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

[Paul E. McKenney]

On Sun, Dec 06, 2015 at 03:37:23PM +0800, Boqun Feng wrote:
> Hi Paul,
> 
> On Fri, Dec 04, 2015 at 08:44:46AM -0800, Paul E. McKenney wrote:
> > On Fri, Dec 04, 2015 at 04:24:54PM +0000, Will Deacon wrote:
> > > 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 ;)
> > 
> > Fortunately, when Peter sent the patch fixing the debug-only
> > cases, he also created wrapper functions for the various types of
> > lock acquisition for rnp->lock.  Of course, the danger is that I
> > might type "raw_spin_lock_irqsave(&rnp->lock, flags)" instead of
> > "raw_spin_lock_irqsave_rcu_node(rnp, flags)" out of force of habit.
> > So I must occasionally scan the RCU source code for "spin_lock.*->lock",
> > which I just now did.  ;-)
> 
> Maybe you can rename ->lock of rnp to ->lock_acquired_on_your_own_risk
> to avoid the force of habit ;-)

Sold!  Though with a shorter alternate name...  And timing will be an
issue.  Probably needs to go into the first post-v4.5 set (due to the
high expected conflict rate), and probably needs to create wrappers for
the spin_unlock functions.

							Thanx, Paul