Re: [PATCH net-next v6 1/8] ptr_ring: add __ptr_ring_full_next() to predict imminent fullness

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 20, 2025 at 04:29:06PM +0100, Simon Schippers wrote:
> Introduce the __ptr_ring_full_next() helper, which lets callers check
> if the ptr_ring will become full after the next insertion. This is useful
> for proactively managing capacity before the ring is actually full.
> Callers must ensure the ring is not already full before using this
> helper. This is because __ptr_ring_discard_one() may zero entries in
> reverse order, the slot after the current producer position may be
> cleared before the current one. This must be considered when using this
> check.
> 
> Note: This function is especially relevant when paired with the memory
> ordering guarantees of __ptr_ring_produce() (smp_wmb()), allowing for
> safe producer/consumer coordination.
> 
> Co-developed-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
> Signed-off-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
> Co-developed-by: Jon Kohler <jon@nutanix.com>
> Signed-off-by: Jon Kohler <jon@nutanix.com>
> Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de>
> ---
>  include/linux/ptr_ring.h | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> index 534531807d95..da141cc8b075 100644
> --- a/include/linux/ptr_ring.h
> +++ b/include/linux/ptr_ring.h
> @@ -96,6 +96,31 @@ static inline bool ptr_ring_full_bh(struct ptr_ring *r)
>  	return ret;
>  }
>  
> +/*
> + * Checks if the ptr_ring will become full after the next insertion.

Is this for the producer or the consumer? A better name would
reflect that.

> + *
> + * Note: Callers must ensure that the ptr_ring is not full before calling
> + * this function,

how?

> as __ptr_ring_discard_one invalidates entries in
> + * reverse order. Because the next entry (rather than the current one)
> + * may be zeroed after an insertion, failing to account for this can
> + * cause false negatives when checking whether the ring will become full
> + * on the next insertion.

this part confuses more than it clarifies.

> + */
> +static inline bool __ptr_ring_full_next(struct ptr_ring *r)
> +{
> +	int p;
> +
> +	if (unlikely(r->size <= 1))
> +		return true;
> +
> +	p = r->producer + 1;
> +
> +	if (unlikely(p >= r->size))
> +		p = 0;
> +
> +	return r->queue[p];
> +}
> +
>  /* Note: callers invoking this in a loop must use a compiler barrier,
>   * for example cpu_relax(). Callers must hold producer_lock.
>   * Callers are responsible for making sure pointer that is being queued
> -- 
> 2.43.0