Atomics

Rust pretty blatantly just inherits the memory model for atomics from C++20. This is not due to this model being particularly excellent or easy to understand. Indeed, this model is quite complex and known to have several flaws. Rather, it is a pragmatic concession to the fact that everyone is pretty bad at modeling atomics. At very least, we can benefit from existing tooling and research around the C/C++ memory model. (You'll often see this model referred to as "C/C++11" or just "C11". C just copies the C++ memory model; and C++11 was the first version of the model but it has received some bugfixes since then.)

Trying to fully explain the model in this book is fairly hopeless. It's defined in terms of madness-inducing causality graphs that require a full book to properly understand in a practical way. If you want all the nitty-gritty details, you should check out the C++20 draft specification (Section 31). Still, we'll try to cover the basics and some of the problems Rust developers face.

The C++ memory model is fundamentally about trying to bridge the gap between the semantics we want, the optimizations compilers want, and the inconsistent chaos our hardware wants. We would like to just write programs and have them do exactly what we said but, you know, fast. Wouldn't that be great?

Compiler Reordering

Compilers fundamentally want to be able to do all sorts of complicated transformations to reduce data dependencies and eliminate dead code. In particular, they may radically change the actual order of events, or make events never occur! If we write something like

x = 1;
y = 3;
x = 2;

The compiler may conclude that it would be best if your program did

x = 2;
y = 3;

This has inverted the order of events and completely eliminated one event. From a single-threaded perspective this is completely unobservable: after all the statements have executed we are in exactly the same state. But if our program is multi-threaded, we may have been relying on x to actually be assigned to 1 before y was assigned. We would like the compiler to be able to make these kinds of optimizations, because they can seriously improve performance. On the other hand, we'd also like to be able to depend on our program doing the thing we said.

Hardware Reordering

On the other hand, even if the compiler totally understood what we wanted and respected our wishes, our hardware might instead get us in trouble. Trouble comes from CPUs in the form of memory hierarchies. There is indeed a global shared memory space somewhere in your hardware, but from the perspective of each CPU core it is so very far away and so very slow. Each CPU would rather work with its local cache of the data and only go through all the anguish of talking to shared memory only when it doesn't actually have that memory in cache.

After all, that's the whole point of the cache, right? If every read from the cache had to run back to shared memory to double check that it hadn't changed, what would the point be? The end result is that the hardware doesn't guarantee that events that occur in the same order on one thread, occur in the same order on another thread. To guarantee this, we must issue special instructions to the CPU telling it to be a bit less smart.

For instance, say we convince the compiler to emit this logic:

initial state: x = 0, y = 1

THREAD 1        THREAD2
y = 3;          if x == 1 {
x = 1;              y *= 2;
                }

Ideally this program has 2 possible final states:

  • y = 3: (thread 2 did the check before thread 1 completed)
  • y = 6: (thread 2 did the check after thread 1 completed)

However there's a third potential state that the hardware enables:

  • y = 2: (thread 2 saw x = 1, but not y = 3, and then overwrote y = 3)

It's worth noting that different kinds of CPU provide different guarantees. It is common to separate hardware into two categories: strongly-ordered and weakly- ordered. Most notably x86/64 provides strong ordering guarantees, while ARM provides weak ordering guarantees. This has two consequences for concurrent programming:

  • Asking for stronger guarantees on strongly-ordered hardware may be cheap or even free because they already provide strong guarantees unconditionally. Weaker guarantees may only yield performance wins on weakly-ordered hardware.

  • Asking for guarantees that are too weak on strongly-ordered hardware is more likely to happen to work, even though your program is strictly incorrect. If possible, concurrent algorithms should be tested on weakly-ordered hardware.

Data Accesses

The C++ memory model attempts to bridge the gap by allowing us to talk about the causality of our program. Generally, this is by establishing a happens before relationship between parts of the program and the threads that are running them. This gives the hardware and compiler room to optimize the program more aggressively where a strict happens-before relationship isn't established, but forces them to be more careful where one is established. The way we communicate these relationships are through data accesses and atomic accesses.

Data accesses are the bread-and-butter of the programming world. They are fundamentally unsynchronized and compilers are free to aggressively optimize them. In particular, data accesses are free to be reordered by the compiler on the assumption that the program is single-threaded. The hardware is also free to propagate the changes made in data accesses to other threads as lazily and inconsistently as it wants. Most critically, data accesses are how data races happen. Data accesses are very friendly to the hardware and compiler, but as we've seen they offer awful semantics to try to write synchronized code with. Actually, that's too weak.

It is literally impossible to write correct synchronized code using only data accesses.

Atomic accesses are how we tell the hardware and compiler that our program is multi-threaded. Each atomic access can be marked with an ordering that specifies what kind of relationship it establishes with other accesses. In practice, this boils down to telling the compiler and hardware certain things they can't do. For the compiler, this largely revolves around re-ordering of instructions. For the hardware, this largely revolves around how writes are propagated to other threads. The set of orderings Rust exposes are:

  • Sequentially Consistent (SeqCst)
  • Release
  • Acquire
  • Relaxed

(Note: We explicitly do not expose the C++ consume ordering)

TODO: negative reasoning vs positive reasoning? TODO: "can't forget to synchronize"

Sequentially Consistent

Sequentially Consistent is the most powerful of all, implying the restrictions of all other orderings. Intuitively, a sequentially consistent operation cannot be reordered: all accesses on one thread that happen before and after a SeqCst access stay before and after it. A data-race-free program that uses only sequentially consistent atomics and data accesses has the very nice property that there is a single global execution of the program's instructions that all threads agree on. This execution is also particularly nice to reason about: it's just an interleaving of each thread's individual executions. This does not hold if you start using the weaker atomic orderings.

The relative developer-friendliness of sequential consistency doesn't come for free. Even on strongly-ordered platforms sequential consistency involves emitting memory fences.

In practice, sequential consistency is rarely necessary for program correctness. However sequential consistency is definitely the right choice if you're not confident about the other memory orders. Having your program run a bit slower than it needs to is certainly better than it running incorrectly! It's also mechanically trivial to downgrade atomic operations to have a weaker consistency later on. Just change SeqCst to Relaxed and you're done! Of course, proving that this transformation is correct is a whole other matter.

Acquire-Release

Acquire and Release are largely intended to be paired. Their names hint at their use case: they're perfectly suited for acquiring and releasing locks, and ensuring that critical sections don't overlap.

Intuitively, an acquire access ensures that every access after it stays after it. However operations that occur before an acquire are free to be reordered to occur after it. Similarly, a release access ensures that every access before it stays before it. However operations that occur after a release are free to be reordered to occur before it.

When thread A releases a location in memory and then thread B subsequently acquires the same location in memory, causality is established. Every write that happened before A's release will be observed by B after its acquisition. However no causality is established with any other threads. Similarly, no causality is established if A and B access different locations in memory.

Basic use of release-acquire is therefore simple: you acquire a location of memory to begin the critical section, and then release that location to end it. For instance, a simple spinlock might look like:

use std::sync::Arc;
use std::sync::atomic::{AtomicBool, Ordering};
use std::thread;

fn main() {
    let lock = Arc::new(AtomicBool::new(false)); // value answers "am I locked?"

    // ... distribute lock to threads somehow ...

    // Try to acquire the lock by setting it to true
    while lock.compare_and_swap(false, true, Ordering::Acquire) { }
    // broke out of the loop, so we successfully acquired the lock!

    // ... scary data accesses ...

    // ok we're done, release the lock
    lock.store(false, Ordering::Release);
}

On strongly-ordered platforms most accesses have release or acquire semantics, making release and acquire often totally free. This is not the case on weakly-ordered platforms.

Relaxed

Relaxed accesses are the absolute weakest. They can be freely re-ordered and provide no happens-before relationship. Still, relaxed operations are still atomic. That is, they don't count as data accesses and any read-modify-write operations done to them occur atomically. Relaxed operations are appropriate for things that you definitely want to happen, but don't particularly otherwise care about. For instance, incrementing a counter can be safely done by multiple threads using a relaxed fetch_add if you're not using the counter to synchronize any other accesses.

There's rarely a benefit in making an operation relaxed on strongly-ordered platforms, since they usually provide release-acquire semantics anyway. However relaxed operations can be cheaper on weakly-ordered platforms.