The Rust project was initiated to solve two thorny problems:

  • How do you do safe systems programming?
  • How do you make concurrency painless?

Initially these problems seemed orthogonal, but to our amazement, the solution turned out to be identical: the same tools that make Rust safe also help you tackle concurrency head-on.

Memory safety bugs and concurrency bugs often come down to code accessing data when it shouldn’t. Rust’s secret weapon is ownership, a discipline for access control that systems programmers try to follow, but that Rust’s compiler checks statically for you.

For memory safety, this means you can program without a garbage collector and without fear of segfaults, because Rust will catch your mistakes.

For concurrency, this means you can choose from a wide variety of paradigms (message passing, shared state, lock-free, purely functional), and Rust will help you avoid common pitfalls.

Here’s a taste of concurrency in Rust:

  • A channel transfers ownership of the messages sent along it, so you can send a pointer from one thread to another without fear of the threads later racing for access through that pointer. Rust’s channels enforce thread isolation.

  • A lock knows what data it protects, and Rust guarantees that the data can only be accessed when the lock is held. State is never accidentally shared. “Lock data, not code” is enforced in Rust.

  • Every data type knows whether it can safely be sent between or accessed by multiple threads, and Rust enforces this safe usage; there are no data races, even for lock-free data structures. Thread safety isn’t just documentation; it’s law.

  • You can even share stack frames between threads, and Rust will statically ensure that the frames remain active while other threads are using them. Even the most daring forms of sharing are guaranteed safe in Rust.

All of these benefits come out of Rust’s ownership model, and in fact locks, channels, lock-free data structures and so on are defined in libraries, not the core language. That means that Rust’s approach to concurrency is open ended: new libraries can embrace new paradigms and catch new bugs, just by adding APIs that use Rust’s ownership features.

The goal of this post is to give you some idea of how that’s done.

Background: ownership

We’ll start with an overview of Rust’s ownership and borrowing systems. If you’re already familiar with these, you can skip the two “background” sections and jump straight into concurrency. If you want a deeper introduction, I can’t recommend Yehuda Katz’s post highly enough. And the Rust book has all the details.

In Rust, every value has an “owning scope,” and passing or returning a value means transferring ownership (“moving” it) to a new scope. Values that are still owned when a scope ends are automatically destroyed at that point.

Let’s look at some simple examples. Suppose we create a vector and push some elements onto it:

fn make_vec() {
    let mut vec = Vec::new(); // owned by make_vec's scope
    vec.push(0);
    vec.push(1);
    // scope ends, `vec` is destroyed
}

The scope that creates a value also initially owns it. In this case, the body of make_vec is the owning scope for vec. The owner can do anything it likes with vec, including mutating it by pushing. At the end of the scope, vec is still owned, so it is automatically deallocated.

Things get more interesting if the vector is returned or passed around:

fn make_vec() -> Vec<i32> {
    let mut vec = Vec::new();
    vec.push(0);
    vec.push(1);
    vec // transfer ownership to the caller
}

fn print_vec(vec: Vec<i32>) {
    // the `vec` parameter is part of this scope, so it's owned by `print_vec`

    for i in vec.iter() {
        println!("{}", i)
    }

    // now, `vec` is deallocated
}

fn use_vec() {
    let vec = make_vec(); // take ownership of the vector
    print_vec(vec);       // pass ownership to `print_vec`
}

Now, just before make_vec’s scope ends, vec is moved out by returning it; it is not destroyed. A caller like use_vec then receives ownership of the vector.

On the other hand, the print_vec function takes a vec parameter, and ownership of the vector is transferred to it by its caller. Since print_vec does not transfer the ownership any further, at the end of its scope the vector is destroyed.

Once ownership has been given away, a value can no longer be used. For example, consider this variant of use_vec:

fn use_vec() {
    let vec = make_vec();  // take ownership of the vector
    print_vec(vec);        // pass ownership to `print_vec`

    for i in vec.iter() {  // continue using `vec`
        println!("{}", i * 2)
    }
}

If you feed this version to the compiler, you’ll get an error:

error: use of moved value: `vec`

for i in vec.iter() {
         ^~~

The compiler is saying vec is no longer available; ownership has been transferred elsewhere. And that’s very good, because the vector has already been deallocated at this point!

Disaster averted.

Background: borrowing

The story so far isn’t totally satisfying, because it’s not our intent for print_vec to destroy the vector it was given. What we really want is to grant print_vec temporary access to the vector, and then continue using the vector afterwards.

This is where borrowing comes in. If you have access to a value in Rust, you can lend out that access to the functions you call. Rust will check that these leases do not outlive the object being borrowed.

To borrow a value, you make a reference to it (a kind of pointer), using the & operator:

fn print_vec(vec: &Vec<i32>) {
    // the `vec` parameter is borrowed for this scope

    for i in vec.iter() {
        println!("{}", i)
    }

    // now, the borrow ends
}

fn use_vec() {
    let vec = make_vec();  // take ownership of the vector
    print_vec(&vec);       // lend access to `print_vec`
    for i in vec.iter() {  // continue using `vec`
        println!("{}", i * 2)
    }
    // vec is destroyed here
}

Now print_vec takes a reference to a vector, and use_vec lends out the vector by writing &vec. Since borrows are temporary, use_vec retains ownership of the vector; it can continue using it after the call to print_vec returns (and its lease on vec has expired).

Each reference is valid for a limited scope, which the compiler will automatically determine. References come in two flavors:

  • Immutable references &T, which allow sharing but not mutation. There can be multiple &T references to the same value simultaneously, but the value cannot be mutated while those references are active.

  • Mutable references &mut T, which allow mutation but not sharing. If there is an &mut T reference to a value, there can be no other active references at that time, but the value can be mutated.

Rust checks these rules at compile time; borrowing has no runtime overhead.

Why have two kinds of references? Consider a function like:

fn push_all(from: &Vec<i32>, to: &mut Vec<i32>) {
    for i in from.iter() {
        to.push(*i);
    }
}

This function iterates over each element of one vector, pushing it onto another. The iterator keeps a pointer into the vector at the current and final positions, stepping one toward the other.

What if we called this function with the same vector for both arguments?

push_all(&vec, &mut vec)

This would spell disaster! As we’re pushing elements onto the vector, it will occasionally need to resize, allocating a new hunk of memory and copying its elements over to it. The iterator would be left with a dangling pointer into the old memory, leading to memory unsafety (with attendant segfaults or worse).

Fortunately, Rust ensures that whenever a mutable borrow is active, no other borrows of the object are active, producing the message:

error: cannot borrow `vec` as mutable because it is also borrowed as immutable
push_all(&vec, &mut vec);
                    ^~~

Disaster averted.

Message passing

Now that we’ve covered the basic ownership story in Rust, let’s see what it means for concurrency.

Concurrent programming comes in many styles, but a particularly simple one is message passing, where threads or actors communicate by sending each other messages. Proponents of the style emphasize the way that it ties together sharing and communication:

Do not communicate by sharing memory; instead, share memory by communicating.

Effective Go

Rust’s ownership makes it easy to turn that advice into a compiler-checked rule. Consider the following channel API (channels in Rust’s standard library are a bit different):

fn send<T: Send>(chan: &Channel<T>, t: T);
fn recv<T: Send>(chan: &Channel<T>) -> T;

Channels are generic over the type of data they transmit (the <T: Send> part of the API). The Send part means that T must be considered safe to send between threads; we’ll come back to that later in the post, but for now it’s enough to know that Vec<i32> is Send.

As always in Rust, passing in a T to the send function means transferring ownership of it. This fact has profound consequences: it means that code like the following will generate a compiler error.

// Suppose chan: Channel<Vec<i32>>

let mut vec = Vec::new();
// do some computation
send(&chan, vec);
print_vec(&vec);

Here, the thread creates a vector, sends it to another thread, and then continues using it. The thread receiving the vector could mutate it as this thread continues running, so the call to print_vec could lead to race condition or, for that matter, a use-after-free bug.

Instead, the Rust compiler will produce an error message on the call to print_vec:

Error: use of moved value `vec`

Disaster averted.

Locks

Another way to deal with concurrency is by having threads communicate through passive, shared state.

Shared-state concurrency has a bad rap. It’s easy to forget to acquire a lock, or otherwise mutate the wrong data at the wrong time, with disastrous results – so easy that many eschew the style altogether.

Rust’s take is that:

  1. Shared-state concurrency is nevertheless a fundamental programming style, needed for systems code, for maximal performance, and for implementing other styles of concurrency.

  2. The problem is really about accidentally shared state.

Rust aims to give you the tools to conquer shared-state concurrency directly, whether you’re using locking or lock-free techniques.

In Rust, threads are “isolated” from each other automatically, due to ownership. Writes can only happen when the thread has mutable access, either by owning the data, or by having a mutable borrow of it. Either way, the thread is guaranteed to be the only one with access at the time. To see how this plays out, let’s look at locks.

Remember that mutable borrows cannot occur simultaneously with other borrows. Locks provide the same guarantee (“mutual exclusion”) through synchronization at runtime. That leads to a locking API that hooks directly into Rust’s ownership system.

Here is a simplified version (the standard library’s is more ergonomic):

// create a new mutex
fn mutex<T: Send>(t: T) -> Mutex<T>;

// acquire the lock
fn lock<T: Send>(mutex: &Mutex<T>) -> MutexGuard<T>;

// access the data protected by the lock
fn access<T: Send>(guard: &mut MutexGuard<T>) -> &mut T;

This lock API is unusual in several respects.

First, the Mutex type is generic over a type T of the data protected by the lock. When you create a Mutex, you transfer ownership of that data into the mutex, immediately giving up access to it. (Locks are unlocked when they are first created.)

Later, you can lock to block the thread until the lock is acquired. This function, too, is unusual in providing a return value, MutexGuard<T>. The MutexGuard automatically releases the lock when it is destroyed; there is no separate unlock function.

The only way to access the lock is through the access function, which turns a mutable borrow of the guard into a mutable borrow of the data (with a shorter lease):

fn use_lock(mutex: &Mutex<Vec<i32>>) {
    // acquire the lock, taking ownership of a guard;
    // the lock is held for the rest of the scope
    let mut guard = lock(mutex);

    // access the data by mutably borrowing the guard
    let vec = access(&mut guard);

    // vec has type `&mut Vec<i32>`
    vec.push(3);

    // lock automatically released here, when `guard` is destroyed
}

There are two key ingredients here:

  • The mutable reference returned by access cannot outlive the MutexGuard it is borrowing from.

  • The lock is only released when the MutexGuard is destroyed.

The result is that Rust enforces locking discipline: it will not let you access lock-protected data except when holding the lock. Any attempt to do otherwise will generate a compiler error. For example, consider the following buggy “refactoring”:

fn use_lock(mutex: &Mutex<Vec<i32>>) {
    let vec = {
        // acquire the lock
        let mut guard = lock(mutex);

        // attempt to return a borrow of the data
        access(&mut guard)

        // guard is destroyed here, releasing the lock
    };

    // attempt to access the data outside of the lock.
    vec.push(3);
}

Rust will generate an error pinpointing the problem:

error: `guard` does not live long enough
access(&mut guard)
            ^~~~~

Disaster averted.

Thread safety and “Send”

It’s typical to distinguish some data types as “thread safe” and others not. Thread safe data structures use enough internal synchronization to be safely used by multiple threads concurrently.

For example, Rust ships with two kinds of “smart pointers” for reference counting:

  • Rc<T> provides reference counting via normal reads/writes. It is not thread safe.

  • Arc<T> provides reference counting via atomic operations. It is thread safe.

The hardware atomic operations used by Arc are more expensive than the vanilla operations used by Rc, so it’s advantageous to use Rc rather than Arc. On the other hand, it’s critical that an Rc<T> never migrate from one thread to another, because that could lead to race conditions that corrupt the count.

Usually, the only recourse is careful documentation; most languages make no semantic distinction between thread-safe and thread-unsafe types.

In Rust, the world is divided into two kinds of data types: those that are Send, meaning they can be safely moved from one thread to another, and those that are !Send, meaning that it may not be safe to do so. If all of a type’s components are Send, so is that type – which covers most types. Certain base types are not inherently thread-safe, though, so it’s also possible to explicitly mark a type like Arc as Send, saying to the compiler: “Trust me; I’ve verified the necessary synchronization here.”

Naturally, Arc is Send, and Rc is not.

We already saw that the Channel and Mutex APIs work only with Send data. Since they are the point at which data crosses thread boundaries, they are also the point of enforcement for Send.

Putting this all together, Rust programmers can reap the benefits of Rc and other thread-unsafe types with confidence, knowing that if they ever do accidentally try to send one to another thread, the Rust compiler will say:

`Rc<Vec<i32>>` cannot be sent between threads safely

Disaster averted.

Sharing the stack: “scoped”

Note: The API mentioned here is an old one which has been moved out of the standard library. You can find equivalent functionality in crossbeam (documentation for scope()) and scoped_threadpool (documentation for scoped())

So far, all the patterns we’ve seen involve creating data structures on the heap that get shared between threads. But what if we wanted to start some threads that make use of data living in our stack frame? That could be dangerous:

fn parent() {
    let mut vec = Vec::new();
    // fill the vector
    thread::spawn(|| {
        print_vec(&vec)
    })
}

The child thread takes a reference to vec, which in turn resides in the stack frame of parent. When parent exits, the stack frame is popped, but the child thread is none the wiser. Oops!

To rule out such memory unsafety, Rust’s basic thread spawning API looks a bit like this:

fn spawn<F>(f: F) where F: 'static, ...

The 'static constraint is a way of saying, roughly, that no borrowed data is permitted in the closure. It means that a function like parent above will generate an error:

error: `vec` does not live long enough

essentially catching the possibility of parent’s stack frame popping. Disaster averted.

But there is another way to guarantee safety: ensure that the parent stack frame stays put until the child thread is done. This is the pattern of fork-join programming, often used for divide-and-conquer parallel algorithms. Rust supports it by providing a “scoped” variant of thread spawning:

fn scoped<'a, F>(f: F) -> JoinGuard<'a> where F: 'a, ...

There are two key differences from the spawn API above:

  • The use a parameter 'a, rather than 'static. This parameter represents a scope that encompasses all the borrows within the closure, f.

  • The return value, a JoinGuard. As its name suggests, JoinGuard ensures that the parent thread joins (waits on) its child, by performing an implicit join in its destructor (if one hasn’t happened explicitly already).

Including 'a in JoinGuard ensures that the JoinGuard cannot escape the scope of any data borrowed by the closure. In other words, Rust guarantees that the parent thread waits for the child to finish before popping any stack frames the child might have access to.

Thus by adjusting our previous example, we can fix the bug and satisfy the compiler:

fn parent() {
    let mut vec = Vec::new();
    // fill the vector
    let guard = thread::scoped(|| {
        print_vec(&vec)
    });
    // guard destroyed here, implicitly joining
}

So in Rust, you can freely borrow stack data into child threads, confident that the compiler will check for sufficient synchronization.

Data races

At this point, we’ve seen enough to venture a strong statement about Rust’s approach to concurrency: the compiler prevents all data races.

A data race is any unsynchronized, concurrent access to data involving a write.

Synchronization here includes things as low-level as atomic instructions. Essentially, this is a way of saying that you cannot accidentally “share state” between threads; all (mutating) access to state has to be mediated by some form of synchronization.

Data races are just one (very important) kind of race condition, but by preventing them, Rust often helps you prevent other, more subtle races as well. For example, it’s often important that updates to different locations appear to take place atomically: other threads see either all of the updates, or none of them. In Rust, having &mut access to the relevant locations at the same time guarantees atomicity of updates to them, since no other thread could possibly have concurrent read access.

It’s worth pausing for a moment to think about this guarantee in the broader landscape of languages. Many languages provide memory safety through garbage collection. But garbage collection doesn’t give you any help in preventing data races.

Rust instead uses ownership and borrowing to provide its two key value propositions:

  • Memory safety without garbage collection.
  • Concurrency without data races.

The future

When Rust first began, it baked channels directly into the language, taking a very opinionated stance on concurrency.

In today’s Rust, concurrency is entirely a library affair; everything described in this post, including Send, is defined in the standard library, and could be defined in an external library instead.

And that’s very exciting, because it means that Rust’s concurrency story can endlessly evolve, growing to encompass new paradigms and catch new classes of bugs. Libraries like syncbox and simple_parallel are taking some of the first steps, and we expect to invest heavily in this space in the next few months. Stay tuned!