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SubjectRe: [PATCH] tools/memory-model: document the "one-time init" pattern
On Fri, Jul 17, 2020 at 09:25:55PM -0400, Alan Stern wrote:
> On Fri, Jul 17, 2020 at 05:58:57PM -0700, Eric Biggers wrote:
> > 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.
>
> Perhaps something like this:
>
> Specifically, if the only way to reach the initialized memory
> involves dereferencing the pointer itself then READ_ONCE() is
> sufficient. This is because there will be an address dependency
> between reading the pointer and accessing the memory, which will
> ensure proper ordering. But if some of the initialized memory
> is reachable some other way (for example, if it is global or
> static data) then there need not be an address dependency,
> merely a control dependency (checking whether the pointer is
> non-NULL). Control dependencies do not always ensure ordering
> -- certainly not for reads, and depending on the compiler,
> possibly not for some writes -- and therefore a load-acquire is
> necessary.
>
> Perhaps this is more wordy than you want, but it does get the important
> ideas across.

I don't think we should worry about wordsmithing this. We should just
say "Use the init_pointer_once API" and then people who want to worry
about optimising the implementation of that API never have to talk to
the people who want to use that API.

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