diff --git a/sys/netinet/tcp_subr.c b/sys/netinet/tcp_subr.c
--- a/sys/netinet/tcp_subr.c
+++ b/sys/netinet/tcp_subr.c
@@ -101,6 +101,9 @@
 #endif
 
 #include <netinet/tcp.h>
+#ifdef INVARIANTS
+#define TCPSTATES
+#endif
 #include <netinet/tcp_fsm.h>
 #include <netinet/tcp_seq.h>
 #include <netinet/tcp_timer.h>
@@ -1755,9 +1758,12 @@
 	int isipv6;
 #endif /* INET6 */
 	int optlen, tlen, win, ulen;
-	bool incl_opts, lock_upgraded;
+	bool incl_opts;
 	uint16_t port;
 	int output_ret;
+#ifdef INVARIANTS
+	int thflags = th->th_flags;
+#endif
 
 	KASSERT(tp != NULL || m != NULL, ("tcp_respond: tp and m both NULL"));
 	NET_EPOCH_ASSERT();
@@ -2088,17 +2094,11 @@
 	TCP_PROBE3(debug__output, tp, th, m);
 	if (flags & TH_RST)
 		TCP_PROBE5(accept__refused, NULL, NULL, m, tp, nth);
-	lock_upgraded = false;
 	lgb = NULL;
 	if ((tp != NULL) && (tp->t_logstate != TCP_LOG_STATE_OFF)) {
 		union tcp_log_stackspecific log;
 		struct timeval tv;
 
-		lock_upgraded = !INP_WLOCKED(inp) && INP_TRY_UPGRADE(inp);
-		/*
-		 *`If we don't already own the write lock and can't upgrade,
-		 * just don't log the event, but still send the response.
-		 */
 		if (INP_WLOCKED(inp)) {
 			memset(&log.u_bbr, 0, sizeof(log.u_bbr));
 			log.u_bbr.inhpts = tp->t_inpcb->inp_in_hpts;
@@ -2109,6 +2109,23 @@
 			log.u_bbr.delivered = 0;
 			lgb = tcp_log_event_(tp, nth, NULL, NULL, TCP_LOG_OUT, ERRNO_UNK,
 			                     0, &log, false, NULL, NULL, 0, &tv);
+		} else {
+			/*
+			 * We can not log the packet, since we only own the
+			 * read lock, but a write lock is needed.
+			 * This happens only for pure SYN segments received in
+			 * the initial CLOSED state or received in a state
+			 * later then listen is the UDP encapsulation port is
+			 * unexpected.
+			 * Ensure that the above description is and stays
+			 * correct.
+			 */
+			KASSERT(((thflags & (TH_ACK|TH_SYN)) == TH_SYN) &&
+			        ((tp->t_state == TCPS_CLOSED) ||
+			         ((tp->t_state > TCPS_LISTEN) && (tp->t_port != port))),
+			        ("%s: Logging of TCP segment with flags 0x%b and UDP encapsulation port %u skipped in state %s",
+			        __func__, thflags, PRINT_TH_FLAGS,
+			        ntohs(port), tcpstates[tp->t_state]));
 		}
 	}
 
@@ -2129,8 +2146,6 @@
 #endif
 	if (lgb != NULL)
 		lgb->tlb_errno = output_ret;
-	if (lock_upgraded)
-		INP_DOWNGRADE(inp);
 }
 
 /*