In the world of concurrency in C++, `std::mutex` plays a critical role in synchronizing access to shared resources. Understanding the consistency model of `std::mutex` is crucial for developers aiming to write robust, concurrent programs. This article delves into whether `std::mutex` is sequentially consistent, providing technical explanations and examples to clarify the concept.

Understanding Sequential Consistency

Sequential consistency is a memory consistency model that ensures the results of execution are as if all operations were performed in some sequential order, and the operations of each individual process appear in this sequence in the order issued by the process. It provides programmers with a straightforward mental model akin to single-threaded execution but can lead to performance bottlenecks.

C++ Memory Model and `std::mutex`

In C++, the memory model provides various levels of operation consistency, with `std::mutex` offering a way to ensure mutual exclusion. The `std::mutex` is part of the standard library designed to protect shared data from being simultaneously accessed by multiple threads, enforcing some level of memory consistency.

Is `std::mutex` Sequentially Consistent?

While `std::mutex` does enforce an order of operations between threads accessing the critical section it protects, it doesn't inherently provide sequential consistency.

1. Mutual Exclusion: `std::mutex` ensures that only one thread can lock the mutex at a time. This guarantees exclusion, preventing race conditions by enforcing an order among threads entering the critical section.
2. Memory Visibility: Upon releasing a mutex (unlocking), all written values are visible to other threads that subsequently acquire a lock on the mutex. This provides a "happens-before" relationship but does not strictly meet the criteria of sequential consistency for operations outside the critical section.
3. Synchronization: While `std::mutex` guarantees order between lock and unlock operations across threads, operations within the critical section can be reordered, making it less restrictive than sequential consistency.

Memory Reordering

Processors may reorder instructions for optimization, which can affect how operations appear in a multi-threaded context, leading to potential inconsistencies. However, using `std::mutex`, the operations inside a critical section appear to execute in the order they were issued, as far as other threads are concerned.

Example

Consider the following example, which illustrates how `std::mutex` handles synchronization but does not provide sequential consistency for operations outside the critical section: