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:

quoted

On Fri, Jul 17, 2020 at 01:53:40PM -0700, Darrick J. Wong wrote:

quoted

quoted

+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.