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

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Message-ID: <ce371d19-e69a-4d8e-a9a0-f3e20439a094@tu-dortmund.de>
Date: Tue, 25 Nov 2025 17:12:35 +0100
From: Simon Schippers <simon.schippers@...dortmund.de>
To: "Michael S. Tsirkin" <mst@...hat.com>
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: [PATCH net-next v6 2/8] ptr_ring: add helper to check if consume
 created space

On 11/25/25 16:01, Michael S. Tsirkin wrote:
> 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
> }

I am a bit confused. You were the one recommending this function to me,
see [1].

Maybe the comments need rework here, but the function should be fine.

Thanks

[1] Link: https://lore.kernel.org/netdev/20250922221553.47802-1-simon.schippers@tu-dortmund.de/T/#mb722e8ae4ceb5df24f74305c6145561883d4e987

> 
> 
> 
>> + */
>> +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
>