xsk: Remove unnecessary READ_ONCE of data

There are two unnecessary READ_ONCE of descriptor data. These are not
needed since the data is written by the producer before it signals
that the data is available by incrementing the producer pointer. As the
access to this producer pointer is serialized and the consumer always
reads the descriptor after it has read and synchronized with the
producer counter, the write of the descriptor will have fully
completed and it does not matter if the consumer has any read tearing.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/1576759171-28550-11-git-send-email-magnus.karlsson@intel.com

net/xdp/xsk_queue.h

@@ -183,7 +183,7 @@ static inline bool xskq_cons_read_addr(struct xsk_queue *q, u64 *addr,
 	while (q->cached_cons != q->cached_prod) {
 		u32 idx = q->cached_cons & q->ring_mask;
 
-		*addr = READ_ONCE(ring->desc[idx]) & q->chunk_mask;
+		*addr = ring->desc[idx] & q->chunk_mask;
 
 		if (umem->flags & XDP_UMEM_UNALIGNED_CHUNK_FLAG) {
 			if (xskq_cons_is_valid_unaligned(q, *addr,
@@ -308,7 +308,7 @@ static inline bool xskq_cons_read_desc(struct xsk_queue *q,
 		struct xdp_rxtx_ring *ring = (struct xdp_rxtx_ring *)q->ring;
 		u32 idx = q->cached_cons & q->ring_mask;
 
-		*desc = READ_ONCE(ring->desc[idx]);
+		*desc = ring->desc[idx];
 		if (xskq_cons_is_valid_desc(q, desc, umem))
 			return true;