sysctl_mq() is wrapped by sysctl_niq() macro and called by ip_sysctl().

Actually it is mp-safe, but I want to use "oldval != newval" style to
avoid `mq_mtx' acquisition while `mq_maxlen' was not changed. It has
type of unsigned int, but protected with `mq_mtx' mutex(9), so I think
to use READ_ONCE() to ensure that `mq_maxlen' actually loaded to
`oldval'. mq_set_maxlen() takes `mq_mtx' mutex(9) within.

Index: sys/kern/uipc_mbuf.c
===================================================================
RCS file: /cvs/src/sys/kern/uipc_mbuf.c,v
retrieving revision 1.300
diff -u -p -r1.300 uipc_mbuf.c
--- sys/kern/uipc_mbuf.c	25 Jun 2025 20:26:32 -0000	1.300
+++ sys/kern/uipc_mbuf.c	15 Jul 2025 22:58:03 -0000
@@ -1839,7 +1839,7 @@ int
 sysctl_mq(int *name, u_int namelen, void *oldp, size_t *oldlenp,
     void *newp, size_t newlen, struct mbuf_queue *mq)
 {
-	unsigned int maxlen;
+	unsigned int oldval, newval;
 	int error;
 
 	/* All sysctl names at this level are terminal. */
@@ -1850,10 +1850,10 @@ sysctl_mq(int *name, u_int namelen, void
 	case IFQCTL_LEN:
 		return (sysctl_rdint(oldp, oldlenp, newp, mq_len(mq)));
 	case IFQCTL_MAXLEN:
-		maxlen = mq->mq_maxlen;
-		error = sysctl_int(oldp, oldlenp, newp, newlen, &maxlen);
-		if (error == 0)
-			mq_set_maxlen(mq, maxlen);
+		oldval = newval = READ_ONCE(mq->mq_maxlen);
+		error = sysctl_int(oldp, oldlenp, newp, newlen, &newval);
+		if (error == 0 && oldval != newval)
+			mq_set_maxlen(mq, newval);
 		return (error);
 	case IFQCTL_DROPS:
 		return (sysctl_rdint(oldp, oldlenp, newp, mq_drops(mq)));