[PATCH] Document Linux's memory barriers

[David Howells]

Alan Cox <alan at lxorguk.ukuu.org.uk> wrote:

> > The LOCK and UNLOCK functions presumably make at least one memory write apiece
> > to manipulate the target lock (on SMP at least).
> 
> No they merely perform the bus transactions neccessary to perform an
> update atomically. They are however "serializing" instructions which
> means they do cause a certain amount of serialization (see the intel
> architecture manual on serializing instructions for detail).
> 
> Athlon and later know how to turn it from locked memory accesses into
> merely an exclusive cache line grab.

So, you're saying that the LOCK and UNLOCK primitives don't actually modify
memory, but rather simply pin the cacheline into the CPU's cache and refuse to
let anyone else touch it?

No... it can't work like that. It *must* make a memory modification - after
all, the CPU doesn't know that what it's doing is a spin_unlock(), say, rather
than an atomic_set().

David