Re: [PATCH] tools/memory-model: remove rb-dep, smp_read_barrier_depends, and lockless_dereference

[Peter Zijlstra]

On Mon, Feb 19, 2018 at 11:41:23AM -0800, Paul E. McKenney wrote:
> On Mon, Feb 19, 2018 at 12:14:45PM -0500, Alan Stern wrote:
> > This leaves us with a question: Do we want to change the kernel by
> > adding memory barriers after unsuccessful RMW operations on Alpha, or
> > do we want to change the model by excluding such operations from
> > address dependencies?
> 
> I vote for adding the barrier on Alpha.  However, I don't know of any
> code in the Linux kernel that relies on read-to-read address dependency
> ordering headed by a failing RMW operation, so I don't feel all that
> strongly about this.

Right, but not knowing doesn't mean doesn't exist, and most certainly
doesn't mean will never exist.

> > Note that operations like atomic_add_unless() already include memory 
> > barriers.
> 
> And I don't see an atomic_add_unless_relaxed(), so we are good on this
> one.  So far, anyway!  ;-)

Not the point, add_unless() is a conditional operation, and therefore
doesn't need to imply anything when failing.