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

[Eric Biggers <ebiggers@kernel.org>]

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.
> 

How about:

	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 the only way to reach the initialized
	memory involves dereferencing the pointer itself, then the data
	dependency barrier provided by READ_ONCE() is sufficient.  However, 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).  READ_ONCE() is *not* sufficient in that case.

	The optimization of replacing smp_load_acquire() with READ_ONCE() is
	discouraged for nontrivial data structures, since it can be difficult to
	determine if it is correct.  In particular, for complex data structures
	the correctness of the READ_ONCE() optimization may depend on internal
	implementation details of other kernel subsystems.