send/receive: use explicit unlikely branch instead of implicit coalescing

It's very unlikely that send will become true. It's nearly always false
between 0 and 120 seconds of a session, and in most cases becomes true
only between 120 and 121 seconds before becoming false again. So,
unlikely(send) is clearly the right option here.

What happened before was that we had this complex boolean expression
with multiple likely and unlikely clauses nested. Since this is
evaluated left-to-right anyway, the whole thing got converted to
unlikely. So, we can clean this up to better represent what's going on.

The generated code is the same.

Suggested-by: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

2 files changed, 12 insertions, 16 deletions

diff --git a/src/receive.c b/src/receive.c
index 6fa012c..c60d2ff 100644
--- a/src/receive.c
+++ b/src/receive.c
@@ -227,21 +227,20 @@ void wg_packet_handshake_receive_worker(struct work_struct *work)
 static void keep_key_fresh(struct wg_peer *peer)
 {
 	struct noise_keypair *keypair;
-	bool send = false;
+	bool send;
 
 	if (peer->sent_lastminute_handshake)
 		return;
 
 	rcu_read_lock_bh();
 	keypair = rcu_dereference_bh(peer->keypairs.current_keypair);
-	if (likely(keypair && READ_ONCE(keypair->sending.is_valid)) &&
-	    keypair->i_am_the_initiator &&
-	    unlikely(wg_birthdate_has_expired(keypair->sending.birthdate,
-			REJECT_AFTER_TIME - KEEPALIVE_TIMEOUT - REKEY_TIMEOUT)))
-		send = true;
+	send = keypair && READ_ONCE(keypair->sending.is_valid) &&
+	       keypair->i_am_the_initiator &&
+	       wg_birthdate_has_expired(keypair->sending.birthdate,
+			REJECT_AFTER_TIME - KEEPALIVE_TIMEOUT - REKEY_TIMEOUT);
 	rcu_read_unlock_bh();
 
-	if (send) {
+	if (unlikely(send)) {
 		peer->sent_lastminute_handshake = true;
 		wg_packet_send_queued_handshake_initiation(peer, false);
 	}
diff --git a/src/send.c b/src/send.c
index e6e89ad..3baae7f 100644
--- a/src/send.c
+++ b/src/send.c
@@ -125,20 +125,17 @@ void wg_packet_send_handshake_cookie(struct wg_device *wg,
 static void keep_key_fresh(struct wg_peer *peer)
 {
 	struct noise_keypair *keypair;
-	bool send = false;
+	bool send;
 
 	rcu_read_lock_bh();
 	keypair = rcu_dereference_bh(peer->keypairs.current_keypair);
-	if (likely(keypair && READ_ONCE(keypair->sending.is_valid)) &&
-	    (unlikely(atomic64_read(&keypair->sending.counter.counter) >
-		      REKEY_AFTER_MESSAGES) ||
-	     (keypair->i_am_the_initiator &&
-	      unlikely(wg_birthdate_has_expired(keypair->sending.birthdate,
-						REKEY_AFTER_TIME)))))
-		send = true;
+	send = keypair && READ_ONCE(keypair->sending.is_valid) &&
+	       (atomic64_read(&keypair->sending.counter.counter) > REKEY_AFTER_MESSAGES ||
+		(keypair->i_am_the_initiator &&
+		 wg_birthdate_has_expired(keypair->sending.birthdate, REKEY_AFTER_TIME)));
 	rcu_read_unlock_bh();
 
-	if (send)
+	if (unlikely(send))
 		wg_packet_send_queued_handshake_initiation(peer, false);
 }