timeout(9) statistics protected by `timeout_mutex', so it could be
easily unlocked. However, I want to introduce additional `tos_lock'
rwlock(9) to disallow simultaneous `timeout_mutex' acquisition from
timeout_sysctl() threads.

Note, we can't use per-cpu counters because timeouts starts to be used
too early.

Index: sys/kern/kern_sysctl.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_sysctl.c,v
diff -u -p -r1.448 kern_sysctl.c
--- sys/kern/kern_sysctl.c	30 Sep 2024 12:32:26 -0000	1.448
+++ sys/kern/kern_sysctl.c	21 Oct 2024 10:38:22 -0000
@@ -564,6 +564,8 @@ kern_sysctl(int *name, u_int namelen, vo
 			return (ENXIO);
 		return (sysctl_rdint(oldp, oldlenp, newp, mp->msg_bufs));
 	}
+	case KERN_TIMEOUT_STATS:
+		return (timeout_sysctl(oldp, oldlenp, newp, newlen));
 	case KERN_OSREV:
 	case KERN_MAXPROC:
 	case KERN_MAXFILES:
@@ -736,8 +738,6 @@ kern_sysctl_locked(int *name, u_int name
 	case KERN_PFSTATUS:
 		return (pf_sysctl(oldp, oldlenp, newp, newlen));
 #endif
-	case KERN_TIMEOUT_STATS:
-		return (timeout_sysctl(oldp, oldlenp, newp, newlen));
 	case KERN_UTC_OFFSET:
 		return (sysctl_utc_offset(oldp, oldlenp, newp, newlen));
 	default:
Index: sys/kern/kern_timeout.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_timeout.c,v
diff -u -p -r1.99 kern_timeout.c
--- sys/kern/kern_timeout.c	11 Aug 2024 00:49:34 -0000	1.99
+++ sys/kern/kern_timeout.c	21 Oct 2024 10:38:22 -0000
@@ -873,14 +873,22 @@ timeout_adjust_ticks(int adj)
 }
 #endif
 
+/*
+ * Disallow simultaneous `timeout_mutex' acquisition to timeout_sysctl()
+ * threads.
+ */
+struct rwlock tos_lock = RWLOCK_INITIALIZER("toslk");
+
 int
 timeout_sysctl(void *oldp, size_t *oldlenp, void *newp, size_t newlen)
 {
 	struct timeoutstat status;
 
+	rw_enter_write(&tos_lock);
 	mtx_enter(&timeout_mutex);
 	memcpy(&status, &tostat, sizeof(status));
 	mtx_leave(&timeout_mutex);
+	rw_exit_write(&tos_lock);
 
 	return sysctl_rdstruct(oldp, oldlenp, newp, &status, sizeof(status));
 }