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: <CACGkMEuE-j0mwHUvDg9uocGCG78HAX4oCXVbt-YS7t5G1LTPfQ@mail.gmail.com>
Date: Wed, 3 Sep 2025 11:13:07 +0800
From: Jason Wang <jasowang@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Simon Schippers <simon.schippers@...dortmund.de>, 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, 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

On Wed, Sep 3, 2025 at 5:13 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> 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.

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