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

[Andrea Parri]

On Tue, Feb 20, 2018 at 06:48:13AM -0800, Paul E. McKenney wrote:
> 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()?

alpha's implementation of __atomic_add_unless() has an unconditional smp_mb()
before returning so, as far as dependencies are concerned, these seem fine.

  Andrea


> 
> 							Thanx, Paul
>