On Tue, Jan 26, 2016 at 12:13:39PM -0800, Paul E. McKenney wrote:

On Tue, Jan 26, 2016 at 11:19:27AM +0100, Peter Zijlstra wrote:

quoted

So isn't smp_mb__after_unlock_lock() exactly such a scenario? And would
not someone trying to implement RCsc locks using locally transitive
RELEASE/ACQUIRE operations need exactly this stuff?

That is, I am afraid we need to cover the mix of local and global
transitive operations at least in overview.

True, but we haven't gotten to locking yet.

The mythical smp_mb__after_release_acquire() then ;-)

(and yes, I know you're going to say we don't have that)

That said, I would argue
that smp_mb__after_unlock_lock() upgrades locks to transitive, and
thus would not be an exception to the "no combining transitive and
non-transitive steps in cycles" rule.

But But But ;-) It does that exactly by combining. I suspect this is
(partly) the source of your SC chains with one PC link example.