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

Paul E. McKenney
2023-01-14T18:15:48Z

On Sat, Jan 14, 2023 at 09:53:43AM -0800, Paul E. McKenney wrote:
> On Fri, Jan 13, 2023 at 08:43:42PM +0000, Jonas Oberhauser wrote:
> >
> >
> > -----Original Message-----
> > From: Paul E. McKenney [mailto:paulmck@kernel.org]
> >
> > > (* Compute matching pairs of Srcu-lock and Srcu-unlock *) let srcu-rscs = ([Srcu-lock] ; rfi ; [Srcu-unlock]) & loc
> >
> > How does the Srcu-unlock read from the Srcu-lock? Is there something in your model or in herd that lets it understand lock and unlock should be treated as writes resp. reads from that specific location?
> >
> > Or do you mean that value given to Srcu-unlock should be the value produced by Srcu-lock?
>
> Yes, and in the Linux kernel one does something like this:
>
> idx = srcu_read_lock(&mysrcu);
> // critical section
> srcu_read_unlock(&mysrcu, idx);
>
> > Perhaps the closest to what you want is to express that as a data dependency if you know how to teach herd that Srcu-unlock is a read and Srcu-lock depends on its second input :D (I have no idea how to do that, hence the questions above)
>
> Given that both you and Alan suggested it, I must try it. ;-)

And it works as desired on these litmus tests:

manual/kernel/C-srcu-nest-*.litmus

In this repository:

https://github.com/paulmckrcu/litmus

However, this has to be dumb luck because herd7 does not yet provide
the second argument to srcu_read_unlock(). My guess is that the herd7
is noting the dependency that is being carried by the pointers to the
srcu_struct structures. This guess stems in part from the fact that
I get "Flag unbalanced-srcu-locking" when I have one SRCU read-side
critical section following another in the same process, both using the
same srcu_struct structure.

Nevertheless, here is the resulting .bell fragment:

------------------------------------------------------------------------

(* Compute matching pairs of Srcu-lock and Srcu-unlock *)
let srcu-rscs = ([Srcu-lock] ; data ; [Srcu-unlock]) & loc

(* Validate nesting *)
flag ~empty Srcu-lock \ domain(srcu-rscs) as unbalanced-srcu-locking
flag ~empty Srcu-unlock \ range(srcu-rscs) as unbalanced-srcu-locking

(* Check for use of synchronize_srcu() inside an RCU critical section *)
flag ~empty rcu-rscs & (po ; [Sync-srcu] ; po) as invalid-sleep

(* Validate SRCU dynamic match *)
flag ~empty different-values(srcu-rscs) as srcu-bad-nesting

------------------------------------------------------------------------

I also created a C-srcu-nest-*.litmus as shown below, and LKMM does
complain about one srcu_read_lock() feeding into multiple instances of
srcu_read_unlock(). The complaint comes from the different_values()
check, which presumably complains about any duplication in the domain
or range of the specified relation.

But still working by accident! ;-)

Thanx, Paul

------------------------------------------------------------------------

C C-srcu-nest-3

(*
* Result: Flag srcu-bad-nesting
*
* This demonstrates erroneous matching of a single srcu_read_lock()
* with multiple srcu_read_unlock() instances.
*)

{}

P0(int *x, int *y, struct srcu_struct *s1, struct srcu_struct *s2)
{
int r1;
int r2;
int r3;
int r4;

r3 = srcu_read_lock(s1);
r2 = READ_ONCE(*y);
r4 = srcu_read_lock(s2);
r5 = srcu_read_lock(s2);
srcu_read_unlock(s1, r3);
r1 = READ_ONCE(*x);
srcu_read_unlock(s2, r4);
}

P1(int *x, int *y, struct srcu_struct *s2)
{
WRITE_ONCE(*y, 1);
synchronize_srcu(s2);
WRITE_ONCE(*x, 1);
}

locations [0:r1]
exists (0:r1=1 /\ 0:r2=0)