Re: [PATCH] ptr_ring: add barriers

[Jason Wang]

On 2017å12æ06æ 10:53, Michael S. Tsirkin wrote:
> On Wed, Dec 06, 2017 at 10:31:39AM +0800, Jason Wang wrote:
> > On 2017å12æ06æ 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 <george.cherian@xxxxxxxxxx>
> > > Suggested-by: Jason Wang <jasowang@xxxxxxxxxx>
> > > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > ---
> > >
> > > 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.

Right.

Acked-by: Jason Wang <jasowang@xxxxxxxxxx>