Re: [PATCH tip/core/rcu 02/40] rcu: Make arch select smp_mb__after_unlock_lock() strength

[Paul E. McKenney]

On Thu, Apr 13, 2017 at 06:37:57PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 13, 2017 at 09:26:51AM -0700, Paul E. McKenney wrote:
> 
> > ARCH_WEAK_RELEASE_ACQUIRE actually works both ways.
> > 
> > To see this, imagine some strange alternate universe in which the Power
> > hardware guys actually did decide to switch PPC to doing RCsc as you
> > suggest.  There would still be a lot of Power hardware out there that
> > still does RCpc.  Therefore, powerpc builds that needed to run on old
> > Power hardware would select ARCH_WEAK_RELEASE_ACQUIRE, while kernels
> > built to run only on the shiny new (but mythical) alternate-universe
> > Power hardware would avoid selecting this Kconfig option.
> 
> Ah, but Power software guys could do it today by replacing an LWSYNC
> with a SYNC in say arch_spin_unlock().
> 
> And yes, I know this isn't a popular suggestion, but it would do the
> trick.

Indeed, there is a fine line between motivating people to move to new
hardware on the one hand and terminally annoying existing users on
the other.  ;-)

> Its just that since there's one (PPC) we can sort of pressure them with
> the pain of being the only ones to hit all the bugs. But the moment more
> appear (and I'm afraid it'll be MIPS, with the excuse that PPC already
> does this) it will be ever so much harder to get rid of it.
> 
> Then again, maybe I should just give up and accept the Linux kernel has
> RCpc locks..

As usual, I must defer to the powerpc maintainers on this one.

							Thanx, Paul