Re: XDP socket rings, and LKMM litmus tests

Paul E. McKenney
2021-03-02T22:37:47Z

On Tue, Mar 02, 2021 at 07:46:27PM +0100, Björn Töpel wrote:
> Hi!
>
> Firstly; The long Cc-list is to reach the LKMM-folks.
>
> Some background; the XDP sockets use a ring-buffer to communicate
> between the kernel and userland. It's a
> single-consumer/single-producer ring, and described in
> net/xdp/xsk_queue.h.
>
> --8<---
> /* The structure of the shared state of the rings are the same as the
> * ring buffer in kernel/events/ring_buffer.c. For the Rx and completion
> * ring, the kernel is the producer and user space is the consumer. For
> * the Tx and fill rings, the kernel is the consumer and user space is
> * the producer.
> *
> * producer consumer
> *
> * if (LOAD ->consumer) { LOAD ->producer
> * (A) smp_rmb() (C)
> * STORE $data LOAD $data
> * smp_wmb() (B) smp_mb() (D)
> * STORE ->producer STORE ->consumer
> * }
> *
> * (A) pairs with (D), and (B) pairs with (C).
> ...
> -->8---
>
> I'd like to replace the smp_{r,w,}mb() barriers with acquire-release
> semantics [1], without breaking existing userspace applications.
>
> So, I figured I'd use herd7 and the LKMM model to build a litmus test
> for the barrier version, then for the acquire-release version, and
> finally permutations of both.
>
> The idea is to use a one element ring, with a state machine outlined
> in the litmus test.
>
> The basic test for the existing smp_{r,w,}mb() barriers looks like:
>
> $ cat spsc-rb+1p1c.litmus
> C spsc-rb+1p1c
>
> // Stupid one entry ring:
> // prod cons allowed action prod cons
> // 0 0 => prod => 1 0
> // 0 1 => cons => 0 0
> // 1 0 => cons => 1 1
> // 1 1 => prod => 0 1
>
> { prod = 1; }
>
> // Here, we start at prod==1,cons==0, data==0, i.e. producer has
> // written data=0, so from here only the consumer can start, and should
> // consume data==0. Afterwards, producer can continue and write 1 to
> // data. Can we enter state prod==0, cons==1, but consumer observerd
> // the write of 1?
>
> P0(int *prod, int *cons, int *data)
> {
> int p;
> int c;
> int cond = 0;
>
> p = READ_ONCE(*prod);
> c = READ_ONCE(*cons);
> if (p == 0)
> if (c == 0)
> cond = 1;
> if (p == 1)
> if (c == 1)
> cond = 1;
>
> if (cond) {
> smp_mb();
> WRITE_ONCE(*data, 1);
> smp_wmb();
> WRITE_ONCE(*prod, p ^ 1);
> }
> }
>
> P1(int *prod, int *cons, int *data)
> {
> int p;
> int c;
> int d = -1;
> int cond = 0;
>
> p = READ_ONCE(*prod);
> c = READ_ONCE(*cons);
> if (p == 1)
> if (c == 0)
> cond = 1;
> if (p == 0)
> if (c == 1)
> cond = 1;
>
> if (cond == 1) {
> smp_rmb();
> d = READ_ONCE(*data);
> smp_mb();
> WRITE_ONCE(*cons, c ^ 1);
> }
> }

Before digging in too deeply, does the following simplification
still capture your intent?

P0(int *prod, int *cons, int *data)
{
int p;
int cond = 0;

p = READ_ONCE(*prod);
if (p == READ_ONCE(*cons))
cond = 1;
if (cond) {
smp_mb();
WRITE_ONCE(*data, 1);
smp_wmb();
WRITE_ONCE(*prod, p ^ 1);
}
}

P1(int *prod, int *cons, int *data)
{
int c;
int d = -1;
int cond = 0;

c = READ_ONCE(*cons);
if (READ_ONCE(*prod) == c)
cond = 1;

if (cond == 1) {
smp_rmb();
d = READ_ONCE(*data);
smp_mb();
WRITE_ONCE(*cons, c ^ 1);
}
}

Thanx, Paul

> exists( 1:d=1 /\ prod=0 /\ cons=1 );
>
> --
>
> The weird state changing if-statements is because that I didn't get
> '&&' and '||' to work with herd.
>
> When this is run:
>
> $ herd7 -conf linux-kernel.cfg litmus-tests/spsc-rb+1p1c.litmus
> Test spsc-rb+1p1c Allowed
> States 2
> 1:d=0; cons=1; prod=0;
> 1:d=0; cons=1; prod=1;
> No
> Witnesses
> Positive: 0 Negative: 2
> Condition exists (1:d=1 /\ prod=0 /\ cons=1)
> Observation spsc-rb+1p1c Never 0 2
> Time spsc-rb+1p1c 0.04
> Hash=b399756d6a1301ca5bda042f32130791
>
> Now to my question; In P0 there's an smp_mb(). Without that, the d==1
> can be observed from P1 (consumer):
>
> $ herd7 -conf linux-kernel.cfg litmus-tests/spsc-rb+1p1c.litmus
> Test spsc-rb+1p1c Allowed
> States 3
> 1:d=0; cons=1; prod=0;
> 1:d=0; cons=1; prod=1;
> 1:d=1; cons=1; prod=0;
> Ok
> Witnesses
> Positive: 1 Negative: 2
> Condition exists (1:d=1 /\ prod=0 /\ cons=1)
> Observation spsc-rb+1p1c Sometimes 1 2
> Time spsc-rb+1p1c 0.04
> Hash=0047fc21fa77da9a9aee15e35ec367ef
>
> In commit c7f2e3cd6c1f ("perf: Optimize ring-buffer write by depending
> on control dependencies") removes the corresponding smp_mb(), and also
> the circular buffer in circular-buffers.txt (pre commit 6c43c091bdc5
> ("documentation: Update circular buffer for
> load-acquire/store-release")) is missing the smp_mb() at the
> producer-side.
>
> I'm trying to wrap my head around why it's OK to remove the smp_mb()
> in the cases above? I'm worried that the current XDP socket ring
> implementation (which is missing smp_mb()) might be broken.
>
>
> If you read this far, thanks! :-)
> Björn
>
>
> [1] https://lore.kernel.org/bpf/20210301104318.263262-2-bjorn.topel@gmail.com/