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

Joel Fernandes
2022-12-20T04:07:40Z

On Sun, Dec 18, 2022 at 2:13 PM Joel Fernandes (Google)
<joel@joelfernandes.org> 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. *)

thanks,

- Joel

>
> kernel/rcu/srcutree.c | 10 ----------
> 1 file changed, 10 deletions(-)
>
> --
> 2.39.0.314.g84b9a713c41-goog
>