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

Alan Stern
2023-01-18T16:50:48Z

On Tue, Jan 17, 2023 at 07:50:41PM -0800, Paul E. McKenney wrote:
> On Tue, Jan 17, 2023 at 03:15:06PM -0500, Alan Stern wrote:
> > On Tue, Jan 17, 2023 at 09:43:08AM -0800, Paul E. McKenney wrote:
> > > On Tue, Jan 17, 2023 at 10:56:34AM -0500, Alan Stern wrote:
> > > > Isn't it true that the current code will flag srcu-bad-nesting if a
> > > > litmus test has non-nested overlapping SRCU read-side critical sections?
> > >
> > > Now that you mention it, it does indeed, flagging srcu-bad-nesting.
> > >
> > > Just to see if I understand, different-values yields true if the set
> > > contains multiple elements with the same value mapping to different
> > > values. Or, to put it another way, if the relation does not correspond
> > > to a function.
> >
> > As I understand it, given a relation r (i.e., a set of pairs of events),
> > different-values(r) returns the sub-relation consisting of those pairs
> > in r for which the value associated with the first event of the pair is
> > different from the value associated with the second event of the pair.
>
> OK, so different-values(r) is different than (r \ id) because the
> former operates on values and the latter on events?

No. Both of these things are relations, not values or events.

Suppose you had:

A: WRITE_ONCE(x, 1);
B: WRITE_ONCE(y, 1);
C: WRITE_ONCE(z, 2);

Then the po relation would consist of the pairs (A,B), (A,C), and (B,C).

The different-values(po) relation would include only (A,C) and (B,C).
It would not include (A,B) because the two events in that pair have the
same value: 1.

And finally, (po \ id) would be the same as po, because the id relation
consists of the pairs (A,A), (B,B), and (C,C) -- and none of those are
in po to begin with, so removing them from po doesn't do anything.

> > Right now the behavior is kind of strange. The following simple litmus
> > test:
> >
> > C test
> > {}
> > P0(int *x)
> > {
> > int r1;
> > r1 = srcu_read_lock(x);
> > srcu_read_unlock(x, r1);
> > }
> > exists (~0:r1=0)
> >
> > produces the following output from herd7:
> >
> > Test test Allowed
> > States 1
> > 0:r1=906;
> > Ok
> > Witnesses
> > Positive: 1 Negative: 0
> > Condition exists (not (0:r1=0))
> > Observation test Always 1 0
> > Time test 0.01
> > Hash=2f42c87ae9c1d267f4e80c66f646b9bb
> >
> > Don't ask me where that 906 value comes from or why it is't 0. Also,
> > herd7's graphical output shows there is no data dependency from the lock
> > to the unlock, but we need to have one.
>
> Is it still the case that any herd7 value greater than 127 is special?

I have no idea.

> > > Given an Srcu-down and an Srcu-up:
> > >
> > > let srcu-rscs = ( return_value(Srcu-lock) ; (dep | rfi)* ;
> > > parameter(Srcu-unlock, 2) ) |
> > > ( return_value(Srcu-down) ; (dep | rf)* ;
> > > parameter(Srcu-up, 2) )
> > >
> > > Seem reasonable, or am I missing yet something else?
> >
> > Not at all reasonable.
> >
> > For one thing, consider this question: Which statements lie inside a
> > read-side critical section?
>
> Here srcu_down_read() and srcu_up_read() are to srcu_read_lock() and
> srcu_read_unlock() as down_read() and up_read() are to mutex_lock()
> and mutex_unlock(). Not that this should be all that much comfort
> given that I have no idea how one would go about modeling down_read()
> and up_read() in LKMM.

It might make sense to work on that first, before trying to do
srcu_down_read() and srcu_up_read().

> > With srcu_read_lock() and a matching srcu_read_unlock(), the answer is
> > clear: All statements po-between the two. With srcu_down_read() and
> > srcu_up_read(), the answer is cloudy in the extreme.
>
> And I agree that it must be clearly specified, and my that previous try
> was completely lacking. Here is a second attempt:
>
> let srcu-rscs = (([Srcu-lock] ; data ; [Srcu-unlock]) & loc) |
> (([Srcu-down] ; (data | rf)* ; [Srcu-up]) & loc)
>
> (And I see your proposal and will try it.)
>
> > Also, bear in mind that the Fundamental Law of RCU is formulated in
> > terms of stores propagating to a critical section's CPU. What are we to
> > make of this when a single critical section can belong to more than one
> > CPU?
>
> One way of answering this question is by analogy with down() and up()
> when used as a cross-task mutex. Another is by mechanically applying
> some of current LKMM. Let's start with this second option.
>
> LKMM works mostly with critical sections, but we also discussed ordering
> based on the set of events po-after an srcu_read_lock() on the one hand
> and the set of events po-before an srcu_read_unlock() on the other.
> Starting here, the critical section is the intersection of these two sets.
>
> In the case of srcu_down_read() and srcu_up_read(), as you say, whatever
> might be a critical section must span processes. So what if instead of
> po, we used (say) xbstar? Then given a set of A such that ([Srcu-down ;
> xbstar ; A) and B such that (B ; xbstar ; [Srcu-up]), then the critical
> section is the intersection of A and B.
>
> One objection to this approach is that a bunch of unrelated events could
> end up being defined as part of the critical section. Except that this
> happens already anyway in real critical sections in the Linux kernel.
>
> So what about down() and up() when used as cross-task mutexes?
> These often do have conceptual critical sections that protect some
> combination of resource, but these critical sections might span tasks
> and/or workqueue handlers. And any reasonable definition of these
> critical sections would be just as likely to pull in unrelated accesses as
> the above intersection approach for srcu_down_read() and srcu_up_read().
>
> But I am just now making all this up, so thoughts?

Maybe we don't really need to talk about read-side critical sections at
all. Once again, here's what explanation.txt currently says:

For any critical section C and any grace period G, at least
one of the following statements must hold:

(1) C ends before G does, and in addition, every store that
propagates to C's CPU before the end of C must propagate to
every CPU before G ends.

(2) G starts before C does, and in addition, every store that
propagates to G's CPU before the start of G must propagate
to every CPU before C starts.

Suppose we change this to:

For any RCU lock operation L and matching unlock operation U,
and any matching grace period G, at least one of the following
statements must hold:

(1) U executes before G ends, and in addition, every store that
propagates to U's CPU before U executes must propagate to
every CPU before G ends.

(2) G starts before L executes, and in addition, every store that
propagates to G's CPU before the start of G must propagate
to every CPU before L executes.

(For SRCU, G matches L and U if it operates on the same srcu structure.)

This can be applied sensibly to regular RCU, regular SRCU, and the
up/down version of SRCU. Maybe it's what we want.

> > Indeed, given:
> >
> > P0(int *x) {
> > srcu_down_read(x);
> > }
> >
> > P1(int *x) {
> > srcu_up_read(x);
> > }
> >
> > what are we to make of executions in which P1 executes before P0?
>
> Indeed, there had better be something else forbidding such executions, or
> this is an invalid use of srcu_down_read() and srcu_up_read(). This might
> become more clear if the example is expanded to include the index returned
> from srcu_down_read() that is to be passed to srcu_up_read():
>
> P0(int *x, int *i) {
> WRITE_ONCE(i, srcu_down_read(x));
> }
>
> P1(int *x, int *i) {
> srcu_up_read(x, READ_ONCE(i));
> }

Hmmm. What happens if you write:

r1 = srcu_down_read(x);
r2 = srcu_down_read(x);
srcu_up_read(x, r1);
srcu_up_read(x, r2);

? I can't even tell what that would be _intended_ to do.

In fact, it seems likely that to make this work, you have to store at
least two values in *x: the value of the up/down counter, and the value
returned by srcu_down_read or stored by srcu_up_read. That means you
can't describe what's happening without using a structure, and herd7
doesn't support structures.

> Which it looks like you in fact to have in your patch, so time for me
> to go try that out.
>
> Thanx, Paul

Alan