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

[Will Deacon]

On Fri, Dec 04, 2015 at 08:07:06AM -0800, Paul E. McKenney wrote:
> 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.  ;-)

It's the ones that it's yet to find that you should be worried about,
and the debug code is all fixed ;)

Will