[PATCH] arm64: spinlock: serialise spin_unlock_wait against concurrent lockers

[Michael Ellerman]

On Fri, 2015-12-04 at 08:13 -0800, Paul E. McKenney wrote:
> On Fri, Dec 04, 2015 at 10:36:26AM +0100, Peter Zijlstra wrote:
> > On Thu, Dec 03, 2015 at 09:22:07AM -0800, Paul E. McKenney wrote:
> > > On the added MBs,
> > > the only alternative I have been able to come up with has even more MBs,
> > > as in on every lock acquisition.  If I am missing something, please do
> > > not keep it a secret!
> > 
> > You're right. And I suppose mpe is still on the fence wrt switching PPC
> > over to RCsc lock order.. which would be all those extra MBs you talk
> > about.
> 
> Yes, I would like to avoid forcing that choice on him.

Yeah I am on the fence :)

Obviously we don't want to be the only chumps finding (or not finding!) the
really subtle locking bugs.

But at the same time I have Anton in my other ear saying "performance
performance!".

cheers