Re: [PATCH bpf v2 7/8] libbpf: ringbuf: Use compiler atomics

[Andrii Nakryiko]

On Thu, Jun 18, 2026 at 5:27 PM Tamir Duberstein <tamird@xxxxxxxxxx> wrote:
>
> Consumer-side ring buffer code uses architecture-specific smp_* helpers
> for shared memory accesses.
>
> Use compiler atomics instead. They provide equivalent acquire and
> release ordering through a portable userspace interface and allow the
> next commit to use compiler fences in the wakeup protocol without mixing
> atomic interfaces.
>
> Reviewed-by: Emil Tsalapatis <emil@xxxxxxxxxxxxxxx>
> Assisted-by: Codex:gpt-5.5
> Signed-off-by: Tamir Duberstein <tamird@xxxxxxxxxx>
> ---
>  tools/lib/bpf/ringbuf.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
> index 1c5bce2b5e12..141f2cbe56eb 100644
> --- a/tools/lib/bpf/ringbuf.c
> +++ b/tools/lib/bpf/ringbuf.c
> @@ -270,14 +270,14 @@ static int64_t ringbuf_process_ring(struct ring *r, size_t n)
>         if (n == 0)
>                 return 0;
>
> -       cons_pos = smp_load_acquire(r->consumer_pos);
> +       cons_pos = __atomic_load_n(r->consumer_pos, __ATOMIC_ACQUIRE);
>         do {
>                 got_new_data = false;
> -               prod_pos = smp_load_acquire(r->producer_pos);
> +               prod_pos = __atomic_load_n(r->producer_pos, __ATOMIC_ACQUIRE);
>                 /* Positions wrap; the consumer cannot logically pass the producer. */
>                 while (cons_pos != prod_pos) {
>                         len_ptr = r->data + (cons_pos & r->mask);
> -                       len = smp_load_acquire(len_ptr);
> +                       len = __atomic_load_n(len_ptr, __ATOMIC_ACQUIRE);
>
>                         /* sample not committed yet, bail out for now */
>                         if (len & BPF_RINGBUF_BUSY_BIT)
> @@ -291,14 +291,16 @@ static int64_t ringbuf_process_ring(struct ring *r, size_t n)
>                                 err = r->sample_cb(r->ctx, sample, len);
>                                 if (err < 0) {
>                                         /* update consumer pos and bail out */
> -                                       smp_store_release(r->consumer_pos,
> -                                                         cons_pos);
> +                                       __atomic_store_n(r->consumer_pos,
> +                                                        cons_pos,
> +                                                        __ATOMIC_RELEASE);

this almost fits 100 characters, I'd keep it as single line

>                                         return err;
>                                 }
>                                 cnt++;
>                         }
>
> -                       smp_store_release(r->consumer_pos, cons_pos);
> +                       __atomic_store_n(r->consumer_pos, cons_pos,
> +                                        __ATOMIC_RELEASE);

this definitely fits 100 characters, let's keep single line for readability

>
>                         if (cnt >= n)
>                                 goto done;
> @@ -413,8 +415,8 @@ struct ring *ring_buffer__ring(struct ring_buffer *rb, unsigned int idx)
>
>  unsigned long ring__consumer_pos(const struct ring *r)
>  {
> -       /* Synchronizes with smp_store_release() in ringbuf_process_ring(). */
> -       return smp_load_acquire(r->consumer_pos);
> +       /* Synchronizes with the release store in ringbuf_process_ring(). */
> +       return __atomic_load_n(r->consumer_pos, __ATOMIC_ACQUIRE);
>  }
>
>  unsigned long ring__producer_pos(const struct ring *r)
> @@ -422,7 +424,7 @@ unsigned long ring__producer_pos(const struct ring *r)
>         /* Synchronizes with smp_store_release() in __bpf_ringbuf_reserve() in
>          * the kernel.
>          */
> -       return smp_load_acquire(r->producer_pos);
> +       return __atomic_load_n(r->producer_pos, __ATOMIC_ACQUIRE);
>  }
>
>  size_t ring__avail_data_size(const struct ring *r)
>
> --
> 2.55.0.rc0.159.gbe5d7338c2
>