Index: head/sys/dev/hyperv/vmbus/hv_channel.c
===================================================================
--- head/sys/dev/hyperv/vmbus/hv_channel.c
+++ head/sys/dev/hyperv/vmbus/hv_channel.c
@@ -37,12 +37,16 @@
 #include <sys/lock.h>
 #include <sys/mutex.h>
 #include <sys/sysctl.h>
+
+#include <machine/atomic.h>
 #include <machine/bus.h>
+
 #include <vm/vm.h>
 #include <vm/vm_param.h>
 #include <vm/pmap.h>
 
 #include <dev/hyperv/vmbus/hv_vmbus_priv.h>
+#include <dev/hyperv/vmbus/vmbus_reg.h>
 #include <dev/hyperv/vmbus/vmbus_var.h>
 
 static int 	vmbus_channel_create_gpadl_header(
@@ -65,11 +69,11 @@
 	if (channel->offer_msg.monitor_allocated) {
 		struct vmbus_softc *sc = vmbus_get_softc();
 		hv_vmbus_monitor_page *monitor_page;
+		uint32_t chanid = channel->offer_msg.child_rel_id;
 
-		/* Each uint32_t represents 32 channels */
-		synch_set_bit((channel->offer_msg.child_rel_id & 31),
-			((uint32_t *)sc->vmbus_tx_evtflags
-				+ ((channel->offer_msg.child_rel_id >> 5))));
+		atomic_set_long(
+		    &sc->vmbus_tx_evtflags[chanid >> VMBUS_EVTFLAG_SHIFT],
+		    1UL << (chanid & VMBUS_EVTFLAG_MASK));
 
 		monitor_page = sc->vmbus_mnf2;
 		synch_set_bit(channel->monitor_bit,
Index: head/sys/dev/hyperv/vmbus/hv_connection.c
===================================================================
--- head/sys/dev/hyperv/vmbus/hv_connection.c
+++ head/sys/dev/hyperv/vmbus/hv_connection.c
@@ -353,12 +353,10 @@
 {
 	struct vmbus_softc *sc = vmbus_get_softc();
 	int ret = 0;
-	uint32_t child_rel_id = channel->offer_msg.child_rel_id;
+	uint32_t chanid = channel->offer_msg.child_rel_id;
 
-	/* Each uint32_t represents 32 channels */
-
-	synch_set_bit(child_rel_id & 31,
-		(((uint32_t *)sc->vmbus_tx_evtflags + (child_rel_id >> 5))));
+	atomic_set_long(&sc->vmbus_tx_evtflags[chanid >> VMBUS_EVTFLAG_SHIFT],
+	    1UL << (chanid & VMBUS_EVTFLAG_MASK));
 	ret = hv_vmbus_signal_event(channel->signal_event_param);
 
 	return (ret);
Index: head/sys/dev/hyperv/vmbus/vmbus.c
===================================================================
--- head/sys/dev/hyperv/vmbus/vmbus.c
+++ head/sys/dev/hyperv/vmbus/vmbus.c
@@ -340,7 +340,7 @@
 	if (evtflags == NULL)
 		return ENOMEM;
 	sc->vmbus_rx_evtflags = (u_long *)evtflags;
-	sc->vmbus_tx_evtflags = evtflags + (PAGE_SIZE / 2);
+	sc->vmbus_tx_evtflags = (u_long *)(evtflags + (PAGE_SIZE / 2));
 	sc->vmbus_evtflags = evtflags;
 
 	sc->vmbus_mnf1 = hyperv_dmamem_alloc(parent_dtag, PAGE_SIZE, 0,
Index: head/sys/dev/hyperv/vmbus/vmbus_reg.h
===================================================================
--- head/sys/dev/hyperv/vmbus/vmbus_reg.h
+++ head/sys/dev/hyperv/vmbus/vmbus_reg.h
@@ -66,6 +66,7 @@
 #define VMBUS_EVTFLAG_SHIFT	5
 #endif
 #define VMBUS_EVTFLAG_LEN	(1 << VMBUS_EVTFLAG_SHIFT)
+#define VMBUS_EVTFLAG_MASK	(VMBUS_EVTFLAG_LEN - 1)
 #define VMBUS_EVTFLAGS_SIZE	256
 
 struct vmbus_evtflags {
Index: head/sys/dev/hyperv/vmbus/vmbus_var.h
===================================================================
--- head/sys/dev/hyperv/vmbus/vmbus_var.h
+++ head/sys/dev/hyperv/vmbus/vmbus_var.h
@@ -63,7 +63,7 @@
 
 struct vmbus_softc {
 	void			(*vmbus_event_proc)(struct vmbus_softc *, int);
-	void			*vmbus_tx_evtflags;
+	u_long			*vmbus_tx_evtflags;
 						/* event flags to host */
 	void			*vmbus_mnf2;	/* monitored by host */