diff --git a/sys/kern/kern_shutdown.c b/sys/kern/kern_shutdown.c
--- a/sys/kern/kern_shutdown.c
+++ b/sys/kern/kern_shutdown.c
@@ -224,8 +224,7 @@
  * Variable panicstr contains argument to first call to panic; used as flag
  * to indicate that the kernel has already called panic.
  */
-const char *panicstr;
-bool __read_frequently panicked;
+const char *panicstr __read_mostly;
 
 int __read_mostly dumping;		/* system is dumping */
 int rebooting;				/* system is rebooting */
@@ -899,6 +898,15 @@
 	int bootopt, newpanic;
 	static char buf[256];
 
+	/*
+	 * 'fmt' must not be NULL as it is put into 'panicstr' which is then
+	 * used as a flag to detect if the kernel has panicked.  Also, although
+	 * vsnprintf() supports a NULL 'fmt' argument, use a more informative
+	 * message.
+	 */
+	if (fmt == NULL)
+		fmt = "<no panic string!>";
+
 	spinlock_enter();
 
 #ifdef SMP
@@ -907,7 +915,7 @@
 	 * concurrently entering panic.  Only the winner will proceed
 	 * further.
 	 */
-	if (panicstr == NULL && !kdb_active) {
+	if (KERNEL_PANICKED() && !kdb_active) {
 		other_cpus = all_cpus;
 		CPU_CLR(PCPU_GET(cpuid), &other_cpus);
 		stop_cpus_hard(other_cpus);
@@ -927,7 +935,6 @@
 	else {
 		bootopt |= RB_DUMP;
 		panicstr = fmt;
-		panicked = true;
 		newpanic = 1;
 	}
 
diff --git a/sys/sys/kassert.h b/sys/sys/kassert.h
--- a/sys/sys/kassert.h
+++ b/sys/sys/kassert.h
@@ -35,8 +35,7 @@
 
 #ifdef _KERNEL
 extern const char *panicstr;	/* panic message */
-extern bool panicked;
-#define	KERNEL_PANICKED()	__predict_false(panicked)
+#define	KERNEL_PANICKED()	__predict_false(panicstr != NULL)
 
 /*
  * Trap accesses going through a pointer. Moreover if kasan is available trap