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SubjectBug in herd7 [Was: Re: Litmus test for question from Al Viro]
On Sun, Oct 04, 2020 at 12:16:31AM +0900, Akira Yokosawa wrote:
> Hi Alan,
>
> Just a minor nit in the litmus test.
>
> On Sat, 3 Oct 2020 09:22:12 -0400, Alan Stern wrote:
> > To expand on my statement about the LKMM's weakness regarding control
> > constructs, here is a litmus test to illustrate the issue. You might
> > want to add this to one of the archives.
> >
> > Alan
> >
> > C crypto-control-data
> > (*
> > * LB plus crypto-control-data plus data
> > *
> > * Expected result: allowed
> > *
> > * This is an example of OOTA and we would like it to be forbidden.
> > * The WRITE_ONCE in P0 is both data-dependent and (at the hardware level)
> > * control-dependent on the preceding READ_ONCE. But the dependencies are
> > * hidden by the form of the conditional control construct, hence the
> > * name "crypto-control-data". The memory model doesn't recognize them.
> > *)
> >
> > {}
> >
> > P0(int *x, int *y)
> > {
> > int r1;
> >
> > r1 = 1;
> > if (READ_ONCE(*x) == 0)
> > r1 = 0;
> > WRITE_ONCE(*y, r1);
> > }
> >
> > P1(int *x, int *y)
> > {
> > WRITE_ONCE(*x, READ_ONCE(*y));
>
> Looks like this one-liner doesn't provide data-dependency of y -> x on herd7.

You're right. This is definitely a bug in herd7.

Luc, were you aware of this?

> When I changed P1 to
>
> P1(int *x, int *y)
> {
> int r1;
>
> r1 = READ_ONCE(*y);
> WRITE_ONCE(*x, r1);
> }
>
> and replaced the WRITE_ONCE() in P0 with smp_store_release(),
> I got the result of:
>
> -----
> Test crypto-control-data Allowed
> States 1
> 0:r1=0;
> No
> Witnesses
> Positive: 0 Negative: 3
> Condition exists (0:r1=1)
> Observation crypto-control-data Never 0 3
> Time crypto-control-data 0.01
> Hash=9b9aebbaf945dad8183d2be0ccb88e11
> -----
>
> Restoring the WRITE_ONCE() in P0, I got the result of:
>
> -----
> Test crypto-control-data Allowed
> States 2
> 0:r1=0;
> 0:r1=1;
> Ok
> Witnesses
> Positive: 1 Negative: 4
> Condition exists (0:r1=1)
> Observation crypto-control-data Sometimes 1 4
> Time crypto-control-data 0.01
> Hash=843eaa4974cec0efae79ce3cb73a1278
> -----

What you should have done was put smp_store_release in P0 and left P1 in
its original form. That test should not be allowed, but herd7 says that
it is.

> As this is the same as the expected result, I suppose you have missed another
> limitation of herd7 + LKMM.

It would be more accurate to say that we all missed it. :-) (And it's
a bug in herd7, not a limitation of either herd7 or LKMM.) How did you
notice it?

> By the way, I think this weakness on control dependency + data dependency
> deserves an entry in tools/memory-model/Documentation/litmus-tests.txt.
>
> In the LIMITATIONS section, item #1 mentions some situation where
> LKMM may not recognize possible losses of control-dependencies by
> compiler optimizations.
>
> What this litmus test demonstrates is a different class of mismatch.

Yes, one in which LKMM does not recognize a genuine dependency because
it can't tell that some optimizations are not valid.

This flaw is fundamental to the way herd7 works. It examines only one
execution at a time, and it doesn't consider the code in a conditional
branch while it's examining an execution where that branch wasn't taken.
Therefore it has no way to know that the code in the unexecuted branch
would prevent a certain optimization. But the compiler does consider
all the code in all branches when deciding what optimizations to apply.

Here's another trivial example:

r1 = READ_ONCE(*x);
if (r1 == 0)
smp_mb();
WRITE_ONCE(*y, 1);

The compiler can't move the WRITE_ONCE before the READ_ONCE or the "if"
statement, because it's not allowed to move shared memory accesses past
a memory barrier -- even if that memory barrier isn't always executed.
Therefore the WRITE_ONCE actually is ordered after the READ_ONCE, but
the memory model doesn't realize it.

> Alan, can you come up with an update in this regard?

I'll write something.

Alan

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