Subject: Re: wmb vs mmiowb

From: Jesse Barnes <[email-masked]>
Date: Thu, 23 Aug 2007 09:56:16 -0700

On Thursday, August 23, 2007 12:27 am Benjamin Herrenschmidt wrote:
> > Of course, the normal memory barrier would usually be a
> > "spin_unlock()" or something like that, not a "wmb()". In fact, I
> > don't think the powerpc implementation (as an example of this) will
> > actually synchronize with anything *but* a spin_unlock().
>
> We are even more sneaky in the sense that we set a per-cpu flag on
> any MMIO write and do the sync automatically in spin_unlock() :-)

Yeah, that's a reasonable thing to do, and in fact I think there's code 
to do something similar when a task is switched out (this keeps user 
level drivers from having do mmiowb() type things).

FWIW, I think I had an earlier version of the patch that used the name 
pioflush() or something similar, the only confusing thing about that 
name is that the primitive doesn't actually force I/Os down to the 
device level, just to the closest bridge.

It'll be interesting to see if upcoming x86 designs share this problem 
(e.g. large HT or CSI topologies).

Jesse