Messages in this thread | | | Date | Thu, 09 Mar 2006 13:38:38 +1100 | From | Nick Piggin <> | Subject | Re: [PATCH] Document Linux's memory barriers [try #2] |
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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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