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SubjectLitmus test for question from Al Viro
Hello!

Al Viro posted the following query:

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

<viro> fun question regarding barriers, if you have time for that
<viro> V->A = V->B = 1;
<viro>
<viro> CPU1:
<viro> to_free = NULL
<viro> spin_lock(&LOCK)
<viro> if (!smp_load_acquire(&V->B))
<viro> to_free = V
<viro> V->A = 0
<viro> spin_unlock(&LOCK)
<viro> kfree(to_free)
<viro>
<viro> CPU2:
<viro> to_free = V;
<viro> if (READ_ONCE(V->A)) {
<viro> spin_lock(&LOCK)
<viro> if (V->A)
<viro> to_free = NULL
<viro> smp_store_release(&V->B, 0);
<viro> spin_unlock(&LOCK)
<viro> }
<viro> kfree(to_free);
<viro> 1) is it guaranteed that V will be freed exactly once and that
no accesses to *V will happen after freeing it?
<viro> 2) do we need smp_store_release() there? I.e. will anything
break if it's replaced with plain V->B = 0?

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

Of course herd7 supports neither structures nor arrays, but I was
crazy enough to try individual variables with made-up address and data
dependencies. This litmus test must also detect use-after-free bugs,
but a simple variable should be able to do that. So here is a
prototype:

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

C C-viro-2020.09.29a

{
int a = 1;
int b = 1;
int v = 1;
}


P0(int *a, int *b, int *v, spinlock_t *l)
{
int r0;
int r1;
int r2 = 2;
int r8;
int r9a = 2;
int r9b = 2;

r0 = 0;
spin_lock(l);
r9a = READ_ONCE(*v); // Use after free?
r8 = r9a - r9a; // Restore address dependency
r8 = b + r8;
r1 = smp_load_acquire(r8);
if (r1 == 0)
r0 = 1;
r9b = READ_ONCE(*v); // Use after free?
WRITE_ONCE(*a, r9b - r9b); // Use data dependency
spin_unlock(l);
if (r0) {
r2 = READ_ONCE(*v);
WRITE_ONCE(*v, 0); /* kfree(). */
}
}

P1(int *a, int *b, int *v, spinlock_t *l)
{
int r0;
int r1;
int r1a;
int r2 = 2;
int r8;
int r9a = 2;
int r9b = 2;
int r9c = 2;

r0 = READ_ONCE(*v);
r9a = r0; // Use after free?
r8 = r9a - r9a; // Restore address dependency
r8 = a + r8;
r1 = READ_ONCE(*r8);
if (r1) {
spin_lock(l);
r9b = READ_ONCE(*v); // Use after free?
r8 = r9b - r9b; // Restore address dependency
r8 = a + r8;
r1a = READ_ONCE(*r8);
if (r1a)
r0 = 0;
r9c = READ_ONCE(*v); // Use after free?
smp_store_release(b, r9c - rc9); // Use data dependency
spin_unlock(l);
}
if (r0) {
r2 = READ_ONCE(*v);
WRITE_ONCE(*v, 0); /* kfree(). */
}
}

locations [a;b;v;0:r1;0:r8;1:r1;1:r8]
exists (0:r0=1:r0 \/ (* Both or neither did kfree(). *)
v=1 \/ (* Neither did kfree, redundant check. *)
0:r2=0 \/ 1:r2=0 \/ (* Both did kfree, redundant check. *)
0:r9a=0 \/ 0:r9b=0 \/ 1:r9a=0 \/ (* CPU1 use after free. *)
1:r9b=0 \/ 1:r9c=0) (* CPU2 use after free. *)

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

This "exists" clause is satisfied:

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

$ herd7 -conf linux-kernel.cfg ~/paper/scalability/LWNLinuxMM/litmus/manual/kernel/C-viro-2020.09.29a.litmus
Test C-viro-2020.09.29a Allowed
States 5
0:r0=0; 0:r1=1; 0:r2=2; 0:r8=b; 0:r9a=0; 0:r9b=0; 1:r0=1; 1:r1=0; 1:r2=1; 1:r8=a; 1:r9a=1; 1:r9b=2; 1:r9c=2; a=0; b=1; v=0;
0:r0=0; 0:r1=1; 0:r2=2; 0:r8=b; 0:r9a=1; 0:r9b=0; 1:r0=1; 1:r1=0; 1:r2=1; 1:r8=a; 1:r9a=1; 1:r9b=2; 1:r9c=2; a=0; b=1; v=0;
0:r0=0; 0:r1=1; 0:r2=2; 0:r8=b; 0:r9a=1; 0:r9b=1; 1:r0=0; 1:r1=1; 1:r2=2; 1:r8=a; 1:r9a=1; 1:r9b=1; 1:r9c=1; a=0; b=1; v=1;
0:r0=0; 0:r1=1; 0:r2=2; 0:r8=b; 0:r9a=1; 0:r9b=1; 1:r0=1; 1:r1=0; 1:r2=1; 1:r8=a; 1:r9a=1; 1:r9b=2; 1:r9c=2; a=0; b=1; v=0;
0:r0=0; 0:r1=1; 0:r2=2; 0:r8=b; 0:r9a=1; 0:r9b=1; 1:r0=1; 1:r1=1; 1:r2=1; 1:r8=a; 1:r9a=1; 1:r9b=1; 1:r9c=1; a=0; b=1; v=0;
Ok
Witnesses
Positive: 3 Negative: 2
Condition exists (0:r0=1:r0 \/ v=1 \/ 0:r2=0 \/ 1:r2=0 \/ 0:r9a=0 \/ 0:r9b=0 \/ 1:r9a=0 \/ 1:r9b=0 \/ 1:r9c=0)
Observation C-viro-2020.09.29a Sometimes 3 2
Time C-viro-2020.09.29a 14.33
Hash=89f74abff4de682ee0bea8ee6dd53134

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

So did we end up with herd7 not respecting "fake" dependencies like
those shown above, or have I just messed up the translation from Al's
example to the litmus test? (Given one thing and another over the past
couple of days, my guess would be that I just messed up the translation,
especially given that I don't see a reference to fake dependencies in
the documentation, but I figured that I should ask.)

Thanx, Paul

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