230: Atomic Operations

by Marc Nieper-Wißkirchen

Status

This SRFI is currently in draft status. Here is an explanation of each status that a SRFI can hold. To provide input on this SRFI, please send email to srfi-230@nospamsrfi.schemers.org. To subscribe to the list, follow these instructions. You can access previous messages via the mailing list archive.

Abstract

This SRFI defines atomic operations for the Scheme programming language. An atomic operation is an operation that, even in the presence of multiple threads, is either executed completely or not at all. Atomic operations can be used to implement mutexes and other synchronization primitives, and they can be used to make concurrent algorithms lock-free. For this, this SRFI defines two data types, atomic flags and atomic (fixnum) boxes, whose contents can be queried and mutated atomically. Moreover, each atomic operation comes with a memory order that defines the level of synchronization with other threads.

Issues

None at present.

Rationale

Virtually all modern multi-threaded CPUs support atomic operations. They can often be used to avoid costly locks in concurrent programming. Many general-purpose programming languages like C, C++, Go, or Rust expose a standard set of atomic operations to the programmer. This SRFI does the same for the Scheme programming language.

This SRFI is particularly interesting for multi-threaded applications, e.g. those based on SRFI 18 (or any other threading library), but can also be used in general library code to make it thread-safe.

Example

The following example shows a typical use of atomic fixnum boxes. If the counter were a regular variable, the read-modify-write operation of increasing the counter might be interrupted by another one in a different thread, resulting in a total count too small.

(import (scheme base)
	(scheme write)
	(srfi 18)
	(srfi 230))

(define *atomic-counter* (make-atomic-fxbox 0))

(define (task)
  (do ((i 0 (+ i 1)))
      ((= i 1000))
    (atomic-fxbox+/fetch! *atomic-counter* 1)))

(define threads (make-vector 10))

(do ((i 0 (+ i 1)))
    ((= i 10))
  (let ((thread (make-thread task)))
    (vector-set! threads i thread)
    (thread-start! thread)))

(do ((i 0 (+ i 1)))
    ((= i 10))
  (thread-join! (vector-ref threads i)))

(display (atomic-fxbox-ref *atomic-counter*))
(newline)

Specification

The procedures and syntax described in this section are exported by the (srfi 230) library in an R7RS system and by both the (srfi :230 atomic) and (srfi :230) libraries in an R6RS system.

Entry format

The following naming conventions imply type restrictions:

obj
expected
desired
any object
fx
fixnum
memory-order
any memory order
atomic-flag
atomic flag
atomic-box
atomic box
atomic-fxbox
atomic fixnum box

Atomicity

Unless otherwise specified, all procedures prefixed with atomic- in the library are executed atomically.

Note: In a safe top-level program (in the sense of R6RS), all reads and writes to the store are necessarily atomic. In particular, a library implementing SRFI 18 is not necessarily safe.

Memory orders

The memory order specifies how accesses, including regular, non-atomic memory accesses, are to be ordered around an atomic operation. When multiple threads simultaneously read and write to several variables, one thread can observe the values change in an order different from the order another thread wrote them. Indeed, the apparent order of changes can even differ among multiple reader threads.

The default behavior of all atomic operations defined in this library provides for sequentially consistent ordering (see below). That default can hurt performance, but the library's atomic operations can be given an additional memory-order argument to specify the exact constraints, beyond atomicity, that the implementation must enforce for that operation.

Atomic operations can be given the following memory orders to modify the default behavior:

relaxed
Relaxed operation: there are no synchronization or ordering constraints imposed on other reads or writes. Only this operation's atomicity is guaranteed.
acquire
A read operation on the store with this memory order performs the acquire operation on the affected memory location: no reads or writes in the current thread can be reordered before this read. All writes in other threads that release the same atomic variable are visible in the current thread.
release
A write operation on the store with this memory order performs the release operation: no reads or writes in the current thread can be reordered after this write. All writes in the current thread are visible in other threads that acquire the same atomic variable.
acquire-release
A read-modify-write operation on the store with this memory order is both an acquire operation and a release operation. No reads or writes on the store in the current thread can be reordered before or after this write. All writes in other threads that release the same atomic variable are visible before the modification and the modification is visible in other threads that acquire the same atomic variable.
sequentially-consistent
A read operation on the store with this memory order performs an acquire operation, a write performs a release operation, and read-modify-write performs both an acquire operation and a release operation, plus a single total order exists in which all threads observe all modifications in the same order.
(memory-order memory-order symbol)syntax

It is a syntax violation if memory-order symbol is not a symbol whose name is one of relaxed, acquire, release, acquire-release, and sequentially-consistent. The result is the corresponding symbol, and specifies the associated memory order.

Note: Only the name of memory-order symbol is significant.

(memory-order? obj)procedure

Returns #t if the argument is a valid memory-order symbol, and returns #f otherwise.

Atomic flags

An atomic flag is tagged with a location that can be either in the clear state or in the set state and which can be changed and tested atomically.

(make-atomic-flag)procedure

Returns a newly allocated atomic flag, which is initially in the clear state.

(atomic-flag? obj)procedure

Returns #t if obj is an atomic flag, and returns #f otherwise.

(atomic-flag-test-and-set! atomic-flag)procedure
(atomic-flag-test-and-set! atomic-flag memory-order)procedure

Returns #t if the state of atomic-flag is set, and returns #f otherwise. Changes the state of atomic-flag to set.

(atomic-flag-clear! atomic-flag)procedure
(atomic-flag-clear! atomic-flag memory-order)procedure

Changes the state of atomic-flag to clear.

Atomic boxes

An atomic box is tagged with a location that can hold any object, the content of the atomic box, and which can be changed and tested atomically.

(make-atomic-box obj)procedure

Returns a newly allocated atomic box, whose initial content is obj.

(atomic-box? obj)procedure

Returns #t if obj is an atomic box, and returns #f otherwise.

(atomic-box-ref atomic-box)procedure
(atomic-box-ref atomic-box memory-order)procedure

Returns the content of atomic-box.

(atomic-box-set! atomic-box obj)procedure
(atomic-box-set! atomic-box obj memory-order)procedure

Sets the content of atomic-box to obj and returns unspecified values.

(atomic-box-swap! atomic-box obj)procedure
(atomic-box-swap! atomic-box obj memory-order)procedure

Sets of the content of atomic-box to obj and returns the previous content.

(atomic-box-compare-and-swap! atomic-box expected desired)procedure
(atomic-box-compare-and-swap! atomic-box expected desired memory-order)procedure

Operationally equivalent to the following, executed atomically:

(let ((actual (atomic-box-ref atomic-box memory-order)))
  (when (eq? actual expected)
    (atomic-box-set! atomic-box desired memory-order))
  actual)

Atomic fixnum boxes

An atomic fixnum box is tagged with a location that can hold a fixnum, the content of the atomic fixnum box, and which can be changed and tested atomically.

(make-atomic-fxbox obj)procedure

Returns a newly allocated atomic fixnum box, whose initial content is obj.

(atomic-fxbox? obj)procedure

Returns #t if obj is an atomic fixnum box, and returns #f otherwise.

(atomic-fxbox-ref atomic-fxbox)procedure
(atomic-fxbox-ref atomic-fxbox memory-order)procedure

Returns the content of atomic-fxbox.

(atomic-fxbox-set! atomic-fxbox fx)procedure
(atomic-fxbox-set! atomic-fxbox fx memory-order)procedure

Sets the content of atomic-fxbox to fx and returns unspecified values.

(atomic-fxbox-swap! atomic-fxbox fx)procedure
(atomic-fxbox-swap! atomic-fxbox fx memory-order)procedure

Sets of the content of atomic-fxbox to fx and returns the previous content.

(atomic-fxbox-compare-and-swap! atomic-fxbox expected desired)procedure
(atomic-fxbox-compare-and-swap! atomic-fxbox expected desired memory-order)procedure

Operationally equivalent to the following, executed atomically:

(let ((actual (atomic-fxbox-ref atomic-fxbox memory-order)))
  (when (fx=? actual expected)
    (atomic-fxbox-set! atomic-fxbox desired memory-order))
  actual)
(atomic-fxbox+/fetch! atomic-fxbox fx)procedure
(atomic-fxbox+/fetch! atomic-fxbox fx memory-order)procedure

Sets the content of atomic-fxbox to the sum of the previous content and fx, and returns the previous content.

(atomic-fxbox-/fetch! atomic-fxbox fx)procedure
(atomic-fxbox-/fetch! atomic-fxbox fx memory-order)procedure

Sets the content of atomic-fxbox to the difference of the previous content and fx, and returns the previous content.

(atomic-fxbox-and/fetch! atomic-fxbox fx)procedure
(atomic-fxbox-and/fetch! atomic-fxbox fx memory-order)procedure

Sets the content of atomic-fxbox to the bitwise “and” of the previous content and fx, and returns the previous content.

(atomic-fxbox-ior/fetch! atomic-fxbox fx)procedure
(atomic-fxbox-ior/fetch! atomic-fxbox fx memory-order)procedure

Sets the content of atomic-fxbox to the bitwise “inclusive or” of the previous content and fx, and returns the previous content.

(atomic-fxbox-xor/fetch! atomic-fxbox fx)procedure
(atomic-fxbox-xor/fetch! atomic-fxbox fx memory-order)procedure

Sets the content of atomic-fxbox to the bitwise “exclusive or” of the previous content and fx, and returns the previous content.

Memory synchronization

(atomic-fence memory-order)procedure

Establishes memory synchronization ordering of non-atomic and relaxed atomic accesses, as instructed by memory-order, without an associated atomic operation. Returns unspecified values.

Note: As this procedure does not store a value into a previous allocated location, its name does not end in “!”.

Implementation

For Schemes that do not support multiple threads, an implementation of this SRFI is trivial as all operations are automatically automatic. In particular, atomic flags and atomic boxes could be represented by SRFI 111 boxes.

For Schemes that support multiple threads, an implementation based on SRFI 18 or SRFI 226 mutexes is possible.

The primitives defined in this SRFI are all supported by modern multi-threaded CPUs and are also implemented in the C and the C++ programming languages. Thus, Scheme systems with a native compiler or Scheme systems with a virtual machine based on C or C++ can easily implement this SRFI efficiently.

The sample implementation is an R7RS implementation based on SRFI 18.

Acknowledgements

This SRFI was inspired by the C11 atomic operations library.

It uses language from the C reference wiki.

© 2021 Marc Nieper-Wißkirchen.

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Editor: Arthur A. Gleckler