[PATCH] Document Linux's memory barriers [try #2]

[Linus Torvalds]

On Thu, 9 Mar 2006, Paul Mackerras wrote:
> 
> A spin_lock does show up on the bus, doesn't it?

Nope.

If the lock entity is in a exclusive cache-line, a spinlock does not show 
up on the bus at _all_. It's all purely in the core. In fact, I think AMD 
does a spinlock in ~15 CPU cycles (that's the serialization overhead in 
the core). I think a P-M core is ~25, while the NetBurst (P4) core is much 
more because they have horrible serialization issues (I think it's on the 
order of 100 cycles there).

Anyway, try doing a spinlock in 15 CPU cycles and going out on the bus for 
it..

(Couple that with spin_unlock basically being free).

Now, if the spinlocks end up _bouncing_ between CPU's, they'll obviously 
be a lot more expensive.

		Linus