Скачать или смотреть Understanding the Behavior of AtomicPtr::compare_exchange in Rust

Dive into the details of `AtomicPtr::compare_exchange` in Rust, exploring its defined behavior and clarifying common misconceptions, particularly for pointer to struct usage.
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Understanding the Behavior of AtomicPtr::compare_exchange in Rust

Rust has gained significant popularity in the realms of systems programming and concurrency due to its focus on safety and performance. One of the key features of Rust’s concurrency model is atomic operations that support safe interactions between threads. However, confusion can arise when dealing with specific functions like AtomicPtr::compare_exchange — especially in contexts like comparing pointers to structs. In this guide, we will explore the behavior of compare_exchange, how it works, and tackle some common questions regarding its use.

The Problem

When using AtomicPtr::compare_exchange, especially with pointers to structures, it can lead to uncertainties around whether the operation produces defined behavior. In the following Rust code snippet, let's dissect a scenario that raises such questions:

[[See Video to Reveal this Text or Code Snippet]]

Main Questions:

Does this usage of compare_exchange produce defined behavior?

If so, what does the defined behavior entail?

Will Rust use double width CAS (Compare-And-Swap) for the operation on supported architectures like x86_64?

A Breakdown of the Solution

1. Does compare_exchange Produce Defined Behavior?

Barring potential bugs in the Rust compiler or standard library, using compare_exchange in the context presented should not result in undefined behavior. It's important to understand the distinction in Rust:

Safe Rust ensures that certain operations cannot cause undefined behavior as seen in languages like C and C+ + . The handling of pointers through atomic operations does encompass safety.

The only time we step into realms requiring unsafe code is when dereferencing pointers or converting pointers to references — here the programmer must confirm conditions that the compiler cannot.

2. What is the Defined Behavior?

The defined behavior aligns with Rust's documentation:

The compare_exchange function compares the address pointed to by some_ptr with that of ptr.

If they match, some_ptr is atomically updated to point to the address provided by other_ptr.

Regardless of the outcome, the previous pointer address is returned.

In the presented code, since some_ptr was initially set to ptr, the operation will successfully update some_ptr to point to other_ptr while returning ptr.

3. Does Rust Use Double Width CAS on Architectures like x86_64?

Notably, AtomicPtr::compare_exchange() only handles the pointer storage which is represented as usize (typically 64 bits on modern computers). There is no need for a double-width CAS operation. Here’s why:

The confusion often stems from terminology within the documentation that uses terms like value of a pointer. It’s essential to clarify that this does not refer to the actual data at that memory location, but instead the representation of the pointer itself.

In simpler terms, the value being compared is merely the address located in the pointer.

Conclusion

Interacting with pointers in Rust can seem complex, particularly when dealing with atomic operations. However, using functions like AtomicPtr::compare_exchange provides defined behaviors as per the specifications, ensuring safe concurrency in your applications. If you're working with pointers to structs, remember that the atomic nature of operations guarantees safety, without the fear of undefined behavior creeping in, provided you remain within the confines of safe Rust.

So next time you utilize compare_exchange, rest assured you are leveraging a powerful primitive correctly!