linux-kernel - Re: [PATCH net-next 05/12] ptr_ring: disallow lockless __ptr_ring

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Date:   Fri, 26 Jan 2018 04:46:12 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        John Fastabend <john.fastabend@...il.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next 05/12] ptr_ring: disallow lockless
 __ptr_ring_full

On Fri, Jan 26, 2018 at 10:38:05AM +0800, Jason Wang wrote:
> 
> 
> On 2018年01月26日 07:36, Michael S. Tsirkin wrote:
> > Similar to bcecb4bbf88a ("net: ptr_ring: otherwise safe empty checks can
> > overrun array bounds") a lockless use of __ptr_ring_full might
> > cause an out of bounds access.
> > 
> > We can fix this, but it's easier to just disallow lockless
> > __ptr_ring_full for now.
> 
> It looks to me that just fix this is better than disallow through doc (which
> is easily to be ignored ...).
> 
> Thanks

lockless is tricky, and I'd rather not sprinkle READ/WRITE_ONCE where
they aren't necessary.

> > 
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > ---
> >   include/linux/ptr_ring.h | 7 ++++---
> >   1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> > index 9a72d8f..f175846 100644
> > --- a/include/linux/ptr_ring.h
> > +++ b/include/linux/ptr_ring.h
> > @@ -45,9 +45,10 @@ struct ptr_ring {
> >   };
> >   /* Note: callers invoking this in a loop must use a compiler barrier,
> > - * for example cpu_relax().  If ring is ever resized, callers must hold
> > - * producer_lock - see e.g. ptr_ring_full.  Otherwise, if callers don't hold
> > - * producer_lock, the next call to __ptr_ring_produce may fail.
> > + * for example cpu_relax().
> > + *
> > + * NB: this is unlike __ptr_ring_empty in that callers must hold producer_lock:
> > + * see e.g. ptr_ring_full.
> >    */
> >   static inline bool __ptr_ring_full(struct ptr_ring *r)
> >   {