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

Paul E. McKenney
2023-01-25T17:18:40Z

On Wed, Jan 25, 2023 at 10:34:40AM -0500, Alan Stern wrote:
> On Wed, Jan 25, 2023 at 07:05:20AM -0800, Paul E. McKenney wrote:
> > On Wed, Jan 25, 2023 at 02:10:08PM +0100, Jonas Oberhauser wrote:
> > >
> > >
> > > On 1/25/2023 3:20 AM, Paul E. McKenney wrote:
> > > > On Tue, Jan 24, 2023 at 08:54:56PM -0500, Alan Stern wrote:
> > > > > On Tue, Jan 24, 2023 at 02:54:49PM -0800, Paul E. McKenney wrote:
> > > > > >
> > > > > > Within the Linux kernel, the rule for a given RCU "domain" is that if
> > > > > > an event follows a grace period in pretty much any sense of the word,
> > > > > > then that event sees the effects of all events in all read-side critical
> > > > > > sections that began prior to the start of that grace period.
> > > > > >
> > > > > > Here the senses of the word "follow" include combinations of rf, fr,
> > > > > > and co, combined with the various acyclic and irreflexive relations
> > > > > > defined in LKMM.
> > > > > The LKMM says pretty much the same thing. In fact, it says the event
> > > > > sees the effects of all events po-before the unlock of (not just inside)
> > > > > any read-side critical section that began prior to the start of the
> > > > > grace period.
> > > > >
> > > > > > > And are these anything the memory model needs to worry about?
> > > > > > Given that several people, yourself included, are starting to use LKMM
> > > > > > to analyze the Linux-kernel RCU implementations, maybe it does.
> > > > > >
> > > > > > Me, I am happy either way.
> > > > > Judging from your description, I don't think we have anything to worry
> > > > > about.
> > > > Sounds good, and let's proceed on that assumption then. We can always
> > > > revisit later if need be.
> > > >
> > > > Thanx, Paul
> > >
> > > FWIW, I currently don't see a need for either RCU nor "base" LKMM to have
> > > this kind of guarantee.
> >
> > In the RCU case, it is because it is far easier to provide this guarantee,
> > even though it is based on hardware and compilers rather than LKMM,
> > than it would be to explain to some random person why the access that
> > is intuitively clearly after the grace period can somehow come before it.
> >
> > > But I'm curious for why it doesn't exist in LKMM -- is it because of Alpha
> > > or some other issues that make it hard to guarantee (like a compiler merging
> > > two threads and optimizing or something?), or is it simply that it seemed
> > > like a complicated guarantee with no discernible upside, or something else?
> >
> > Because to the best of my knowledge, no one has ever come up with a
> > use for 2+2W and friends that isn't better handled by some much more
> > straightforward pattern of accesses. So we did not guarantee it in LKMM.
> >
> > Yes, you could argue that my "ease of explanation" paragraph above is
> > a valid use case, but I am not sure that this is all that compelling of
> > an argument. ;-)
>
> Are we all talking about the same thing? There were two different
> guarantees mentioned above:
>
> The RCU guarantee about writes in a read-side critical section
> becoming visible to all CPUs before a later grace period ends;
>
> The guarantee about the 2+2W pattern and friends being
> forbidden.
>
> The LKMM includes the first of these but not the second (for the reason
> Paul stated).

I am not sure whether or not we are talking about the same thing,
but given this litmus test:

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

C C-srcu-observed-4

(*
* Result: Sometimes
*
* The Linux-kernel implementation is suspected to forbid this.
*)

{}

P0(int *x, int *y, int *z, struct srcu_struct *s)
{
int r1;

r1 = srcu_read_lock(s);
WRITE_ONCE(*y, 2);
WRITE_ONCE(*x, 1);
srcu_read_unlock(s, r1);
}

P1(int *x, int *y, int *z, struct srcu_struct *s)
{
int r1;

WRITE_ONCE(*y, 1);
synchronize_srcu(s);
WRITE_ONCE(*z, 2);
}

P2(int *x, int *y, int *z, struct srcu_struct *s)
{
WRITE_ONCE(*z, 1);
smp_store_release(x, 2);
}

exists (x=1 /\ y=1 /\ z=1)

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

We get the following from herd7:

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

$ herd7 -conf linux-kernel.cfg C-srcu-observed-4.litmus
Test C-srcu-observed-4 Allowed
States 8
x=1; y=1; z=1;
x=1; y=1; z=2;
x=1; y=2; z=1;
x=1; y=2; z=2;
x=2; y=1; z=1;
x=2; y=1; z=2;
x=2; y=2; z=1;
x=2; y=2; z=2;
Ok
Witnesses
Positive: 1 Negative: 7
Condition exists (x=1 /\ y=1 /\ z=1)
Observation C-srcu-observed-4 Sometimes 1 7
Time C-srcu-observed-4 0.02
Hash=8b6020369b73ac19070864a9db00bbf8

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

This does not seem to me to be consistent with your "The RCU guarantee
about writes in a read-side critical section becoming visible to all
CPUs before a later grace period ends".

So what am I missing here?

Again, I am OK with LKMM allowing C-srcu-observed-4.litmus, as long as
the actual Linux-kernel implementation forbids it.

Thanx, Paul