nordic-dev.net/rust
nordic-dev.net/rust
2
0
Fork 0
mirror of https://github.com/rust-lang/rust.git synced 2024-11-25 16:24:46 +00:00
Code Issues Packages Projects Releases Wiki Activity

compiler_fence documentation: emphasize synchronization, not reordering

Browse Source
This commit is contained in:
Ralf Jung 2024-09-01 16:46:11 +02:00
parent 1a1cc050d8
commit 32a30dd005
1 changed files with 29 additions and 29 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

58
library/core/src/sync/atomic.rs
View File

@ -3570,10 +3570,9 @@ unsafe fn atomic_umin<T: Copy>(dst: *mut T, val: T, order: Ordering) -> T {
/// An atomic fence.
///
/// Depending on the specified order, a fence prevents the compiler and CPU from
/// reordering certain types of memory operations around it.
/// That creates synchronizes-with relationships between it and atomic operations
/// or fences in other threads.
/// Fences create synchronization between themselves and atomic operations or fences in other
/// threads. To achieve this, a fence prevents the compiler and CPU from reordering certain types of
/// memory operations around it.
///
/// A fence 'A' which has (at least) [`Release`] ordering semantics, synchronizes
/// with a fence 'B' with (at least) [`Acquire`] semantics, if and only if there
@ -3594,6 +3593,12 @@ unsafe fn atomic_umin<T: Copy>(dst: *mut T, val: T, order: Ordering) -> T {
/// }
/// ```
///
/// Note that in the example above, it is crucial that the accesses to `x` are atomic. Fences cannot
/// be used to establish synchronization among non-atomic accesses in different threads. However,
/// thanks to the happens-before relationship between A and B, any non-atomic accesses that
/// happen-before A are now also properly synchronized with any non-atomic accesses that
/// happen-after B.
///
/// Atomic operations with [`Release`] or [`Acquire`] semantics can also synchronize
/// with a fence.
///
@ -3659,33 +3664,30 @@ pub fn fence(order: Ordering) {
}
}
/// A compiler memory fence.
/// A "compiler-only" atomic fence.
///
/// `compiler_fence` does not emit any machine code, but restricts the kinds
/// of memory re-ordering the compiler is allowed to do. Specifically, depending on
/// the given [`Ordering`] semantics, the compiler may be disallowed from moving reads
/// or writes from before or after the call to the other side of the call to
/// `compiler_fence`. Note that it does **not** prevent the *hardware*
/// from doing such re-ordering. This is not a problem in a single-threaded,
/// execution context, but when other threads may modify memory at the same
/// time, stronger synchronization primitives such as [`fence`] are required.
/// Like [`fence`], this function establishes synchronization with other atomic operations and
/// fences. However, unlike [`fence`], `compiler_fence` only establishes synchronization with
/// operations *in the same thread*. This may at first sound rather useless, since code within a
/// thread is typically already totally ordered and does not need any further synchronization.
/// However, there are cases where code can run on the same thread without being ordered:
/// - The most common case is that of a *signal handler*: a signal handler runs in the same thread
/// as the code it interrupted, but it is not ordered with respect to that code. `compiler_fence`
/// can be used to establish synchronization between a thread and its signal handler, the same way
/// that `fence` can be used to establish synchronization across threads.
/// - Similar situations can arise in embedded programming with interrupt handlers, or in custom
/// implementations of preemptive green threads. In general, `compiler_fence` can establish
/// synchronization with code that is guaranteed to run on the same hardware CPU.
///
/// The re-ordering prevented by the different ordering semantics are:
/// See [`fence`] for how a fence can be used to achieve synchronization. Note that just like
/// [`fence`], synchronization still requires atomic operations to be used in both threads -- it is
/// not possible to perform synchronization entirely with fences and non-atomic operations.
///
/// - with [`SeqCst`], no re-ordering of reads and writes across this point is allowed.
/// - with [`Release`], preceding reads and writes cannot be moved past subsequent writes.
/// - with [`Acquire`], subsequent reads and writes cannot be moved ahead of preceding reads.
/// - with [`AcqRel`], both of the above rules are enforced.
/// `compiler_fence` does not emit any machine code, but restricts the kinds of memory re-ordering
/// the compiler is allowed to do. `compiler_fence` corresponds to [`atomic_signal_fence`] in C and
/// C++.
///
/// `compiler_fence` is generally only useful for preventing a thread from
/// racing *with itself*. That is, if a given thread is executing one piece
/// of code, and is then interrupted, and starts executing code elsewhere
/// (while still in the same thread, and conceptually still on the same
/// core). In traditional programs, this can only occur when a signal
/// handler is registered. In more low-level code, such situations can also
/// arise when handling interrupts, when implementing green threads with
/// pre-emption, etc. Curious readers are encouraged to read the Linux kernel's
/// discussion of [memory barriers].
/// [`atomic_signal_fence`]: https://en.cppreference.com/w/cpp/atomic/atomic_signal_fence
///
/// # Panics
///
@ -3723,8 +3725,6 @@ pub fn fence(order: Ordering) {
/// }
/// }
/// ```
///
/// [memory barriers]: https://www.kernel.org/doc/Documentation/memory-barriers.txt
#[inline]
#[stable(feature = "compiler_fences", since = "1.21.0")]
#[rustc_diagnostic_item = "compiler_fence"]