Htykuut

I want to some work on a vector shared by multiple threads but I don't want to use a Mutex because it is not wait-free.

The code below is written as I would in C.

#![feature(core_intrinsics, ptr_internals)]



use std::intrinsics::atomic_xadd_rel;

use std::ptr::Unique;

use std::thread::spawn;



fn main() {

    let mut data = [0; 8];

    let mut pool = Vec::with_capacity(8);

    for index in 0..8 {

        let data_ptr = Unique::new(data.as_mut_ptr());

        pool.push(spawn(move || {

            println!("Thread {} -> {}", index, unsafe {

                atomic_xadd_rel(

                    data_ptr

                        .unwrap()

                        .as_ptr()

                        .add(if index % 2 != 0 { index - 1 } else { index }),

                    1,

                )

            });

        }));

    }

    for work in pool {

        work.join().unwrap();

    }

    println!("Data {:?}", data);

}

I've also written the code using only the stable API:

use std::iter::repeat_with;

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

use std::sync::Arc;

use std::thread::spawn;



fn main() {

    let data = Arc::new(

        repeat_with(|| AtomicUsize::new(0))

            .take(8)

            .collect::<Vec<_>>(),

    );

    let mut pool = Vec::with_capacity(8);

    for index in 0..8 {

        let data_clone = data.clone();

        pool.push(spawn(move || {

            let offset = index - (index % 2 != 0) as usize;

            println!(

                "Thread {} -> {}",

                index,

                data_clone[offset].fetch_add(1, Relaxed)

            );

        }));

    }

    for work in pool {

        work.join().unwrap();

    }

    println!("Data {:?}", data);

}

This code returns

Thread 0 -> 0

Thread 1 -> 1

Thread 3 -> 0

Thread 5 -> 1

Thread 7 -> 1

Thread 2 -> 1

Thread 6 -> 0

Thread 4 -> 0

Data [2, 0, 2, 0, 2, 0, 2, 0]

Is there is a proper way to do this in Rust?

I do not think this is a duplicate of How do I pass disjoint slices from a vector to different threads? because my vector / slice elements overlap between threads. In my sample, each odd index of the slice is incremented twice by two different threads.

Assuming that each thread has unique access to a particular element or sub-slice of your vector, this would be a case to use split_at (or one of the similar functions). split_at splits a mutable slice into two independent mutable slices; you can call it multiple times to split your slice into the correct number of segments, and pass each sub-slice to a separate thread.

The best way to pass the sub-slices to a thread would be to use something like the scoped threads in crossbeam.