mirror of https://github.com/rust-lang/nomicon
You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
67 lines
3.0 KiB
67 lines
3.0 KiB
% Data Races and Race Conditions
|
|
|
|
Safe Rust guarantees an absence of data races, which are defined as:
|
|
|
|
* two or more threads concurrently accessing a location of memory
|
|
* one of them is a write
|
|
* one of them is unsynchronized
|
|
|
|
A data race has Undefined Behaviour, and is therefore impossible to perform
|
|
in Safe Rust. Data races are *mostly* prevented through rust's ownership system:
|
|
it's impossible to alias a mutable reference, so it's impossible to perform a
|
|
data race. Interior mutability makes this more complicated, which is largely why
|
|
we have the Send and Sync traits (see below).
|
|
|
|
However Rust *does not* prevent general race conditions. This is
|
|
pretty fundamentally impossible, and probably honestly undesirable. Your hardware
|
|
is racy, your OS is racy, the other programs on your computer are racy, and the
|
|
world this all runs in is racy. Any system that could genuinely claim to prevent
|
|
*all* race conditions would be pretty awful to use, if not just incorrect.
|
|
|
|
So it's perfectly "fine" for a Safe Rust program to get deadlocked or do
|
|
something incredibly stupid with incorrect synchronization. Obviously such a
|
|
program isn't very good, but Rust can only hold your hand so far. Still, a
|
|
race condition can't violate memory safety in a Rust program on
|
|
its own. Only in conjunction with some other unsafe code can a race condition
|
|
actually violate memory safety. For instance:
|
|
|
|
```rust
|
|
use std::thread;
|
|
use std::sync::atomic::{AtomicUsize, Ordering};
|
|
use std::sync::Arc;
|
|
|
|
let data = vec![1, 2, 3, 4];
|
|
// Arc so that the memory the AtomicUsize is stored in still exists for
|
|
// the other thread to increment, even if we completely finish executing
|
|
// before it. Rust won't compile the program without it, because of the
|
|
// lifetime requirements of thread::spawn!
|
|
let idx = Arc::new(AtomicUsize::new(0));
|
|
let other_idx = idx.clone();
|
|
|
|
// `move` captures other_idx by-value, moving it into this thread
|
|
thread::spawn(move || {
|
|
// It's ok to mutate idx because this value
|
|
// is an atomic, so it can't cause a Data Race.
|
|
other_idx.fetch_add(10, Ordering::SeqCst);
|
|
});
|
|
|
|
// Index with the value loaded from the atomic. This is safe because we
|
|
// read the atomic memory only once, and then pass a *copy* of that value
|
|
// to the Vec's indexing implementation. This indexing will be correctly
|
|
// bounds checked, and there's no chance of the value getting changed
|
|
// in the middle. However our program may panic if the thread we spawned
|
|
// managed to increment before this ran. A race condition because correct
|
|
// program execution (panicing is rarely correct) depends on order of
|
|
// thread execution.
|
|
println!("{}", data[idx.load(Ordering::SeqCst)]);
|
|
|
|
if idx.load(Ordering::SeqCst) < data.len() {
|
|
unsafe {
|
|
// Incorrectly loading the idx *after* we did the bounds check.
|
|
// It could have changed. This is a race condition, *and dangerous*
|
|
// because we decided to do `get_unchecked`, which is `unsafe`.
|
|
println!("{}", data.get_unchecked(idx.load(Ordering::SeqCst)));
|
|
}
|
|
}
|
|
```
|