Re: Memory Barrier Definitions

[Anton Blanchard <anton@samba.org>]

> You have
> 
> 	Compiler ordering
> 	CPU v CPU memory ordering
> 	CPU v I/O memory ordering
> 	I/O v I/O memory ordering

Yep. Maybe we could have:

CPU v CPU	smp_*mb or cpu_*mb 
CPU v I/O	*mb
I/O v I/O	io_*mb

Then again before Linus hits me on the head for hoarding vowels,

http://hypermail.spyroid.com/linux-kernel/archived/2001/week41/1270.html

I should suggest we make these a little less cryptic:

CPU v CPU	cpu_{read,write,memory}_barrier
CPU v I/O	{read,write,memory}_barrier
I/O v I/O	io_{read,write,memory}_barrier

> and our current heirarchy is a little bit more squashed than that. I'd 
> agree. We actually hit a corner case of this on the IDT winchip x86 where
> we run relaxed store ordering and have to define wmb() as a locked add of
> zero to the top of stack - which does have a penalty that isnt needed
> for CPU ordering.
> 
> How much of this impacts Mips64 ?

I remember some ia64 implementations have issues. Jesse, could you
fill us in again? I think you have problems with out of order
loads/stores to noncacheable space, right?

Anton