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

Linus Torvalds torvalds at osdl.org
Thu Mar 9 12:27:05 EST 2006

On Thu, 9 Mar 2006, Paul Mackerras wrote:
> ... and x86 mmiowb is a no-op.  It's not x86 that I think is buggy.

x86 mmiowb would have to be a real op too if there were any multi-pathed 
PCI buses out there for x86, methinks.

Basically, the issue boils down to one thing: no "normal" barrier will 
_ever_ show up on the bus on x86 (ie ia64, afaik). That, together with any 
situation where there are multiple paths to one physical device means that 
mmiowb() _has_ to be a special op, and no spinlocks etc will _ever_ do the 
serialization you look for.

Put another way: the only way to avoid mmiowb() being special is either 
one of:
 (a) have the bus fabric itself be synchronizing
 (b) pay a huge expense on the much more critical _regular_ barriers

Now, I claim that (b) is just broken. I'd rather take the hit when I need 
to, than every time. 

Now, (a) is trivial for small cases, but scales badly unless you do some 
fancy footwork. I suspect you could do some scalable multi-pathable 
version with using similar approaches to resolving device conflicts as the 
cache coherency protocol does (or by having a token-passing thing), but it 
seems SGI's solution was fairly well thought out. 

That said, when I heard of the NUMA IO issues on the SGI platform, I was 
initially pretty horrified. It seems to have worked out ok, and as long as 
we're talking about machines where you can concentrate on validating just 
a few drivers, it seems to be a good tradeoff.

Would I want the hard-to-think-about IO ordering on a regular desktop 
platform? No.


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