Clippy lint · Performance · checked with Clippy 0.1.99

clippy::missing_spin_loop: busy-waiting loop should at least have a spin loop hint

What it meansChecks for empty spin loops It is in the performance group and warns by default.

What you see

warning: busy-waiting loop should at least have a spin loop hint
 --> main.rs:6:33
  |
6 | while b.load(Ordering::Acquire) {};
  |                                 ^^ help: try: `{ std::hint::spin_loop() }`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#missing_spin_loop
  = note: `#[warn(clippy::missing_spin_loop)]` on by default

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

The loop body should have something like thread::park() or at least std::hint::spin_loop() to avoid needlessly burning cycles and conserve energy. Perhaps even better use an actual lock, if possible.

Code that could be written to run faster. Warns by default.

How to fix it

Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.

Flagged

use core::sync::atomic::{AtomicBool, Ordering};
let b = AtomicBool::new(true);
// give a ref to `b` to another thread,wait for it to become false
while b.load(Ordering::Acquire) {};

Write this instead

while b.load(Ordering::Acquire) {
    std::hint::spin_loop()
}

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

On one item

#[allow(clippy::missing_spin_loop)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
missing_spin_loop = "allow"

One run

cargo clippy -- -A clippy::missing-spin-loop

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Performancegroup

warndefault level

yesfixed by cargo clippy --fix

1.61.0added in Rust

Clippy's own documentation

What it does

Checks for empty spin loops

Why is this bad?

The loop body should have something like thread::park() or at least std::hint::spin_loop() to avoid needlessly burning cycles and conserve energy. Perhaps even better use an actual lock, if possible.

Known problems

This lint doesn’t currently trigger on while let or loop { match .. { .. } } loops, which would be considered idiomatic in combination with e.g. AtomicBool::compare_exchange_weak.

Example

use core::sync::atomic::{AtomicBool, Ordering};
let b = AtomicBool::new(true);
// give a ref to `b` to another thread,wait for it to become false
while b.load(Ordering::Acquire) {};

Use instead:

while b.load(Ordering::Acquire) {
    std::hint::spin_loop()
}

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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