Re: [PATCH memory-model 2/4] tools/memory-model: Add example for heuristic lockless reads

On Fri, Jul 23, 2021 at 01:28:11PM -0700, Paul E. McKenney wrote:

On Fri, Jul 23, 2021 at 02:11:38PM -0400, Alan Stern wrote:

quoted

In other words, if the second read races with the WRITE_ONCE, it needs 

to

quoted

get either the value before the write or the value after the write;
nothing else will do because it isn't a heuristic here.  Fair point.

quoted

 (If the value changes immediately after being read, the fact that
->f_lock is held prevents begin_global() from completing.)

This seems like something worth explaining in the document.  That
"IMPORTANT" comment doesn't really get the full point across.

How about this comment instead?

      /* This works even if data_race() returns nonsense. */

That's somewhat better.


On Fri, Jul 23, 2021 at 01:32:48PM -0700, Paul E. McKenney wrote:

On Fri, Jul 23, 2021 at 01:08:20PM -0400, Alan Stern wrote:

quoted

This doesn't mention the reason for the acquire-release
synchronization of global_flag.  It's needed because work done between
begin_global() and end_global() can affect a foo structure without
holding its private f_lock member, and we want all such work to be
visible to other threads when they call do_something_locked() later.

Like this added paragraph at the end?

	The smp_load_acquire() and smp_store_release() are required
	because changes to a foo structure between calls to begin_global()
	and end_global() are carried out without holding that structure's
	->f_lock.  The smp_load_acquire() and smp_store_release()
	ensure that the next invocation of do_something() from the call
	to do_something_locked() that acquires that ->f_lock will see
	those changes.

I'd shorten the last sentence:

	The smp_load_acquire() and smp_store_release() ensure that the
	next invocation of do_something() from do_something_locked()
	will see those changes.

Alan