Re: [PATCH] ptr_ring: add barriers

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 <redacted>
Suggested-by: Jason Wang <redacted>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

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;
  }