Conversation
colesbury
added
topic-free-threading
needs backport to 3.13
Contributor
|
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).
colesbury
force-pushed
the
gh-121368-seq-lock
branch
from
9a68c4c to
a408e64
Compare
Contributor
Author
|
Here's the relevant portion of a Rust seqlock implementation for reference: |
Contributor
Author
|
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
Outdated
| 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. |
|
Thanks @colesbury for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13. |
miss-islington
pushed a commit
to miss-islington/cpython
that referenced
this pull request
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 <colesbury@gmail.com>
|
GH-121505 is a backport of this pull request to the 3.13 branch. |
colesbury
added a commit
that referenced
this pull request
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 <colesbury@gmail.com>
noahbkim
pushed a commit
to hudson-trading/cpython
that referenced
this pull request
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
pushed a commit
to estyxx/cpython
that referenced
this pull request
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.
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
3 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
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.