`msgbufp' and `consbufp' are immutable, such as `msg_magic' and
`msg_bufs'. initmsgbuf() and initconsbuf() which initialize this buffers
are called during kernel bootstrap, when concurrent sysctl(2) is
impossible, so they don't need to be reordered or use barriers.

ok?

Index: sys/kern/kern_sysctl.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_sysctl.c,v
diff -u -p -r1.434 kern_sysctl.c
--- sys/kern/kern_sysctl.c	6 Aug 2024 12:36:54 -0000	1.434
+++ sys/kern/kern_sysctl.c	7 Aug 2024 09:22:27 -0000
@@ -533,6 +533,18 @@ kern_sysctl(int *name, u_int namelen, vo
 		return (sysctl_rdstruct(oldp, oldlenp, newp,
 		    &mbs, sizeof(mbs)));
 	}
+	case KERN_MSGBUFSIZE:
+	case KERN_CONSBUFSIZE: {
+		struct msgbuf *mp;
+		mp = (name[0] == KERN_MSGBUFSIZE) ? msgbufp : consbufp;
+		/*
+		 * deal with cases where the message buffer has
+		 * become corrupted.
+		 */
+		if (!mp || mp->msg_magic != MSG_MAGIC)
+			return (ENXIO);
+		return (sysctl_rdint(oldp, oldlenp, newp, mp->msg_bufs));
+	}
 	case KERN_OSREV:
 	case KERN_NFILES:
 	case KERN_TTYCOUNT:
@@ -616,18 +628,6 @@ kern_sysctl_locked(int *name, u_int name
 		error =  sysctl_int(oldp, oldlenp, newp, newlen, &inthostid);
 		hostid = inthostid;
 		return (error);
-	case KERN_MSGBUFSIZE:
-	case KERN_CONSBUFSIZE: {
-		struct msgbuf *mp;
-		mp = (name[0] == KERN_MSGBUFSIZE) ? msgbufp : consbufp;
-		/*
-		 * deal with cases where the message buffer has
-		 * become corrupted.
-		 */
-		if (!mp || mp->msg_magic != MSG_MAGIC)
-			return (ENXIO);
-		return (sysctl_rdint(oldp, oldlenp, newp, mp->msg_bufs));
-	}
 	case KERN_CONSBUF:
 		if ((error = suser(p)))
 			return (error);
@@ -635,7 +635,10 @@ kern_sysctl_locked(int *name, u_int name
 	case KERN_MSGBUF: {
 		struct msgbuf *mp;
 		mp = (name[0] == KERN_MSGBUF) ? msgbufp : consbufp;
-		/* see note above */
+		/*
+		 * deal with cases where the message buffer has
+		 * become corrupted.
+		 */
 		if (!mp || mp->msg_magic != MSG_MAGIC)
 			return (ENXIO);
 		return (sysctl_rdstruct(oldp, oldlenp, newp, mp,