From 275d5c82518897d465ae1c9d1d4cc0e6d426a4d1 Mon Sep 17 00:00:00 2001 From: Ralf Jung Date: Sat, 9 Sep 2023 17:06:10 +0200 Subject: [PATCH] wording --- library/core/src/sync/atomic.rs | 22 +++++++++++++--------- 1 file changed, 13 insertions(+), 9 deletions(-) diff --git a/library/core/src/sync/atomic.rs b/library/core/src/sync/atomic.rs index 6adf199a763..71ee578f8b4 100644 --- a/library/core/src/sync/atomic.rs +++ b/library/core/src/sync/atomic.rs @@ -87,18 +87,21 @@ //! atomic `load`s might be implemented using compare-exchange operations, even a `load` can fault //! on read-only memory. //! -//! For the purpose of this section, "read-only memory" is defined as memory that is read-only in +//! (For the purpose of this section, "read-only memory" is defined as memory that is read-only in //! the underlying target, i.e., the pages are mapped with a read-only flag and any attempt to write //! will cause a page fault. In particular, an `&u128` reference that points to memory that is //! read-write mapped is *not* considered to point to "read-only memory". In Rust, almost all memory //! is read-write; the only exceptions are memory created by `const` items or `static` items without -//! interior mutability. +//! interior mutability, and memory that was specifically marked as read-only by the operating +//! system via platform-specific APIs.) //! -//! However, as an exception from this general rule, Rust guarantees that "sufficiently small" -//! atomic loads are implemented in a way that works on read-only memory. This threshold of -//! "sufficiently small" depends on the architecture: +//! However, as an exception from this general rule, "sufficiently small" atomic loads are +//! implemented in a way that works on read-only memory. The exact threshold for what makes a load +//! "sufficiently small" varies depending on the architecture and feature flags, but Rust guarantees +//! that atomic loads that do not exceed the size documented in the following table are guaranteed +//! to be read-only: //! -//! | Target architecture | Maximal atomic `load` size that is guaranteed read-only | +//! | Target architecture | Atomic loads no larger than this are guaranteed read-only | //! |---------------|---------| //! | `x86` | 4 bytes | //! | `x86_64` | 8 bytes | @@ -106,10 +109,11 @@ //! | `aarch64` | 8 bytes | //! | `riscv32` | 4 bytes | //! | `riscv64` | 8 bytes | -//! | `powerpc64le` | 8 bytes | +//! | `powerpc64` | 8 bytes | //! -//! Any atomic `load` on read-only memory larger than the given size are Undefined Behavior. For -//! architectures not listed above, all atomic `load` on read-only memory are Undefined Behavior. +//! Atomics loads that are larger than this threshold (and *all* atomic loads on targets not listed +//! in the table) might still be read-only under certain conditions, but that is not a stable +//! guarantee and should not be relied upon. //! //! # Examples //!