[PATCH net-next v2 17/39] rxrpc: Don't need barrier for ->tx_bottom and ->acks_hard_ack

[David Howells]

We don't need a barrier for the ->tx_bottom value (which indicates the
lowest sequence still in the transmission queue) and the ->acks_hard_ack
value (which tracks the DATA packets hard-ack'd by the latest ACK packet
received and thus indicates which DATA packets can now be discarded) as the
app thread doesn't use either value as a reference to memory to access.
Rather, the app thread merely uses these as a guide to how much space is
available in the transmission queue

Change the code to use READ/WRITE_ONCE() instead.

Also, change rxrpc_check_tx_space() to use the same value for tx_bottom
throughout.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
cc: Eric Dumazet <edumazet@xxxxxxxxxx>
cc: Jakub Kicinski <kuba@xxxxxxxxxx>
cc: Paolo Abeni <pabeni@xxxxxxxxxx>
cc: linux-afs@xxxxxxxxxxxxxxxxxxx
cc: netdev@xxxxxxxxxxxxxxx
---
 net/rxrpc/sendmsg.c | 8 +++++---
 net/rxrpc/txbuf.c   | 4 ++--
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/net/rxrpc/sendmsg.c b/net/rxrpc/sendmsg.c
index 786c1fb1369a..467c9402882e 100644
--- a/net/rxrpc/sendmsg.c
+++ b/net/rxrpc/sendmsg.c
@@ -94,9 +94,11 @@ static int rxrpc_wait_to_be_connected(struct rxrpc_call *call, long *timeo)
  */
 static bool rxrpc_check_tx_space(struct rxrpc_call *call, rxrpc_seq_t *_tx_win)
 {
+	rxrpc_seq_t tx_bottom = READ_ONCE(call->tx_bottom);
+
 	if (_tx_win)
-		*_tx_win = call->tx_bottom;
-	return call->tx_prepared - call->tx_bottom < 256;
+		*_tx_win = tx_bottom;
+	return call->tx_prepared - tx_bottom < 256;
 }
 
 /*
@@ -138,7 +140,7 @@ static int rxrpc_wait_for_tx_window_waitall(struct rxrpc_sock *rx,
 		rtt = 2;
 
 	timeout = rtt;
-	tx_start = smp_load_acquire(&call->acks_hard_ack);
+	tx_start = READ_ONCE(call->acks_hard_ack);
 
 	for (;;) {
 		set_current_state(TASK_UNINTERRUPTIBLE);
diff --git a/net/rxrpc/txbuf.c b/net/rxrpc/txbuf.c
index 8b7c854ed3d7..0cc8f49d69a9 100644
--- a/net/rxrpc/txbuf.c
+++ b/net/rxrpc/txbuf.c
@@ -214,14 +214,14 @@ void rxrpc_shrink_call_tx_buffer(struct rxrpc_call *call)
 
 	while ((txb = list_first_entry_or_null(&call->tx_buffer,
 					       struct rxrpc_txbuf, call_link))) {
-		hard_ack = smp_load_acquire(&call->acks_hard_ack);
+		hard_ack = call->acks_hard_ack;
 		if (before(hard_ack, txb->seq))
 			break;
 
 		if (txb->seq != call->tx_bottom + 1)
 			rxrpc_see_txbuf(txb, rxrpc_txbuf_see_out_of_step);
 		ASSERTCMP(txb->seq, ==, call->tx_bottom + 1);
-		smp_store_release(&call->tx_bottom, call->tx_bottom + 1);
+		WRITE_ONCE(call->tx_bottom, call->tx_bottom + 1);
 		list_del_rcu(&txb->call_link);
 
 		trace_rxrpc_txqueue(call, rxrpc_txqueue_dequeue);