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

[Matthew Wilcox]

On Wed, Mar 08, 2006 at 11:26:41AM -0800, Linus Torvalds wrote:
> > I presume there only needs to be an mmiowb() here if you've got the
> > appropriate CPU's I/O memory window set up to be weakly ordered.
> 
> Actually, since the different NUMA things may have different paths to the 
> PCI thing, I don't think even the mmiowb() will really help. It has 
> nothing to serialize _with_.
> 
> It only orders mmio from within _one_ CPU and "path" to the destination. 
> The IO might be posted somewhere on a PCI bridge, and and depending on the 
> posting rules, the mmiowb() just isn't relevant for IO coming through 
> another path.

Looking at the SGI implementation, it's smarter than you think.  Looks
like there's a register in the local I/O hub that lets you determine
when this write has been queued in the appropriate host->pci bridge.
So by the time __sn_mmiowb() returns, you're guaranteed no other CPU
can bypass the write because the write's got far enough.