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

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Message-ID: <20251125120122-mutt-send-email-mst@kernel.org>
Date: Tue, 25 Nov 2025 12:18:51 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Simon Schippers <simon.schippers@...dortmund.de>
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: Re: [PATCH net-next v6 2/8] ptr_ring: add helper to check if consume
 created space

On Tue, Nov 25, 2025 at 05:12:35PM +0100, Simon Schippers wrote:
> 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


I see, (an element was consumed) part confused, instead of clarifying.
That is not the question - it was consumed.



Let me try:

Returns true if the last call to __ptr_ring_consume() 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
does not guarantee that the next call to __ptr_ring_produce
will succeed.

Note2: This function is only valid immediately after a single call to
__ptr_ring_consume(). If multiple calls to ptr_ring_consume*() have
been made, and you want to know whether any of them created space,
it is not enough to call this after the last __ptr_ring_consume -
instead, this check must be performed after each call individually.
Likewise, do not use this function after calling
__ptr_ring_consume_batched().




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