Fix stacked borrows violations in epoch & skiplist, fix CI failures by taiki-e · Pull Request #871 · crossbeam-rs/crossbeam

taiki-e · 2022-07-20T02:00:11Z

Fixes #545
Fixes #878

Also rollups CI failures fixes from (the third and subsequent commits):
Closes #1218
Closes #1196

See #545 (comment) for the reported stacked borrows violations.

Note: this is a breaking change because changes API of Pointable trait

796: epoch: Remove ptr-to-int casts r=taiki-e a=taiki-e Use [this hack](rust-lang/miri#1866 (comment)) to fix compatibility issues with Miri (see #490 (comment) for details). Due to the #545, still not compatible with stacked borrows. This will be fixed by the subsequent PR (#871). Note: this is a breaking change because changes API of Pointable and Pointer traits Fixes #579 881: Remove deprecated items r=taiki-e a=taiki-e This removes the following deprecated items: - crossbeam-epoch: - `CompareAndSetError` - `CompareAndSetOrdering` - `Atomic::compare_and_set` - `Atomic::compare_and_set_weak` - crossbeam-utils: - `AtomicCell::compare_and_swap` Co-authored-by: Taiki Endo <te316e89@gmail.com>

796: epoch: Remove ptr-to-int casts r=taiki-e a=taiki-e Use [this hack](rust-lang/miri#1866 (comment)) to fix compatibility issues with Miri (see #490 (comment) for details). Due to the #545, still not compatible with stacked borrows. This will be fixed by the subsequent PR (#871). Note: this is a breaking change because changes API of Pointable and Pointer traits Fixes #579 Co-authored-by: Taiki Endo <te316e89@gmail.com>

taiki-e · 2022-11-20T08:33:28Z

crossbeam-epoch/src/atomic.rs

This is no longer a dereference, but I'm not sure what is a good API name and description.

taiki-e · 2023-06-17T11:45:44Z

(Given that Miri appears to be trying to move to TB in the long term, I don't see the need to rush to merge this.)

saethlin · 2025-08-03T18:49:27Z

(Given that Miri appears to be trying to move to TB in the long term, I don't see the need to rush to merge this.)

That's wrong. There is no plan to move to TB entirely, and in any case TB doesn't help here because the implementation in Miri (currently, at least) doesn't support the int-to-ptr that crossbeam-epoch currently relies on.

taiki-e · 2025-08-03T19:53:05Z

(Given that Miri appears to be trying to move to TB in the long term, I don't see the need to rush to merge this.)

That's wrong. There is no plan to move to TB entirely,

That comment was my understanding based on reading several discussions and blog posts at 2023-06 and before, and I may have misunderstood something. (My understanding at the time was that they wanted to adopt a somewhat stricter version of TB. EDIT: Ralf's recent comment seem to suggest the future adoption of a stricter version of TB.)

Well, to be honest, that comment should no longer be relevant to this PR. The remaining issues with the epoch and issues in skiplist that made me hesitate to progress this PR at the time have been fixed in the last two commits in this PR (thanks @Imberflur for the skiplist fix), so there is no longer any need to consider TB-only support.

and in any case TB doesn't help here because the implementation in Miri (currently, at least) doesn't support the int-to-ptr that crossbeam-epoch currently relies on.

That issue has already been resolved in the master branch: #796

Copilot

Pull request overview

This PR fixes stacked borrows violations in crossbeam's epoch-based garbage collection and skiplist implementations, originally reported in issues #545 and #878. The changes involve significant API modifications to avoid creating intermediate references that violate Miri's stacked borrows model, allowing the code to pass under default Miri checks without requiring Tree Borrows. Additionally, the PR includes CI configuration updates and bumps the MSRV to 1.74 to support newly used standard library features.

Changes:

Refactored Pointable trait to return raw pointers instead of references to avoid stacked borrows violations
Modified IsElement trait to use raw pointers throughout the API
Introduced TowerRef and NodeRef wrapper types in skiplist to preserve provenance for dynamically-sized trailing arrays
Updated Local::release_handle and Local::finalize to accept raw pointers
Replaced manual ceiling division with div_ceil method (requires Rust 1.73+)
Updated MSRV from 1.61/1.60 to 1.74 across affected crates
Removed Tree Borrows requirement from CI Miri tests
Added new targets to no_atomic.rs list

Reviewed changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
crossbeam-epoch/src/atomic.rs	Breaking API change: Pointable trait methods now return raw pointers instead of references; added helper methods to Shared for safe pointer access
crossbeam-epoch/src/internal.rs	Changed Local::release_handle and finalize to use raw pointers to avoid stacked borrows violations
crossbeam-epoch/src/guard.rs	Updated call to Local::release_handle to work with new signature
crossbeam-epoch/src/sync/list.rs	Breaking API change: IsElement trait now uses raw pointers; updated all implementations and call sites
crossbeam-skiplist/src/base.rs	Major refactoring: introduced TowerRef and NodeRef types to preserve provenance; removed Index trait usage; updated all node traversal logic
crossbeam-deque/src/deque.rs	Replaced manual ceiling division `(n+1)/2` with `div_ceil(2)` method
no_atomic.rs	Added armv4t-none-eabi, armv5te-none-eabi, thumbv4t-none-eabi, thumbv5te-none-eabi targets
ci/miri.sh	Removed Tree Borrows flag since Stacked Borrows violations are now fixed
.github/workflows/ci.yml	Updated MSRV to 1.74, upgraded Ubuntu ARM runner to 24.04, removed vm.mmap_rnd_bits workaround
Multiple Cargo.toml and README.md files	Updated rust-version and MSRV documentation from 1.60-1.61 to 1.74
ci/check-features.sh	Updated comment and cargo command to reflect new MSRV requirement for atomic feature
Cargo.toml (root)	Allowed manual_map clippy lint

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crossbeam-skiplist/src/base.rs

Co-authored-by: Imbris <imbrisf@gmail.com>

``` error: use `core::ptr::eq` when comparing raw pointers --> crossbeam-epoch/src/sync/list.rs:343:17 | 343 | assert!(maybe_e3.unwrap().unwrap() as *const Entry == e3.as_raw()); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `core::ptr::eq(maybe_e3.unwrap().unwrap(), e3.as_raw())` | = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_eq = note: `-D clippy::ptr-eq` implied by `-D warnings` = help: to override `-D warnings` add `#[allow(clippy::ptr_eq)]` error: use `core::ptr::eq` when comparing raw pointers --> crossbeam-epoch/src/sync/list.rs:346:17 | 346 | assert!(maybe_e2.unwrap().unwrap() as *const Entry == e2.as_raw()); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `core::ptr::eq(maybe_e2.unwrap().unwrap(), e2.as_raw())` | = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_eq error: use `core::ptr::eq` when comparing raw pointers --> crossbeam-epoch/src/sync/list.rs:349:17 | 349 | assert!(maybe_e1.unwrap().unwrap() as *const Entry == e1.as_raw()); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `core::ptr::eq(maybe_e1.unwrap().unwrap(), e1.as_raw())` | = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_eq error: use `core::ptr::eq` when comparing raw pointers --> crossbeam-epoch/src/sync/list.rs:382:17 | 382 | assert!(maybe_e3.unwrap().unwrap() as *const Entry == e3.as_raw()); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `core::ptr::eq(maybe_e3.unwrap().unwrap(), e3.as_raw())` | = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_eq error: use `core::ptr::eq` when comparing raw pointers --> crossbeam-epoch/src/sync/list.rs:385:17 | 385 | assert!(maybe_e1.unwrap().unwrap() as *const Entry == e1.as_raw()); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `core::ptr::eq(maybe_e1.unwrap().unwrap(), e1.as_raw())` | = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_eq ```

``` error: non-canonical implementation of `partial_cmp` on an `Ord` type --> crossbeam-epoch/src/atomic.rs:1552:1 | 1552 | / impl<'g, T: ?Sized + Pointable> PartialOrd<Shared<'g, T>> for Shared<'g, T> { 1553 | | fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> { | | __________________________________________________________________- 1554 | || Some(self.data.cmp(&other.data)) 1555 | || } | ||_____- help: change this to: `{ Some(self.cmp(other)) }` 1556 | | } | |__^ | = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.93.0/index.html#non_canonical_partial_ord_impl = note: `-D clippy::non-canonical-partial-ord-impl` implied by `-D warnings` = help: to override `-D warnings` add `#[allow(clippy::non_canonical_partial_ord_impl)]` ```

``` error: manually reimplementing `div_ceil` --> crossbeam-deque/src/deque.rs:774:35 | 774 | let batch_size = cmp::min((len as usize + 1) / 2, limit); | ^^^^^^^^^^^^^^^^^^^^^^ help: consider using `.div_ceil()`: `(len as usize).div_ceil(2)` | = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.93.0/index.html#manual_div_ceil = note: `-D clippy::manual-div-ceil` implied by `-D warnings` = help: to override `-D warnings` add `#[allow(clippy::manual_div_ceil)]` error: manually reimplementing `div_ceil` --> crossbeam-deque/src/deque.rs:1626:27 | 1626 | advance = ((len + 1) / 2).min(limit); | ^^^^^^^^^^^^^^^ help: consider using `.div_ceil()`: `len.div_ceil(2)` | = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.93.0/index.html#manual_div_ceil error: manually reimplementing `div_ceil` --> crossbeam-deque/src/deque.rs:1823:27 | 1823 | advance = ((len + 1) / 2).min(limit); | ^^^^^^^^^^^^^^^ help: consider using `.div_ceil()`: `len.div_ceil(2)` | = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.93.0/index.html#manual_div_ceil ```

This reverts commit 75d24e0.

This reverts commit 8131228.

…e inline asm support

taiki-e added crossbeam-epoch breaking change labels Jul 20, 2022

taiki-e marked this pull request as draft July 20, 2022 02:00

taiki-e force-pushed the epoch-fix-sb-violations branch from fcafef5 to 8e29940 Compare July 20, 2022 16:30

taiki-e marked this pull request as ready for review July 21, 2022 01:20

taiki-e force-pushed the epoch-fix-sb-violations branch 3 times, most recently from f25a4cd to a81e099 Compare July 22, 2022 15:24

taiki-e mentioned this pull request Jul 22, 2022

epoch: Remove ptr-to-int casts #796

Merged

taiki-e force-pushed the epoch-fix-sb-violations branch from a81e099 to e7c8fbf Compare July 22, 2022 16:54

This was referenced Jul 22, 2022

epoch: Miri reports SB violation #545

Closed

skiplist: Miri reports SB violation #878

Closed

taiki-e force-pushed the epoch-fix-sb-violations branch from e7c8fbf to 86c1680 Compare July 22, 2022 18:14

taiki-e force-pushed the epoch-fix-sb-violations branch from 86c1680 to 525ae4f Compare July 23, 2022 08:50

taiki-e requested a review from jeehoonkang July 23, 2022 10:53

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taiki-e force-pushed the epoch-fix-sb-violations branch from 48e0dfd to be5bcc4 Compare November 20, 2022 08:04

taiki-e commented Nov 20, 2022

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taiki-e marked this pull request as draft November 20, 2022 08:34

taiki-e marked this pull request as ready for review November 20, 2022 08:37

taiki-e force-pushed the epoch-fix-sb-violations branch from be5bcc4 to 87eecad Compare December 16, 2022 13:41

taiki-e force-pushed the epoch-fix-sb-violations branch from 87eecad to f0f7323 Compare December 27, 2022 14:43

taiki-e mentioned this pull request Jan 30, 2023

Pounds of miri provenance errors #957

Closed

taiki-e mentioned this pull request Jun 17, 2023

Question (epoch): How to avoid Miri to report data race on an Atomic pointer? #993

Closed

taiki-e force-pushed the epoch-fix-sb-violations branch from f0f7323 to 6bd2fda Compare June 17, 2023 11:20

cuviper mentioned this pull request Aug 19, 2025

miri raised on any build rayon-rs/rayon#1267

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taiki-e force-pushed the epoch-fix-sb-violations branch from 6ddfad6 to 85e491b Compare February 14, 2026 08:57

taiki-e changed the title ~~epoch: Fix stacked borrows violations~~ Fix stacked borrows violations in epoch & skiplist, fix CI failures Feb 14, 2026

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taiki-e and others added 10 commits February 14, 2026 18:51

epoch: Fix stacked borrows violations

daf1d10

skiplist: Fix stacked borrows violations

966f9bd

Co-authored-by: Imbris <imbrisf@gmail.com>

Update no_atomic.rs

0a85420

Bump MSRV of atomic feature to 1.74

3cf0a5d

Revert "ci: Use ubuntu-22.04-arm instead of ubuntu-24.04-arm"

9e5370d

This reverts commit 75d24e0.

Revert "ci: Work around sanitizer issue on latest Linux kernel"

8264d0e

This reverts commit 8131228.

ci: Test both stacked borrows and tree borrows

0407e1b

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utils: Fix AtomicCell::is_lock_free doctest with target without stabl…

bb14b07

…e inline asm support

taiki-e force-pushed the epoch-fix-sb-violations branch from 9587ed2 to bb14b07 Compare February 14, 2026 10:29

taiki-e merged commit b3fea26 into master Feb 14, 2026
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taiki-e deleted the epoch-fix-sb-violations branch February 14, 2026 10:52

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Fix stacked borrows violations in epoch & skiplist, fix CI failures#871

Fix stacked borrows violations in epoch & skiplist, fix CI failures#871
taiki-e merged 11 commits intomasterfrom
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