[spacechild1]

> An atomic read-modify-write.

No, this also applies to (non-relaxed) atomic loads and stores, depending on the platform.

> Atomic non-seq-cst load/stores can be cheap.

Relaxed atomic loads and stores are always cheap, but anything above requires additional memory order instructions on many platforms, most notably on ARM.

Here we are talking specifically about mutexes, which follow acquire release semantics.

To be clear: locking an uncontented mutex is indeed much, much cheaper than an actual call into the kernel, but it is not free either.

[gpderetta]

Ok, technically we both used the weasel word 'can' so we are both right.

But even on ARM, these days store releases and load acquires, while not as free as on x86 are very cheap.

To make my statement more precise, typically what is still expensive pretty much everywhere is anything with #StoreLoad barrier semantics, which is what you need to acquire a mutex.