Hi,

Sysctl KERN_MAXCLUSTERS only needs a mutex to keep nmbclust and
mbuf_mem_limit in sync.  All other read access happens atomically.

ok?

bluhm

Index: kern/kern_sysctl.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/kern/kern_sysctl.c,v
diff -u -p -r1.469 kern_sysctl.c
--- kern/kern_sysctl.c	1 Jun 2025 03:43:48 -0000	1.469
+++ kern/kern_sysctl.c	2 Jun 2025 18:27:49 -0000
@@ -553,6 +553,15 @@ kern_sysctl(int *name, u_int namelen, vo
 		microboottime(&bt);
 		return (sysctl_rdstruct(oldp, oldlenp, newp, &bt, sizeof bt));
 	}
+	case KERN_MAXCLUSTERS: {
+		int oval, nval;
+
+		oval = nval = atomic_load_long(&nmbclust);
+		error = sysctl_int(oldp, oldlenp, newp, newlen, &nval);
+		if (error == 0 && oval != nval)
+			error = nmbclust_update(nval);
+		return (error);
+	}
 	case KERN_MBSTAT: {
 		uint64_t counters[MBSTAT_COUNT];
 		struct mbstat mbs;
@@ -758,13 +767,6 @@ kern_sysctl_locked(int *name, u_int name
 			return (EINVAL);
 		stackgap_random = stackgap;
 		return (0);
-	case KERN_MAXCLUSTERS: {
-		int val = nmbclust;
-		error = sysctl_int(oldp, oldlenp, newp, newlen, &val);
-		if (error == 0 && val != nmbclust)
-			error = nmbclust_update(val);
-		return (error);
-	}
 	case KERN_CACHEPCT: {
 		u_int64_t dmapages;
 		int opct, pgs;
Index: kern/uipc_mbuf.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/kern/uipc_mbuf.c,v
diff -u -p -r1.296 uipc_mbuf.c
--- kern/uipc_mbuf.c	1 Jan 2025 13:44:22 -0000	1.296
+++ kern/uipc_mbuf.c	2 Jun 2025 18:27:49 -0000
@@ -129,6 +129,9 @@ struct	mutex m_extref_mtx = MUTEX_INITIA
 void	m_extfree(struct mbuf *);
 void	m_zero(struct mbuf *);
 
+/* keep nmbclust and mbuf_mem_limit in sync */
+struct mutex mbuf_nmbclust_mtx = MUTEX_INITIALIZER(IPL_NET);
+
 unsigned long mbuf_mem_limit;	/* [a] how much memory can be allocated */
 unsigned long mbuf_mem_alloc;	/* [a] how much memory has been allocated */
 
@@ -218,8 +221,10 @@ nmbclust_update(long newval)
 	if (newval <= 0 || newval > LONG_MAX / MCLBYTES)
 		return ERANGE;
 	/* update the global mbuf memory limit */
-	nmbclust = newval;
-	atomic_store_long(&mbuf_mem_limit, nmbclust * MCLBYTES);
+	mtx_enter(&mbuf_nmbclust_mtx);
+	atomic_store_long(&nmbclust, newval);
+	atomic_store_long(&mbuf_mem_limit, newval * MCLBYTES);
+	mtx_leave(&mbuf_nmbclust_mtx);
 
 	pool_wakeup(&mbpool);
 	for (i = 0; i < nitems(mclsizes); i++)
Index: net/pf_ioctl.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/net/pf_ioctl.c,v
diff -u -p -r1.421 pf_ioctl.c
--- net/pf_ioctl.c	22 May 2025 06:34:03 -0000	1.421
+++ net/pf_ioctl.c	2 Jun 2025 18:27:49 -0000
@@ -2134,7 +2134,8 @@ pfioctl(dev_t dev, u_long cmd, caddr_t a
 			goto fail;
 		}
 		/* Fragments reference mbuf clusters. */
-		if (pl->index == PF_LIMIT_FRAGS && pl->limit > nmbclust) {
+		if (pl->index == PF_LIMIT_FRAGS && pl->limit >
+		    (long)atomic_load_long(&nmbclust)) {
 			error = EINVAL;
 			PF_UNLOCK();
 			goto fail;