[alloc+atomic] Make allocation and deallocation paths of slab allocators mutex free

[williamwutq]

Description: Under the flag atomic, making the two slab allocators (SlabBStackAllocator and CheckedSlabBStackAllocator) operations atomic and mutex free with new atomic operations on the free list. Also adds the Discard option to Rust BStackGenOp, which is C's pop with null pointer input.

Important Feature: No
Type: Allocator - Optimization; Allocator - Concurrent
Tests: Included
Feature Flags: alloc + set + atomic
Breaking change: No
New Types: None
Rust Only: No
Fuzz: Yes
Safety Review: Needed: Crash Safety, Invariants, Thread Safety

[Copilot]

Pull request overview

This PR updates the two slab allocators to remove allocator-level mutexes under the atomic feature by making free-list push/pop operations linearizable using BStack::cross_exchange and BStack::process_gen, aiming to make allocation/deallocation paths mutex-free while preserving (or improving) crash/thread-safety guarantees.

Changes:

• Replaced mutex-serialized free-list operations in SlabBStackAllocator with process_gen (pop) and cross_exchange (push/splice) under atomic.
• Updated CheckedSlabBStackAllocator free-list operations similarly; retained a mutex only to make recover() single-flight.
• Reworked CheckedSlabBStackAllocator::recover() in atomic builds into a single process_gen scan pass plus lock-free per-block reclaim.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/alloc/slab.rs	Removes allocator mutex under atomic and makes free-list push/pop mutex-free via cross_exchange/process_gen.
src/alloc/checked_slab.rs	Makes alloc/dealloc/realloc free-list paths mutex-free under atomic, retains mutex only for single-flight recovery, and rewrites recover() to use a locked process_gen scan.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

[williamwutq]

LGTM