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@xxxxxxxxxxxxxx>
> Signed-off-by: Tim Gebauer <tim.gebauer@xxxxxxxxxxxxxx>
> Signed-off-by: Simon Schippers <simon.schippers@xxxxxxxxxxxxxx>
> ---
> 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
