folly/SmallLocks.h
This module is currently x64 only.
This header defines two very small mutex types. These are useful in highly memory-constrained environments where contention is unlikely. The purpose of these is to allow fine-grained locking in massive data structures where memory is at a premium. Often, each record may have a spare bit or byte lying around, so sometimes these can be tacked on with no additional memory cost.
There are two types exported from this header. MicroSpinLock is a single byte lock, and PicoSpinLock can be wrapped around an integer to use a single bit as a lock. Why do we have both? Because you can't use x64 bts on a single byte, sosizeof(MicroSpinLock) is smaller than sizeof(PicoSpinLock) can be, giving it some use cases.
Both the locks in this header model the C++11 Lockable concept. So you can use std::lock_guard or std::unique_lock to lock them in an RAII way if you want.
Additional information is in the header.