[PATCH 6/6] powerpc: Use lwsync for acquire barrier if CPU supports it

[Benjamin Herrenschmidt]

> Yes I suppose the branch can be executed "non speculatively" before the 
> lwsync is completed. Wheras the larx/stcwx will have to complete before
> the branch outcome can be known. I suppose probably not worthwhile
> avoiding the full IO sync by adding yet more crap to make this work.
> 
> Thanks for putting my mind to rest though :)
> 
> I'd still be interested to know how expensive the full sync is when you
> have a lot of IOs in flight.
> 
> Question, are you going to do the hint and isync->lwsync thing for
> userspace as well? Too bad the kernel doesn't export synchronisation
> primitives to userspace...

We'd love to.... the vdso inherits from all the dynamic patching
features of the kernel and more...

It's really a matter of getting glibc to grok it, and doing so
efficiently (ie, not just having a function point in glibc, might be a
good use of the IFUNC stuff).

Cheers,
Ben.