Re: [PATCH] tools/memory-model: document the "one-time init" pattern

[Eric Biggers]

On Fri, Jul 17, 2020 at 01:53:40PM -0700, Darrick J. Wong wrote:
> > +There are also cases in which the smp_load_acquire() can be replaced by
> > +the more lightweight READ_ONCE().  (smp_store_release() is still
> > +required.)  Specifically, if all initialized memory is transitively
> > +reachable from the pointer itself, then there is no control dependency
> 
> I don't quite understand what "transitively reachable from the pointer
> itself" means?  Does that describe the situation where all the objects
> reachable through the object that the global struct foo pointer points
> at are /only/ reachable via that global pointer?
> 

The intent is that "transitively reachable" means that all initialized memory
can be reached by dereferencing the pointer in some way, e.g. p->a->b[5]->c.

It could also be the case that allocating the object initializes some global or
static data, which isn't reachable in that way.  Access to that data would then
be a control dependency, which a data dependency barrier wouldn't work for.

It's possible I misunderstood something.  (Note the next paragraph does say that
using READ_ONCE() is discouraged, exactly for this reason -- it can be hard to
tell whether it's correct.)  Suggestions of what to write here are appreciated.

- Eric