Re: [PATCH 1/4] ptr_ring_spare: Helper to check if spare capacity of size cnt is available

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

On Tue, Sep 02, 2025 at 10:09:54AM +0200, Simon Schippers wrote:
> The implementation is inspired by ptr_ring_empty.
> 
> Co-developed-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
> Signed-off-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
> Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de>
> ---
>  include/linux/ptr_ring.h | 71 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 71 insertions(+)
> 
> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> index 551329220e4f..6b8cfaecf478 100644
> --- a/include/linux/ptr_ring.h
> +++ b/include/linux/ptr_ring.h
> @@ -243,6 +243,77 @@ static inline bool ptr_ring_empty_bh(struct ptr_ring *r)
>  	return ret;
>  }
>  
> +/*
> + * Check if a spare capacity of cnt is available without taking any locks.

Not sure what "spare" means here. I think you mean

Check if the ring has enough space to produce a given
number of entries.

> + *
> + * If cnt==0 or cnt > r->size it acts the same as __ptr_ring_empty.

Logically, cnt = 0 should always be true, cnt > size should always be
false then?

Why do you want it to act as __ptr_ring_empty?


> + *
> + * The same requirements apply as described for __ptr_ring_empty.


Which is:

 * However, if some other CPU consumes ring entries at the same time, the value
 * returned is not guaranteed to be correct.


but it's not right here yes? consuming entries will just add more
space ...

Also:
 * In this case - to avoid incorrectly detecting the ring
 * as empty - the CPU consuming the ring entries is responsible
 * for either consuming all ring entries until the ring is empty,
 * or synchronizing with some other CPU and causing it to
 * re-test __ptr_ring_empty and/or consume the ring enteries
 * after the synchronization point.

how would you apply this here?


> + */
> +static inline bool __ptr_ring_spare(struct ptr_ring *r, int cnt)
> +{
> +	int size = r->size;
> +	int to_check;
> +
> +	if (unlikely(!size || cnt < 0))
> +		return true;
> +
> +	if (cnt > size)
> +		cnt = 0;
> +
> +	to_check = READ_ONCE(r->consumer_head) - cnt;
> +
> +	if (to_check < 0)
> +		to_check += size;
> +
> +	return !r->queue[to_check];
> +}
> +

I will have to look at how this is used to understand if it's
correct. But I think we need better documentation.


> +static inline bool ptr_ring_spare(struct ptr_ring *r, int cnt)
> +{
> +	bool ret;
> +
> +	spin_lock(&r->consumer_lock);
> +	ret = __ptr_ring_spare(r, cnt);
> +	spin_unlock(&r->consumer_lock);
> +
> +	return ret;


I don't understand why you take the consumer lock here.
If a producer is running it will make the value wrong,
if consumer is running it will just create more space.


> +}
> +
> +static inline bool ptr_ring_spare_irq(struct ptr_ring *r, int cnt)
> +{
> +	bool ret;
> +
> +	spin_lock_irq(&r->consumer_lock);
> +	ret = __ptr_ring_spare(r, cnt);
> +	spin_unlock_irq(&r->consumer_lock);
> +
> +	return ret;
> +}
> +
> +static inline bool ptr_ring_spare_any(struct ptr_ring *r, int cnt)
> +{
> +	unsigned long flags;
> +	bool ret;
> +
> +	spin_lock_irqsave(&r->consumer_lock, flags);
> +	ret = __ptr_ring_spare(r, cnt);
> +	spin_unlock_irqrestore(&r->consumer_lock, flags);
> +
> +	return ret;
> +}
> +
> +static inline bool ptr_ring_spare_bh(struct ptr_ring *r, int cnt)
> +{
> +	bool ret;
> +
> +	spin_lock_bh(&r->consumer_lock);
> +	ret = __ptr_ring_spare(r, cnt);
> +	spin_unlock_bh(&r->consumer_lock);
> +
> +	return ret;
> +}
> +
>  /* Must only be called after __ptr_ring_peek returned !NULL */
>  static inline void __ptr_ring_discard_one(struct ptr_ring *r)
>  {
> -- 
> 2.43.0