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Rollup merge of #56023 - vorner:doc/atomic-ordering-strip, r=@stjepang

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atomic::Ordering: Get rid of misleading parts of intro

Remove the parts of atomic::Ordering's intro that wrongly claimed that
SeqCst prevents all reorderings around it.

Closes #55196

This is a (minimal) alternative to #55233.

I also wonder if it would be worth adding at least some warnings that atomics are often a footgun/hard to use correctly, similarly like `mem::transmute` or other functions have.
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Guillaume Gomez 2018-11-29 13:10:30 +01:00 committed by GitHub
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1 changed files with 9 additions and 7 deletions
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16
src/libcore/sync/atomic.rs
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@ -173,11 +173,11 @@ unsafe impl<T> Sync for AtomicPtr<T> {}
/// Atomic memory orderings
///
/// Memory orderings limit the ways that both the compiler and CPU may reorder
/// instructions around atomic operations. At its most restrictive,
/// "sequentially consistent" atomics allow neither reads nor writes
/// to be moved either before or after the atomic operation; on the other end
/// "relaxed" atomics allow all reorderings.
/// Memory orderings specify the way atomic operations synchronize memory.
/// In its weakest [`Relaxed`][Ordering::Relaxed], only the memory directly touched by the
/// operation is synchronized. On the other hand, a store-load pair of [`SeqCst`][Ordering::SeqCst]
/// operations synchronize other memory while additionally preserving a total order of such
/// operations across all threads.
///
/// Rust's memory orderings are [the same as
/// LLVM's](https://llvm.org/docs/LangRef.html#memory-model-for-concurrent-operations).
@ -185,6 +185,8 @@ unsafe impl<T> Sync for AtomicPtr<T> {}
/// For more information see the [nomicon].
///
/// [nomicon]: ../../../nomicon/atomics.html
/// [Ordering::Relaxed]: #variant.Relaxed
/// [Ordering::SeqCst]: #variant.SeqCst
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Copy, Clone, Debug)]
#[non_exhaustive]
@ -234,8 +236,8 @@ pub enum Ordering {
/// For loads it uses [`Acquire`] ordering. For stores it uses the [`Release`] ordering.
///
/// Notice that in the case of `compare_and_swap`, it is possible that the operation ends up
/// not performing any store and hence it has just `Acquire` ordering. However,
/// `AcqRel` will never perform [`Relaxed`] accesses.
/// not performing any store and hence it has just [`Acquire`] ordering. However,
/// [`AcqRel`][`AcquireRelease`] will never perform [`Relaxed`] accesses.
///
/// This ordering is only applicable for operations that combine both loads and stores.
///