ptr_ring: prevent queue load/store tearing

In theory compiler could tear queue loads or stores in two. It does not
seem to be happening in practice but it seems easier to convert the
cases where this would be a problem to READ/WRITE_ONCE than worry about
it.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ptr_ring.h

@@ -114,7 +114,7 @@ static inline int __ptr_ring_produce(struct ptr_ring *r, void *ptr)
 	/* Pairs with smp_read_barrier_depends in __ptr_ring_consume. */
 	smp_wmb();
 
-	r->queue[r->producer++] = ptr;
+	WRITE_ONCE(r->queue[r->producer++], ptr);
 	if (unlikely(r->producer >= r->size))
 		r->producer = 0;
 	return 0;
@@ -173,7 +173,7 @@ static inline int ptr_ring_produce_bh(struct ptr_ring *r, void *ptr)
 static inline void *__ptr_ring_peek(struct ptr_ring *r)
 {
 	if (likely(r->size))
-		return r->queue[r->consumer_head];
+		return READ_ONCE(r->queue[r->consumer_head]);
 	return NULL;
 }