Module atomic

Atomic types

Atomic types provide primitive shared-memory communication between threads, and are the building blocks of other concurrent types.

This module defines atomic versions of a select number of primitive types, including AtomicBool, AtomicIsize, AtomicUsize, AtomicI8, AtomicU16, etc. Atomic types present operations that, when used correctly, synchronize updates between threads.

Atomic variables are safe to share between threads (they implement Sync) but they do not themselves provide the mechanism for sharing and follow the threading model of Rust. The most common way to share an atomic variable is to put it into an Arc (an atomically-reference-counted shared pointer).

Atomic types may be stored in static variables, initialized using the constant initializers like AtomicBool::new. Atomic statics are often used for lazy global initialization.

Memory model for atomic accesses

Rust atomics currently follow the same rules as C++20 atomics, specifically the rules from the intro.races section, without the “consume” memory ordering. Since C++ uses an object-based memory model whereas Rust is access-based, a bit of translation work has to be done to apply the C++ rules to Rust: whenever C++ talks about “the value of an object”, we understand that to mean the resulting bytes obtained when doing a read. When the C++ standard talks about “the value of an atomic object”, this refers to the result of doing an atomic load (via the operations provided in this module). A “modification of an atomic object” refers to an atomic store.

The end result is almost equivalent to saying that creating a shared reference to one of the Rust atomic types corresponds to creating an atomic_ref in C++, with the atomic_ref being destroyed when the lifetime of the shared reference ends. The main difference is that Rust permits concurrent atomic and non-atomic reads to the same memory as those cause no issue in the C++ memory model, they are just forbidden in C++ because memory is partitioned into “atomic objects” and “non-atomic objects” (with atomic_ref temporarily converting a non-atomic object into an atomic object).

The most important aspect of this model is that data races are undefined behavior. A data race is defined as conflicting non-synchronized accesses where at least one of the accesses is non-atomic. Here, accesses are conflicting if they affect overlapping regions of memory and at least one of them is a write. They are non-synchronized if neither of them happens-before the other, according to the happens-before order of the memory model.

The other possible cause of undefined behavior in the memory model are mixed-size accesses: Rust inherits the C++ limitation that non-synchronized conflicting atomic accesses may not partially overlap. In other words, every pair of non-synchronized atomic accesses must be either disjoint, access the exact same memory (including using the same access size), or both be reads.

Each atomic access takes an Ordering which defines how the operation interacts with the happens-before order. These orderings behave the same as the corresponding C++20 atomic orderings. For more information, see the nomicon.

use std::sync::atomic::{AtomicU16, AtomicU8, Ordering};
use std::mem::transmute;
use std::thread;

let atomic = AtomicU16::new(0);

thread::scope(|s| {
    // This is UB: conflicting non-synchronized accesses, at least one of which is non-atomic.
    s.spawn(|| atomic.store(1, Ordering::Relaxed)); // atomic store
    s.spawn(|| unsafe { atomic.as_ptr().write(2) }); // non-atomic write
});

thread::scope(|s| {
    // This is fine: the accesses do not conflict (as none of them performs any modification).
    // In C++ this would be disallowed since creating an `atomic_ref` precludes
    // further non-atomic accesses, but Rust does not have that limitation.
    s.spawn(|| atomic.load(Ordering::Relaxed)); // atomic load
    s.spawn(|| unsafe { atomic.as_ptr().read() }); // non-atomic read
});

thread::scope(|s| {
    // This is fine: `join` synchronizes the code in a way such that the atomic
    // store happens-before the non-atomic write.
    let handle = s.spawn(|| atomic.store(1, Ordering::Relaxed)); // atomic store
    handle.join().unwrap(); // synchronize
    s.spawn(|| unsafe { atomic.as_ptr().write(2) }); // non-atomic write
});

thread::scope(|s| {
    // This is UB: non-synchronized conflicting differently-sized atomic accesses.
    s.spawn(|| atomic.store(1, Ordering::Relaxed));
    s.spawn(|| unsafe {
        let differently_sized = transmute::<&AtomicU16, &AtomicU8>(&atomic);
        differently_sized.store(2, Ordering::Relaxed);
    });
});

thread::scope(|s| {
    // This is fine: `join` synchronizes the code in a way such that
    // the 1-byte store happens-before the 2-byte store.
    let handle = s.spawn(|| atomic.store(1, Ordering::Relaxed));
    handle.join().unwrap();
    s.spawn(|| unsafe {
        let differently_sized = transmute::<&AtomicU16, &AtomicU8>(&atomic);
        differently_sized.store(2, Ordering::Relaxed);
    });
});

Portability

All atomic types in this module are guaranteed to be lock-free if they’re available. This means they don’t internally acquire a global mutex. Atomic types and operations are not guaranteed to be wait-free. This means that operations like fetch_or may be implemented with a compare-and-swap loop.

Atomic operations may be implemented at the instruction layer with larger-size atomics. For example some platforms use 4-byte atomic instructions to implement AtomicI8. Note that this emulation should not have an impact on correctness of code, it’s just something to be aware of.

The atomic types in this module might not be available on all platforms. The atomic types here are all widely available, however, and can generally be relied upon existing. Some notable exceptions are:

• PowerPC and MIPS platforms with 32-bit pointers do not have AtomicU64 or AtomicI64 types.
• ARM platforms like armv5te that aren’t for Linux only provide load and store operations, and do not support Compare and Swap (CAS) operations, such as swap, fetch_add, etc. Additionally on Linux, these CAS operations are implemented via operating system support, which may come with a performance penalty.
• ARM targets with thumbv6m only provide load and store operations, and do not support Compare and Swap (CAS) operations, such as swap, fetch_add, etc.

Note that future platforms may be added that also do not have support for some atomic operations. Maximally portable code will want to be careful about which atomic types are used. AtomicUsize and AtomicIsize are generally the most portable, but even then they’re not available everywhere. For reference, the std library requires AtomicBools and pointer-sized atomics, although core does not.

The #[cfg(target_has_atomic)] attribute can be used to conditionally compile based on the target’s supported bit widths. It is a key-value option set for each supported size, with values “8”, “16”, “32”, “64”, “128”, and “ptr” for pointer-sized atomics.

Atomic accesses to read-only memory

In general, all atomic accesses on read-only memory are undefined behavior. For instance, attempting to do a compare_exchange that will definitely fail (making it conceptually a read-only operation) can still cause a segmentation fault if the underlying memory page is mapped read-only. Since atomic loads 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 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, and memory that was specifically marked as read-only by the operating system via platform-specific APIs.

As an exception from the general rule stated above, “sufficiently small” atomic loads with Ordering::Relaxed are implemented in a way that works on read-only memory, and are hence not undefined behavior. The exact size limit for what makes a load “sufficiently small” varies depending on the target:

target_arch	Size limit
x86, arm, mips, mips32r6, powerpc, riscv32, sparc, hexagon	4 bytes
x86_64, aarch64, loongarch64, mips64, mips64r6, powerpc64, riscv64, sparc64, s390x	8 bytes

Atomics loads that are larger than this limit as well as atomic loads with ordering other than Relaxed, as well as 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.

If you need to do an acquire load on read-only memory, you can do a relaxed load followed by an acquire fence instead.

Examples

A simple spinlock:

use std::sync::Arc;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::{hint, thread};

fn main() {
    let spinlock = Arc::new(AtomicUsize::new(1));

    let spinlock_clone = Arc::clone(&spinlock);

    let thread = thread::spawn(move || {
        spinlock_clone.store(0, Ordering::Release);
    });

    // Wait for the other thread to release the lock
    while spinlock.load(Ordering::Acquire) != 0 {
        hint::spin_loop();
    }

    if let Err(panic) = thread.join() {
        println!("Thread had an error: {panic:?}");
    }
}

Keep a global count of live threads:

use std::sync::atomic::{AtomicUsize, Ordering};

static GLOBAL_THREAD_COUNT: AtomicUsize = AtomicUsize::new(0);

// Note that Relaxed ordering doesn't synchronize anything
// except the global thread counter itself.
let old_thread_count = GLOBAL_THREAD_COUNT.fetch_add(1, Ordering::Relaxed);
// Note that this number may not be true at the moment of printing
// because some other thread may have changed static value already.
println!("live threads: {}", old_thread_count + 1);

Structs

• AtomicBool
  A boolean type which can be safely shared between threads.
• AtomicI8
  An integer type which can be safely shared between threads.
• AtomicI16
  An integer type which can be safely shared between threads.
• AtomicI32
  An integer type which can be safely shared between threads.
• AtomicI64
  An integer type which can be safely shared between threads.
• AtomicIsize
  An integer type which can be safely shared between threads.
• AtomicPtr
  A raw pointer type which can be safely shared between threads.
• AtomicU8
  An integer type which can be safely shared between threads.
• AtomicU16
  An integer type which can be safely shared between threads.
• AtomicU32
  An integer type which can be safely shared between threads.
• AtomicU64
  An integer type which can be safely shared between threads.
• AtomicUsize
  An integer type which can be safely shared between threads.

Enums

• Ordering
  Atomic memory orderings

Constants

• ATOMIC_BOOL_INITDeprecated
  An AtomicBool initialized to false.
• ATOMIC_ISIZE_INITDeprecated
  An AtomicIsize initialized to 0.
• ATOMIC_USIZE_INITDeprecated
  An AtomicUsize initialized to 0.

Functions

• compiler_fence
  A “compiler-only” atomic fence.
• fence
  An atomic fence.
• spin_loop_hintDeprecated
  Signals the processor that it is inside a busy-wait spin-loop (“spin lock”).