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

[Paul E. McKenney]

On Mon, Feb 19, 2018 at 06:44:13PM +0100, Peter Zijlstra wrote:
> On Mon, Feb 19, 2018 at 12:14:45PM -0500, Alan Stern wrote:
> > Note that operations like atomic_add_unless() already include memory 
> > barriers.
> 
> It is valid for atomic_add_unless() to not imply any barriers when the
> addition doesn't happen.

Agreed, given that atomic_add_unless() just returns 0 or 1, not the
pointer being added.  Of course, the __atomic_add_unless() function
that it calls is another story, as it does return the old value.  Sigh.
And __atomic_add_unless() is called directly from some code.  All of
which looks to be counters rather than pointers, thankfully.

So, do we want to rely on atomic_add_unless() always being
invoked on counters rather than pointers, or does it need an
smp_read_barrier_depends()?

							Thanx, Paul