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

Paul E. McKenney
2023-01-16T22:16:03Z

On Mon, Jan 16, 2023 at 02:20:57PM -0500, Alan Stern wrote:
> On Mon, Jan 16, 2023 at 11:06:52AM -0800, Paul E. McKenney wrote:
> > On Mon, Jan 16, 2023 at 01:11:41PM -0500, Alan Stern wrote:
>
> > > Why do you want to prohibit nesting? Why would that be a better
> > > approximation?
> >
> > Because the current LKMM gives wrong answers for nested critical
> > sections.
>
> I don't agree. Or at least, it depends on whose definition of "nested
> critical sections" you adopt.

Fair point, and I have therefore updated the test's header comment
to read as follows:

(*
* Result: Sometimes
*
* This demonstrates non-nested overlapping of SRCU read-side critical
* sections. Unlike RCU, SRCU critical sections do not unconditionally
* nest.
*)

> > For example, for the litmus test shown below, mainline
> > LKMM will incorrectly report "Never". The two SRCU read-side critical
> > sections are independent, so the fact that P1()'s synchronize_srcu() is
> > guaranteed to wait for the first on to complete says nothing about the
> > second having completed. Therefore, in Linux-kernel SRCU, the "exists"
> > clause could be satisfied.
> >
> > In contrast, the proposed change flags this as having nesting.
>
> In fact, this litmus test has overlapping critical sections, not nested
> ones. But the current LKML incorrectly _thinks_ they are nested,
> because it matches each lock with the first unmatched unlock.
>
> If you write a litmus test that has properly nested (not overlapping!)
> read-side critical sections, the current LKMM will match the locks and
> unlocks correctly and will give the right answer.
>
> So what you really want to do is rule out overlapping, not nesting. But
> I guess there's no way to do one without the other.

None that I could see!

Thanx, Paul

> Alan
>
> > Thaxn, Paul
> >
> > ------------------------------------------------------------------------
> >
> > C C-srcu-nest-5
> >
> > (*
> > * Result: Sometimes
> > *
> > * This demonstrates non-nesting of SRCU read-side critical sections.
> > * Unlike RCU, SRCU critical sections do not nest.
> > *)
> >
> > {}
> >
> > P0(int *x, int *y, struct srcu_struct *s1)
> > {
> > int r1;
> > int r2;
> > int r3;
> > int r4;
> >
> > r3 = srcu_read_lock(s1);
> > r2 = READ_ONCE(*y);
> > r4 = srcu_read_lock(s1);
> > srcu_read_unlock(s1, r3);
> > r1 = READ_ONCE(*x);
> > srcu_read_unlock(s1, r4);
> > }
> >
> > P1(int *x, int *y, struct srcu_struct *s1)
> > {
> > WRITE_ONCE(*y, 1);
> > synchronize_srcu(s1);
> > WRITE_ONCE(*x, 1);
> > }
> >
> > locations [0:r1]
> > exists (0:r1=1 /\ 0:r2=0)