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.
+ *
+ * If cnt==0 or cnt > r->size it acts the same as __ptr_ring_empty.
+ *
+ * The same requirements apply as described for __ptr_ring_empty.
+ */
+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];
+}
+
+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;
+}
+
+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

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

Michael S. Tsirkin wrote:
> 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.
> 
>

I agree, I messed up the ptr_ring helper.
Your proposed approach is way superior and I will use that one instead.

The idea behind the cnt was to have an option if the producer may produce
multiple entries like tap_handle_frame with GSO. But of course this should
be in a different patch since I will not cover tap_handle_frame, which is
used by ipvtap and macvtap, in this patch series.

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

cnt >= r->size?

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

Does !size ever happen. Also no need for preconditions for trivial
errors that never happen, like passing negative values. Or prefer
an unsigned type.

> +
> +	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];
> +}
> +
> +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;
> +}
> +
> +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;
> +}

Please only introduce the variants actually used.

On Wed, Sep 3, 2025 at 5:13 AM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> 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.
> > + *
> > + * If cnt==0 or cnt > r->size it acts the same as __ptr_ring_empty.
>
> cnt >= r->size?
>
> > + *
> > + * The same requirements apply as described for __ptr_ring_empty.
> > + */
> > +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;
>
> Does !size ever happen.

Yes, see 982fb490c298 ("ptr_ring: support zero length ring"). The
reason is tun reuse dev->tx_queue_len for ptr_ring size.

> Also no need for preconditions for trivial
> errors that never happen, like passing negative values. Or prefer
> an unsigned type.

+1.

Thanks