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

[Boqun Feng]

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 ;-)

Regards,
Boqun

> 								Thanx, Paul
> 
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