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

Petr Mladek
2019-08-26T14:11:02Z

On Mon 2019-08-26 10:34:36, Andrea Parri wrote:
> > > + /*
> > > + * bA:
> > > + *
> > > + * Setup the node to be a list terminator: next_id == id.
> > > + */
> > > + WRITE_ONCE(n->next_id, id);
> >
> > Do we need WRITE_ONCE() here?
> > Both "n" and "id" are given as parameters and do not change.
> > The assigment must be done before "id" is set as nl->head_id.
> > The ordering is enforced by cmpxchg_release().
>
> (Disclaimer: this is still a very much debated issue...)
>
> According to the LKMM, this question boils down to the question:
>
> Is there "ordering"/synchronization between the above access and
> the "matching accesses" bF and aA' to the same location?
>
> Again according to the LKMM's analysis, such synchronization is provided
> by the RELEASE -> "reads-from" -> ADDR relation. (Encoding address dep.
> in litmus tests is kind of tricky but possible, e.g., for the pattern in
> question, we could write/model as follows:
>
> 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?

> 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?

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?

Best Regards,
Petr