Example Mutex in `std::sync::atomic::fence` is wrong

[pca006132]

Page: https://doc.rust-lang.org/std/sync/atomic/fn.fence.html

use std::sync::atomic::AtomicBool;
use std::sync::atomic::fence;
use std::sync::atomic::Ordering;

// A mutual exclusion primitive based on spinlock.
pub struct Mutex {
    flag: AtomicBool,
}

impl Mutex {
    pub fn new() -> Mutex {
        Mutex {
            flag: AtomicBool::new(false),
        }
    }

    pub fn lock(&self) {
        while !self.flag.compare_and_swap(false, true, Ordering::Relaxed) {}
        // This fence synchronizes-with store in `unlock`.
        fence(Ordering::Acquire);
    }

    pub fn unlock(&self) {
        self.flag.store(false, Ordering::Release);
    }
}

The CAS function would return the original value of the flag, so the loop would exit when the original value of flag is true, i.e. when it is locked.

[RalfJung]

That's a good catch, thanks!