linux-kernel - Re: [PATCH 1/4] ptr_ring_spare: Helper to check if spare capacity of size cnt is available

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Message-ID: <willemdebruijn.kernel.6b96c721c235@gmail.com>
Date: Tue, 02 Sep 2025 17:13:17 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Simon Schippers <simon.schippers@...dortmund.de>, 
 willemdebruijn.kernel@...il.com, 
 jasowang@...hat.com, 
 mst@...hat.com, 
 eperezma@...hat.com, 
 stephen@...workplumber.org, 
 netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 virtualization@...ts.linux.dev, 
 kvm@...r.kernel.org
Cc: Simon Schippers <simon.schippers@...dortmund.de>, 
 Tim Gebauer <tim.gebauer@...dortmund.de>
Subject: Re: [PATCH 1/4] ptr_ring_spare: Helper to check if spare capacity of
 size cnt is available

Simon Schippers wrote:
> The implementation is inspired by ptr_ring_empty.
> 
> Co-developed-by: Tim Gebauer <tim.gebauer@...dortmund.de>
> Signed-off-by: Tim Gebauer <tim.gebauer@...dortmund.de>
> Signed-off-by: Simon Schippers <simon.schippers@...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.