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

["Paul E. McKenney" <paulmck@kernel.org>]

On Fri, Jul 23, 2021 at 02:11:38PM -0400, Alan Stern wrote:
> On Fri, Jul 23, 2021 at 10:30:10AM -0700, Paul E. McKenney wrote:
> > On Fri, Jul 23, 2021 at 12:59:47PM -0400, Alan Stern wrote:
> > > On Fri, Jul 23, 2021 at 09:24:31AM -0700, Paul E. McKenney wrote:
> > > > On Thu, Jul 22, 2021 at 10:08:46PM -0400, Alan Stern wrote:
> > > 
> > > > > > +	void do_something_locked(struct foo *fp)
> > > > > > +	{
> > > > > > +		bool gf = true;
> > > > > > +
> > > > > > +		/* IMPORTANT: Heuristic plus spin_lock()! */
> > > > > > +		if (!data_race(global_flag)) {
> > > > > > +			spin_lock(&fp->f_lock);
> > > > > > +			if (!smp_load_acquire(&global_flag)) {
> > > 
> > > > > > +	void begin_global(void)
> > > > > > +	{
> > > > > > +		int i;
> > > > > > +
> > > > > > +		spin_lock(&global_lock);
> > > > > > +		WRITE_ONCE(global_flag, true);
> > > > > 
> > > > > Why does this need to be WRITE_ONCE?  It still races with the first read 
> > > > > of global_flag above.
> > > > 
> > > > But also with the smp_load_acquire() of global_flag, right?
> > > 
> > > What I'm curious about is why, given these two races, you notate one of 
> > > them by changing a normal write to WRITE_ONCE and you notate the other 
> > > by changing a normal read to a data_race() read.  Why not handle them 
> > > both the same way?
> > 
> > Because the code can tolerate the first read returning complete nonsense,
> > but needs the value from the second read to be exact at that point in
> > time.
> 
> In other words, if the second read races with the WRITE_ONCE, it needs to 
> 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.
> 
> >  (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. */

							Thanx, Paul