Re: [PATCH tip/locking/core v4 1/6] powerpc: atomic: Make *xchg and *cmpxchg a full barrier
From: Boqun Feng
Date: Sun Oct 25 2015 - 22:07:43 EST
On Wed, Oct 21, 2015 at 12:36:38PM -0700, Paul E. McKenney wrote:
> On Wed, Oct 21, 2015 at 10:18:33AM +0200, Peter Zijlstra wrote:
> > On Tue, Oct 20, 2015 at 02:28:35PM -0700, Paul E. McKenney wrote:
> > > I am not seeing a sync there, but I really have to defer to the
> > > maintainers on this one. I could easily have missed one.
> > So x86 implies a full barrier for everything that changes the CPL; and
> > some form of implied ordering seems a must if you change the privilege
> > level unless you tag every single load/store with the priv level at that
> > time, which seems the more expensive option.
> And it is entirely possible that there is some similar operation
> somewhere in the powerpc entry/exit code. I would not trust myself
> to recognize it, though.
> > So I suspect the typical implementation will flush all load/stores,
> > change the effective priv level and continue.
> > This can of course be implemented at a pure per CPU ordering (RCpc),
> > which would be in line with the rest of Power, in which case you do
> > indeed need an explicit sync to make it visible to other CPUs.
> > But yes, if Michael or Ben could clarify this it would be good.
Michael and Ben, ping for this, thank you ;-)
> > Back then I talked to Ralf about what MIPS says on this, and MIPS arch
> > spec is entirely quiet on this, it allows implementations full freedom
> > IIRC.
> :-) ;-) ;-)
> > </ramble>
> Thanx, Paul
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