Re: [Xen-devel] [v3,11/41] mips: reuse asm-generic/barrier.h

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Mon, Jan 25, 2016 at 10:03:22PM -0800, Paul E. McKenney wrote:
> On Mon, Jan 25, 2016 at 04:42:43PM +0000, Will Deacon wrote:
> > On Fri, Jan 15, 2016 at 01:58:53PM -0800, Paul E. McKenney wrote:
> > > On Fri, Jan 15, 2016 at 10:27:14PM +0100, Peter Zijlstra wrote:

> > > > Yes, that seems a good start. But yesterday you raised the 'fun' point
> > > > of two globally ordered sequences connected by a single local link.
> > > 
> > > The conclusion that I am slowly coming to is that litmus tests should
> > > not be thought of as linear chains, but rather as cycles.  If you think
> > > of it as a cycle, then it doesn't matter where the local link is, just
> > > how many of them and how they are connected.
> > 
> > Do you have some examples of this? I'm struggling to make it work in my
> > mind, or are you talking specifically in the context of the kernel
> > memory model?
> 
> Now that you mention it, maybe it would be best to keep the transitive
> and non-transitive separate for the time being anyway.  Just because it
> might be possible to deal with does not necessarily mean that we should
> be encouraging it.  ;-)

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.

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