diff --git a/library/core/src/sync/atomic.rs b/library/core/src/sync/atomic.rs index 495d9191a9f85..b06a3bd4487d3 100644 --- a/library/core/src/sync/atomic.rs +++ b/library/core/src/sync/atomic.rs @@ -3570,10 +3570,9 @@ unsafe fn atomic_umin(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(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"]