Index: head/sys/amd64/amd64/efirt.c
===================================================================
--- head/sys/amd64/amd64/efirt.c
+++ head/sys/amd64/amd64/efirt.c
@@ -46,6 +46,7 @@
 #include <sys/sysctl.h>
 #include <sys/systm.h>
 #include <sys/vmmeter.h>
+#include <isa/rtc.h>
 #include <machine/fpu.h>
 #include <machine/efi.h>
 #include <machine/metadata.h>
@@ -445,7 +446,7 @@
 	efi_status status;
 	int error;
 
-	mtx_assert(&resettodr_lock, MA_OWNED);
+	mtx_assert(&atrtc_time_lock, MA_OWNED);
 	error = efi_enter();
 	if (error != 0)
 		return (error);
@@ -462,9 +463,9 @@
 
 	if (efi_runtime == NULL)
 		return (ENXIO);
-	mtx_lock(&resettodr_lock);
+	mtx_lock(&atrtc_time_lock);
 	error = efi_get_time_locked(tm);
-	mtx_unlock(&resettodr_lock);
+	mtx_unlock(&atrtc_time_lock);
 	return (error);
 }
 
@@ -487,7 +488,7 @@
 	efi_status status;
 	int error;
 
-	mtx_assert(&resettodr_lock, MA_OWNED);
+	mtx_assert(&atrtc_time_lock, MA_OWNED);
 	error = efi_enter();
 	if (error != 0)
 		return (error);
@@ -504,9 +505,9 @@
 
 	if (efi_runtime == NULL)
 		return (ENXIO);
-	mtx_lock(&resettodr_lock);
+	mtx_lock(&atrtc_time_lock);
 	error = efi_set_time_locked(tm);
-	mtx_unlock(&resettodr_lock);
+	mtx_unlock(&atrtc_time_lock);
 	return (error);
 }
 
Index: head/sys/isa/rtc.h
===================================================================
--- head/sys/isa/rtc.h
+++ head/sys/isa/rtc.h
@@ -113,6 +113,7 @@
 
 #ifdef _KERNEL
 extern  struct mtx clock_lock;
+extern  struct mtx atrtc_time_lock;
 extern	int atrtcclock_disable;
 int	rtcin(int reg);
 void	atrtc_restore(void);
Index: head/sys/x86/isa/atrtc.c
===================================================================
--- head/sys/x86/isa/atrtc.c
+++ head/sys/x86/isa/atrtc.c
@@ -53,9 +53,17 @@
 #include <machine/intr_machdep.h>
 #include "clock_if.h"
 
+/*
+ * clock_lock protects low-level access to individual hardware registers.
+ * atrtc_time_lock protects the entire sequence of accessing multiple registers
+ * to read or write the date and time.
+ */
 #define	RTC_LOCK	do { if (!kdb_active) mtx_lock_spin(&clock_lock); } while (0)
 #define	RTC_UNLOCK	do { if (!kdb_active) mtx_unlock_spin(&clock_lock); } while (0)
 
+struct mtx atrtc_time_lock;
+MTX_SYSINIT(atrtc_lock_init, &atrtc_time_lock, "atrtc", MTX_DEF);
+
 int	atrtcclock_disable = 0;
 
 static	int	rtc_reg = -1;
@@ -163,6 +171,8 @@
 
 	clock_ts_to_ct(ts, &ct);
 
+	mtx_lock(&atrtc_time_lock);
+
 	/* Disable RTC updates and interrupts. */
 	writertc(RTC_STATUSB, RTCSB_HALT | RTCSB_24HR);
 
@@ -181,6 +191,8 @@
 	/* Re-enable RTC updates and interrupts. */
 	writertc(RTC_STATUSB, rtc_statusb);
 	rtcin(RTC_INTR);
+
+	mtx_unlock(&atrtc_time_lock);
 }
 
 /**********************************************************************
@@ -352,6 +364,7 @@
 	 * to make sure that no more than 240us pass after we start reading,
 	 * and try again if so.
 	 */
+	mtx_lock(&atrtc_time_lock);
 	while (rtcin(RTC_STATUSA) & RTCSA_TUP)
 		continue;
 	critical_enter();
@@ -369,6 +382,7 @@
 	ct.year += (ct.year < 80 ? 2000 : 1900);
 #endif
 	critical_exit();
+	mtx_unlock(&atrtc_time_lock);
 	/* Set dow = -1 because some clocks don't set it correctly. */
 	ct.dow = -1;
 	return (clock_ct_to_ts(&ct, ts));