Subject: Re: [PATCH net-next v7 2/9] ptr_ring: add helper to detect newly freed space on consume

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 9 Jan 2026 02:22:44 -0500

On Wed, Jan 07, 2026 at 10:04:41PM +0100, Simon Schippers wrote:
> This proposed function checks whether __ptr_ring_zero_tail() was invoked
> within the last n calls to __ptr_ring_consume(), which indicates that new
> free space was created. Since __ptr_ring_zero_tail() moves the tail to
> the head - and no other function modifies either the head or the tail,
> aside from the wrap-around case described below - detecting such a
> movement is sufficient to detect the invocation of
> __ptr_ring_zero_tail().
> 
> The implementation detects this movement by checking whether the tail is
> at most n positions behind the head. If this condition holds, the shift
> of the tail to its current position must have occurred within the last n
> calls to __ptr_ring_consume(), indicating that __ptr_ring_zero_tail() was
> invoked and that new free space was created.
> 
> This logic also correctly handles the wrap-around case in which
> __ptr_ring_zero_tail() is invoked and the head and the tail are reset
> to 0. Since this reset likewise moves the tail to the head, the same
> detection logic applies.
> 
> Co-developed-by: Tim Gebauer <[email-masked]>
> Signed-off-by: Tim Gebauer <[email-masked]>
> Signed-off-by: Simon Schippers <[email-masked]>
> ---
>  include/linux/ptr_ring.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> index a5a3fa4916d3..7cdae6d1d400 100644
> --- a/include/linux/ptr_ring.h
> +++ b/include/linux/ptr_ring.h
> @@ -438,6 +438,19 @@ static inline int ptr_ring_consume_batched_bh(struct ptr_ring *r,
>  	return ret;
>  }
>  
> +/* Returns true if the consume of the last n elements has created space
> + * in the ring buffer (i.e., a new element can be produced).
> + *
> + * Note: Because of batching, a successful call to __ptr_ring_consume() /
> + * __ptr_ring_consume_batched() does not guarantee that the next call to
> + * __ptr_ring_produce() will succeed.

I think the issue is it does not say what is the actual guarantee.

Another issue is that the "Note" really should be more prominent,
it really is part of explaining what the functions does.

Hmm. Maybe we should tell it how many entries have been consumed and
get back an indication of how much space this created?

fundamentally
	 n - (r->consumer_head - r->consumer_tail)?

does the below sound good maybe?

/* Returns the amound of space (number of new elements that can be
 * produced) that calls to ptr_ring_consume created.
 *
 * Getting n entries from calls to ptr_ring_consume() /
 * ptr_ring_consume_batched() does *not* guarantee that the next n calls to
 * ptr_ring_produce() will succeed.
 *
 * Use this function after consuming n entries to get a hint about
 * how much space was actually created.

> + */
> +static inline bool __ptr_ring_consume_created_space(struct ptr_ring *r,
> +						    int n)
> +{
> +	return r->consumer_head - r->consumer_tail < n;
> +}
> +
>  /* Cast to structure type and call a function without discarding from FIFO.
>   * Function must return a value.
>   * Callers must take consumer_lock.
> -- 
> 2.43.0