Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

Alan Stern
2023-01-16T18:25:07Z

On Sun, Jan 15, 2023 at 08:23:29PM -0800, Paul E. McKenney wrote:
> On Sun, Jan 15, 2023 at 03:46:10PM -0500, Alan Stern wrote:
> > On Sun, Jan 15, 2023 at 10:10:52AM -0800, Paul E. McKenney wrote:
> > > On Sun, Jan 15, 2023 at 11:23:31AM -0500, Alan Stern wrote:
> > > > On Sat, Jan 14, 2023 at 09:15:10PM -0800, Paul E. McKenney wrote:
> > > > > What am I missing here?
> > > >
> > > > I don't think you're missing anything. This is a matter for Boqun or
> > > > Luc; it must have something to do with the way herd treats the
> > > > srcu_read_lock() and srcu_read_unlock() primitives.
> > >
> > > It looks like we need something that tracks (data | rf)* between
> > > the return value of srcu_read_lock() and the second parameter of
> > > srcu_read_unlock(). The reason for rf rather than rfi is the upcoming
> > > srcu_down_read() and srcu_up_read().
> >
> > Or just make herd treat srcu_read_lock(s) as an annotated equivalent of
> > READ_ONCE(&s) and srcu_read_unlock(s, v) as an annotated equivalent of
> > WRITE_ONCE(s, v). But with some special accomodation to avoid
> > interaction with the new carry-dep relation.
>
> This is a modification to herd7 you are suggesting? Otherwise, I am
> suffering a failure of imagination on how to properly sort it from the
> other READ_ONCE() and WRITE_ONCE() instances.

srcu_read_lock and srcu_read_unlock events would be distinguished from
other marked loads and stores by belonging to the Srcu-lock and
Srcu-unlock sets. But I don't know whether this result can be
accomplished just by modifying the .def file -- it might require changes
to herd7. (In fact, as far as I know there is no documentation at all
for the double-underscore operations used in linux-kernel.def. Hint
hint!)

As mentioned earlier, we should ask Luc or Boqun.

> > > Or is there some better intermediate position that could be taken?
> >
> > Do you mean go back to the current linux-kernel.bell? The code you
> > wrote above is different, since it prohibits nesting.
>
> Not to the current linux-kernel.bell, but, as you say, making the change
> to obtain a better approximation by prohibiting nesting.

Why do you want to prohibit nesting? Why would that be a better
approximation?

Alan