linux-kernel - Re: [PATCH net-next v6 2/8] ptr_ring: add helper to check if consume created space

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Message-ID: <20251125095650-mutt-send-email-mst@kernel.org>
Date: Tue, 25 Nov 2025 10:01:58 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Simon Schippers <simon.schippers@...dortmund.de>
Cc: willemdebruijn.kernel@...il.com, jasowang@...hat.com,
	andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, eperezma@...hat.com,
	jon@...anix.com, tim.gebauer@...dortmund.de, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	virtualization@...ts.linux.dev
Subject: Re: [PATCH net-next v6 2/8] ptr_ring: add helper to check if consume
 created space

On Thu, Nov 20, 2025 at 04:29:07PM +0100, Simon Schippers wrote:
> Add __ptr_ring_consume_created_space() to check whether the previous
> __ptr_ring_consume() call successfully consumed an element and created
> space in the ring buffer. This enables callers to conditionally notify
> producers when space becomes available.
> 
> The function is only valid immediately after a single consume operation
> and should not be used after calling __ptr_ring_consume_batched().
> 
> Co-developed-by: Tim Gebauer <tim.gebauer@...dortmund.de>
> Signed-off-by: Tim Gebauer <tim.gebauer@...dortmund.de>
> Co-developed by: Jon Kohler <jon@...anix.com>
> Signed-off-by: Jon Kohler <jon@...anix.com>
> Signed-off-by: Simon Schippers <simon.schippers@...dortmund.de>
> ---
>  include/linux/ptr_ring.h | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> index da141cc8b075..76d6840b45a3 100644
> --- a/include/linux/ptr_ring.h
> +++ b/include/linux/ptr_ring.h
> @@ -453,6 +453,23 @@ static inline int ptr_ring_consume_batched_bh(struct ptr_ring *r,
>  	return ret;
>  }
>  
> +/*
> + * Check if the previous consume operation created space

space?

what does this mean?

> + *
> + * Returns true if the last call to __ptr_ring_consume() has created
> + * space in the ring buffer (i.e., an element was consumed).
> + *
> + * Note: This function is only valid immediately after a single call to
> + * __ptr_ring_consume(). If multiple calls to ptr_ring_consume*() have
> + * been made, this check must be performed after each call individually.
> + * Likewise, do not use this function after calling
> + * __ptr_ring_consume_batched().

API-wise, it is a really weird function.  So is 

{
	p = __ptr_ring_consume

	return !!p
}

guaranteed to be equivalent to 

{
	p = __ptr_ring_consume

	return !!__ptr_ring_consume_created_space
}



> + */
> +static inline bool __ptr_ring_consume_created_space(struct ptr_ring *r)
> +{
> +	return r->consumer_tail >= r->consumer_head;
> +}
> +
>  /* 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