c++ - Does std::mutex create a fence?

Question

If I lock a std::mutex will I always get a memory fence? I am unsure if it implies or enforces you to get the fence.

Update:

Found this reference following up on RMF's comments.

Multithreaded programming and memory visibility

Answer 1

As I understand this is covered in:

1.10 Multi-threaded executions and data races

Para 5:

The library de?nes a number of atomic operations (Clause 29) and operations on mutexes (Clause 30) that are specially identi?ed as synchronization operations. These operations play a special role in making assignments in one thread visible to another. A synchronization operation on one or more memory locations is either a consume operation, an acquire operation, a release operation, or both an acquire and release operation. A synchronization operation without an associated memory location is a fence and can be either an acquire fence, a release fence, or both an acquire and release fence. In addition, there are relaxed atomic operations, which are not synchronization operations, and atomic read-modify-write operations, which have special characteristics. [Note: For example, a call that acquires a mutex will perform an acquire operation on the locations comprising the mutex. Correspondingly, a call that releases the same mutex will perform a release operation on those same locations. Informally, performing a release operation on A forces prior side e?ects on other memory locations to become visible to other threads that later perform a consume or an acquire operation on A. “Relaxed” atomic operations are not synchronization operations even though, like synchronization operations, they cannot contribute to data races. —end note]