Subject: Re: [PATCH] ptr_ring: add barriers

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 6 Dec 2017 04:53:56 +0200

On Wed, Dec 06, 2017 at 10:31:39AM +0800, Jason Wang wrote:
> 
> 
> On 2017年12月06日 03:29, Michael S. Tsirkin wrote:
> > Users of ptr_ring expect that it's safe to give the
> > data structure a pointer and have it be available
> > to consumers, but that actually requires an smb_wmb
> > or a stronger barrier.
> > 
> > In absence of such barriers and on architectures that reorder writes,
> > consumer might read an un=initialized value from an skb pointer stored
> > in the skb array.  This was observed causing crashes.
> > 
> > To fix, add memory barriers.  The barrier we use is a wmb, the
> > assumption being that producers do not need to read the value so we do
> > not need to order these reads.
> > 
> > Reported-by: George Cherian <[email-masked]>
> > Suggested-by: Jason Wang <[email-masked]>
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > ---
> > 
> > George, could you pls report whether this patch fixes
> > the issue for you?
> > 
> > This seems to be needed in stable as well.
> > 
> > 
> > 
> > 
> >   include/linux/ptr_ring.h | 9 +++++++++
> >   1 file changed, 9 insertions(+)
> > 
> > diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> > index 37b4bb2..6866df4 100644
> > --- a/include/linux/ptr_ring.h
> > +++ b/include/linux/ptr_ring.h
> > @@ -101,12 +101,18 @@ static inline bool ptr_ring_full_bh(struct ptr_ring *r)
> >   /* Note: callers invoking this in a loop must use a compiler barrier,
> >    * for example cpu_relax(). Callers must hold producer_lock.
> > + * Callers are responsible for making sure pointer that is being queued
> > + * points to a valid data.
> >    */
> >   static inline int __ptr_ring_produce(struct ptr_ring *r, void *ptr)
> >   {
> >   	if (unlikely(!r->size) || r->queue[r->producer])
> >   		return -ENOSPC;
> > +	/* Make sure the pointer we are storing points to a valid data. */
> > +	/* Pairs with smp_read_barrier_depends in __ptr_ring_consume. */
> > +	smp_wmb();
> > +
> >   	r->queue[r->producer++] = ptr;
> >   	if (unlikely(r->producer >= r->size))
> >   		r->producer = 0;
> > @@ -275,6 +281,9 @@ static inline void *__ptr_ring_consume(struct ptr_ring *r)
> >   	if (ptr)
> >   		__ptr_ring_discard_one(r);
> > +	/* Make sure anyone accessing data through the pointer is up to date. */
> > +	/* Pairs with smp_wmb in __ptr_ring_produce. */
> > +	smp_read_barrier_depends();
> >   	return ptr;
> >   }
> 
> I was thinking whether or not it's better to move those to the callers. Then
> we can save lots of barriers in e.g batch consuming.
> 
> Thanks

Batch consumers only do smp_read_barrier_depends which is free on
non-alpha. I suggest we do the simple thing for stable and reserve
optimizations for later.

-- 
MST