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

[Paul E. McKenney]

On Mon, Dec 07, 2015 at 11:34:55AM +0100, Peter Zijlstra wrote:
> On Mon, Dec 07, 2015 at 08:45:04AM +0800, Boqun Feng wrote:
> > > > Or maybe, we introduce another address space of sparse like:
> > > > 
> > > > 	# define __private	__attribute__((noderef, address_space(6)))
> > > > 
> > > > and macro to dereference private
> > > > 
> > > > 	# define private_dereference(p) ((typeof(*p) *) p)
> > > > 
> > > > and define struct rcu_node like:
> > > > 
> > > > 	struct rcu_node {
> > > > 		raw_spinlock_t __private lock;	/* Root rcu_node's lock protects some */
> > > > 		...
> > > > 	};
> > > > 
> > > > and finally raw_spin_{un}lock_rcu_node() like:
> > > > 
> > > > 	static inline void raw_spin_lock_rcu_node(struct rcu_node *rnp)
> > > > 	{
> > > > 		raw_spin_lock(private_dereference(&rnp->lock));
> > > > 		smp_mb__after_unlock_lock();
> > > > 	}
> > > > 
> > > > 	static inline void raw_spin_unlock_rcu_node(struct rcu_node *rnp)
> > > > 	{
> > > > 		raw_spin_unlock(private_dereference(&rnp->lock));
> > > > 	}
> > > > 
> > > > This __private mechanism also works for others who wants to private
> > > > their fields of struct, which is not supported by C.
> > > > 
> > > > I will send two patches(one introduces __private and one uses it for
> > > > rcu_node->lock) if you think this is not a bad idea ;-)
> 
> > If rcu_node->lock is the only user then this is probably a bad idea, but
> > if others also want to have a way to privatize some fields of the
> > structure, this may be not that bad?
> 
> Thomas might also want this for things like
> irq_common_data::state_use_accessors for instance.
> 
> And I'm fairly sure there's more out there.

If Thomas takes it, I will consider also applying it to RCU.

							Thanx, Paul