Re: numlist_push() barriers Re: [RFC PATCH v4 1/9] printk-rb: add a new printk ringbuffer implementation

Andrea Parri
2019-08-26T16:01:24Z

> > C S+ponarelease+addroncena
> >
> > {
> > int *y = &a;
> > }
> >
> > P0(int *x, int **y, int *a)
> > {
> > int *r0;
> >
> > *x = 2;
> > r0 = cmpxchg_release(y, a, x);
> > }
> >
> > P1(int *x, int **y)
> > {
> > int *r0;
> >
> > r0 = READ_ONCE(*y);
> > *r0 = 1;
> > }
> >
> > exists (1:r0=x /\ x=2)
>
> Which r0 the above exists rule refers to, please?
> Do both P0 and P1 define r0 by purpose?

"1:r0" is the value returned by the above READ_ONCE(*y), following the
convention [thread number]:[local variable]; but yes, I could probably
have saved you this question by picking a different name, ;-) sorry.

>
> > Then
> >
> > $ herd7 -conf linux-kernel.cfg S+ponarelease+addroncena
> > Test S+ponarelease+addroncena Allowed
> > States 2
> > 1:r0=a; x=2;
> > 1:r0=x; x=1;
> > No
> > Witnesses
> > Positive: 0 Negative: 2
> > Condition exists (1:r0=x /\ x=2)
> > Observation S+ponarelease+addroncena Never 0 2
> > Time S+ponarelease+addroncena 0.01
> > Hash=7eaf7b5e95419a3c352d7fd50b9cd0d5
> >
> > that is, the test is not racy and the "exists" clause is not satisfiable
> > in the LKMM. Notice that _if the READ_ONCE(*y) in P1 were replaced by a
> > plain read, then we would obtain:
> >
> > Test S+ponarelease+addrnana Allowed
> > States 2
> > 1:r0=x; x=1;
> > 1:r0=x; x=2;
>
> Do you have any explanation how r0=x; x=2; could happen, please?

I should have remarked: the states listed here lose their significance
when there is a data race: "data race" is LKMM's way of saying "I give
up, I'm unable to list all the reachable states; your call...". ;-)

This example is "complicated", e.g., by the tearing of the plain read,
tearing which is envisaged/modelled by the LKMM: however, this tearing
doesn't explain the "1:r0=x; x=2;" state by itself, AFAICT.

Said this, I'm not sure how I copied this output... For completeness,
I report the full/intended test at the bottom of my email.

>
> Does the ommited READ_ONCE allows to do r0 = (*y) twice
> before and after *r0 = 1?
> Or the two operations P1 can be called in any order?
>
> I am sorry if it obvious. Feel free to ask me to re-read Paul's
> articles on LWN more times or point me to another resources.
>
>
>
> > Ok
> > Witnesses
> > Positive: 1 Negative: 1
> > Flag data-race [ <-- the LKMM warns about a data-race ]
> > Condition exists (1:r0=x /\ x=2)
> > Observation S+ponarelease+addrnana Sometimes 1 1
> > Time S+ponarelease+addrnana 0.00
> > Hash=a61acf2e8e51c2129d33ddf5e4c76a49
> >
> > N.B. This analysis generally depends on the assumption that every marked
> > access (e.g., the cmpxchg_release() called out above and the READ_ONCE()
> > heading the address dependencies) are _single-copy atomic, an assumption
> > which has been recently shown to _not be valid in such generality:
> >
> > https://lkml.kernel.org/r/20190821103200.kpufwtviqhpbuv2n@willie-the-truck
>
> So, it might be even worse. Do I get it correctly?

Worse than I was hoping..., definitely! ;-)

Andrea

---
C S+ponarelease+addrnana

{
int *y = &a;
}

P0(int *x, int **y, int *a)
{
int *r0;

*x = 2;
r0 = cmpxchg_release(y, a, x);
}

P1(int *x, int **y)
{
int *r0;

r0 = *y;
*r0 = 1;
}

exists (1:r0=x /\ x=2)