On 1/9/26 09:31, Michael S. Tsirkin wrote:
> On Fri, Jan 09, 2026 at 08:35:31AM +0100, Simon Schippers wrote:
>> On 1/9/26 08:22, Michael S. Tsirkin wrote:
>>> 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 protected]>
>>>> Signed-off-by: Tim Gebauer <[email protected]>
>>>> Signed-off-by: Simon Schippers <[email protected]>
>>>> ---
>>>> 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)?
>>
>> No, that is wrong from my POV.
>>
>> It always creates the same amount of space which is the batch size or
>> multiple batch sizes (or something less in the wrap-around case). That is
>> of course only if __ptr_ring_zero_tail() was executed at least once,
>> else it creates zero space.
>
> exactly, and caller does not know, and now he wants to know so
> we add an API for him to find out?
>
> I feel the fact it's a binary (batch or 0) is an implementation
> detail better hidden from user.
I agree, and I now understood your logic :)
So it should be:
static inline int __ptr_ring_consume_created_space(struct ptr_ring *r,
int n)
{
return max(n - (r->consumer_head - r->consumer_tail), 0);
}
Right?
>
>
>
>>>
>>>
>>> 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
>>>
>
