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

[Paul E. McKenney]

On Fri, Dec 04, 2015 at 10:21:10AM +0100, Peter Zijlstra wrote:
> On Thu, Dec 03, 2015 at 09:22:07AM -0800, Paul E. McKenney wrote:
> > >   2. Only PowerPC is going to see the (very occassional) failures, so
> > >      testing this is nigh on impossible :(
> > 
> > Indeed, we clearly cannot rely on normal testing, witness rcutorture
> > failing to find the missing smp_mb__after_unlock_lock() instances that
> > Peter found by inspection.  So I believe that augmented testing is
> > required, perhaps as suggested above.
> 
> To be fair, those were in debug code and non critical for correctness
> per se. That is, at worst the debug print would've observed an incorrect
> value.

True enough, but there is still risk from people repurposing debug code
for non-debug uses.  Still, thank you, I don't feel -quite- so bad about
rcutorture's failure to find these.  ;-)

							Thanx, Paul