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

[Nick Piggin]

Linus Torvalds wrote:

>
>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. 
>

I'm not very driver-minded; would it make sense to have io versions of
locks, which can provide critical sections for IO operations?

The number of (uncommented) memory barriers sprinkled around drivers
looks pretty scary...

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