diff --git a/src/races.md b/src/races.md index cb78ac65..552cd1fb 100644 --- a/src/races.md +++ b/src/races.md @@ -2,30 +2,32 @@ Safe Rust guarantees an absence of data races, which are defined as: -* two or more threads concurrently accessing a location of memory -* one or more of them is a write -* one or more of them is unsynchronized +- two or more threads concurrently accessing a location of memory +- one or more of them is a write +- one or more of them is unsynchronized -A data race has Undefined Behavior, and is therefore impossible to perform -in Safe Rust. Data races are *mostly* prevented through Rust's ownership system: +A data race has Undefined Behavior, 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). +we have the Send and Sync traits (see the next section for more on this). **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. +This is mathematically impossible in situations where you do not control the +scheduler, which is true for the normal OS environment. If you do control +preemption, it _can be_ possible to prevent general races - this technique is +used by frameworks such as [RTIC](https://github.com/rtic-rs/rtic). However, +actually having control over scheduling is a very uncommon case. -So it's perfectly "fine" for a Safe Rust program to get deadlocked or do -something nonsensical 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: +For this reason, it is considered "safe" for Rust to get deadlocked or do +something nonsensical with incorrect synchronization: this is known as a general +race condition or resource race. Obviously such a program isn't very good, but +Rust of course cannot prevent all logic errors. + +In any case, a race condition cannot 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, a correct program looks like this: ```rust,no_run use std::thread; @@ -58,6 +60,9 @@ thread::spawn(move || { println!("{}", data[idx.load(Ordering::SeqCst)]); ``` +We can cause a data race if we instead do the bound check in advance, and then +unsafely access the data with an unchecked value: + ```rust,no_run use std::thread; use std::sync::atomic::{AtomicUsize, Ordering};