Re: [RFC 0/2] srcu: Remove pre-flip memory barrier

[Frederic Weisbecker]

On Mon, Dec 19, 2022 at 11:07:17PM -0500, Joel Fernandes wrote:
> On Sun, Dec 18, 2022 at 2:13 PM Joel Fernandes (Google)
> <joel@xxxxxxxxxxxxxxxxx> wrote:
> >
> > Hello, I believe the pre-flip memory barrier is not required. The only reason I
> > can say to remove it, other than the possibility that it is unnecessary, is to
> > not have extra code that does not help. However, since we are issuing a fully
> > memory-barrier after the flip, I cannot say that it hurts to do it anyway.
> >
> > For this reason, please consider these patches as "informational", than a
> > "please merge". :-) Though, feel free to consider merging if you agree!
> >
> > All SRCU scenarios pass with these, with 6 hours of testing.
> >
> > thanks,
> >
> >  - Joel
> >
> > Joel Fernandes (Google) (2):
> > srcu: Remove comment about prior read lock counts
> > srcu: Remove memory barrier "E" as it is not required
> 
> And litmus tests confirm that "E" does not really do what the comments
> say, PTAL:
> Test 1:
> C mbe
> (*
>  * Result: sometimes
>  * Does previous scan see old reader's lock count, if a new reader saw
> the new srcu_idx?
>  *)
> 
> {}
> 
> P0(int *lockcount, int *srcu_idx) // updater
> {
>         int r0;
>         r0 = READ_ONCE(*lockcount);
>         smp_mb();       // E
>         WRITE_ONCE(*srcu_idx, 1);
> }
> 
> P1(int *lockcount, int *srcu_idx) // reader
> {
>         int r0;
>         WRITE_ONCE(*lockcount, 1); // previous reader
>         smp_mb();       // B+C
>         r0 = READ_ONCE(*srcu_idx); // new reader
> }
> exists (0:r0=0 /\ 1:r0=1) (* Bad outcome. *)
> 
> Test 2:
> C mbe2
> 
> (*
>  * Result: sometimes
>  * If updater saw reader's lock count, was that reader using the old idx?
>  *)
> 
> {}
> 
> P0(int *lockcount, int *srcu_idx) // updater
> {
>         int r0;
>         r0 = READ_ONCE(*lockcount);
>         smp_mb();       // E
>         WRITE_ONCE(*srcu_idx, 1);
> }
> 
> P1(int *lockcount, int *srcu_idx) // reader
> {
>         int r0;
>         int r1;
>         r1 = READ_ONCE(*srcu_idx); // previous reader
>         WRITE_ONCE(*lockcount, 1); // previous reader
>         smp_mb();       // B+C
>         r0 = READ_ONCE(*srcu_idx); // new reader
> }
> exists (0:r0=1 /\ 1:r1=1) (* Bad outcome. *)

Actually, starring at this some more, there is some form of dependency
on the idx in order to build the address where the reader must write the
lockcount to. Litmus doesn't support arrays but assuming that
&ssp->sda->srcu_lock_count == 0 (note the & in the beginning), it
could be modelized that way (I'm eluding the unlock part to simplify):

---
C w-depend-r

{
	PLOCK=LOCK0;
}

// updater
P0(int *LOCK0, int *LOCK1, int **PLOCK)
{
	int lock1;

	lock1 = READ_ONCE(*LOCK1); // READ from inactive idx
	smp_mb();
	WRITE_ONCE(*PLOCK, LOCK1); // Flip idx
}

// reader
P1(int **PLOCK)
{
	int *plock;

	plock = READ_ONCE(*PLOCK); 	// Read active idx
	WRITE_ONCE(*plock, 1); // Write to active idx
}

exists (0:lock0=1) // never happens
---

* Is it an address dependency? But the LKMM refers to that only in
  terms of LOAD - LOAD ordering.

* Is it a control dependency? But the LKMM refers to that only in
  terms of branch with "if".

So I don't know the name of the pattern but it makes litmus happy.

Thanks.