Clippy lint · Performance · checked with Clippy 0.1.99

clippy::readonly_write_lock: this write lock is used only for reading

What it meansLooks for calls to RwLock::write where the lock is only used for reading. It is in the performance group and warns by default.

What you see

warning: this write lock is used only for reading
 --> main.rs:5:15
  |
5 |     let num = lock.write().unwrap();
  |               ^^^^^^^^^^^^ help: consider using a read lock instead: `lock.read()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#readonly_write_lock
  = note: `#[warn(clippy::readonly_write_lock)]` 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 write portion of RwLock is exclusive, meaning that no other thread can access the lock while this writer is active.

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

How to fix it

Clippy does not fix this one automatically (its suggestion may change behaviour, so it only shows it). Change the code by hand:

Flagged

use std::sync::RwLock;
fn assert_is_zero(lock: &RwLock<i32>) {
    let num = lock.write().unwrap();
    assert_eq!(*num, 0);
}

Write this instead

use std::sync::RwLock;
fn assert_is_zero(lock: &RwLock<i32>) {
    let num = lock.read().unwrap();
    assert_eq!(*num, 0);
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
readonly_write_lock = "allow"

One run

cargo clippy -- -A clippy::readonly-write-lock

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

nofixed by cargo clippy --fix

1.73.0added in Rust

Clippy's own documentation

What it does

Looks for calls to RwLock::write where the lock is only used for reading.

Why is this bad?

The write portion of RwLock is exclusive, meaning that no other thread can access the lock while this writer is active.

Example

use std::sync::RwLock;
fn assert_is_zero(lock: &RwLock<i32>) {
    let num = lock.write().unwrap();
    assert_eq!(*num, 0);
}

Use instead:

use std::sync::RwLock;
fn assert_is_zero(lock: &RwLock<i32>) {
    let num = lock.read().unwrap();
    assert_eq!(*num, 0);
}

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

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