Index: head/share/man/man9/EVENTHANDLER.9
===================================================================
--- head/share/man/man9/EVENTHANDLER.9
+++ head/share/man/man9/EVENTHANDLER.9
@@ -23,7 +23,7 @@
 .\" SUCH DAMAGE.
 .\" $FreeBSD$
 .\"
-.Dd March 27, 2017
+.Dd October 1, 2017
 .Dt EVENTHANDLER 9
 .Os
 .Sh NAME
@@ -37,6 +37,7 @@
 .Ft eventhandler_tag
 .Fn EVENTHANDLER_REGISTER name func arg priority
 .Fn EVENTHANDLER_DEREGISTER name tag
+.Fn EVENTHANDLER_DEREGISTER_NOWAIT name tag
 .Ft eventhandler_tag
 .Fo eventhandler_register
 .Fa "struct eventhandler_list *list"
@@ -50,6 +51,11 @@
 .Fa "struct eventhandler_list *list"
 .Fa "eventhandler_tag tag"
 .Fc
+.Ft void
+.Fo eventhandler_deregister_nowait
+.Fa "struct eventhandler_list *list"
+.Fa "eventhandler_tag tag"
+.Fc
 .Ft "struct eventhandler_list *"
 .Fn eventhandler_find_list "const char *name"
 .Ft void
@@ -121,6 +127,18 @@
 .Fa tag
 from the event handler named by argument
 .Fa name .
+It waits until no threads are running handlers for this event before
+returning, making it safe to unload a module immediately upon return
+from this function.
+.It Fn EVENTHANDLER_DEREGISTER_NOWAIT
+This macro removes a previously registered callback associated with tag
+.Fa tag
+from the event handler named by argument
+.Fa name .
+Upon return, one or more threads could still be running the removed
+function(s), but no new calls will be made.
+To remove a handler function from within that function, use this
+version of deregister, to avoid a deadlock.
 .It Fn EVENTHANDLER_INVOKE
 This macro is used to invoke all the callbacks associated with event
 handler
@@ -176,6 +194,21 @@
 .Fn eventhandler_deregister
 to remove the particular callback function.
 .It Fn eventhandler_deregister
+The
+.Fn eventhandler_deregister
+function removes the callback associated with tag
+.Fa tag
+from the event handler list pointed to by
+.Fa list .
+If
+.Fa tag
+is
+.Va NULL ,
+all callback functions for the event are removed.
+This function will not return until all threads have exited from the
+removed handler callback function(s).
+This function is not safe to call from inside an event handler callback.
+.It Fn eventhandler_deregister_nowait
 The
 .Fn eventhandler_deregister
 function removes the callback associated with tag
Index: head/sys/kern/subr_eventhandler.c
===================================================================
--- head/sys/kern/subr_eventhandler.c
+++ head/sys/kern/subr_eventhandler.c
@@ -180,8 +180,9 @@
 }
 #endif
 
-void
-eventhandler_deregister(struct eventhandler_list *list, eventhandler_tag tag)
+static void
+_eventhandler_deregister(struct eventhandler_list *list, eventhandler_tag tag,
+    bool wait)
 {
     struct eventhandler_entry	*ep = tag;
 
@@ -215,9 +216,24 @@
 		ep->ee_priority = EHE_DEAD_PRIORITY;
 	}
     }
-    while (list->el_runcount > 0)
+    while (wait && list->el_runcount > 0)
 	    mtx_sleep(list, &list->el_lock, 0, "evhrm", 0);
     EHL_UNLOCK(list);
+}
+
+void
+eventhandler_deregister(struct eventhandler_list *list, eventhandler_tag tag)
+{
+
+	_eventhandler_deregister(list, tag, true);
+}
+
+void
+eventhandler_deregister_nowait(struct eventhandler_list *list,
+    eventhandler_tag tag)
+{
+
+	_eventhandler_deregister(list, tag, false);
 }
 
 /*
Index: head/sys/sys/eventhandler.h
===================================================================
--- head/sys/sys/eventhandler.h
+++ head/sys/sys/eventhandler.h
@@ -141,11 +141,20 @@
 	if ((_el = eventhandler_find_list(#name)) != NULL)		\
 		eventhandler_deregister(_el, tag);			\
 } while(0)
-	
 
+#define EVENTHANDLER_DEREGISTER_NOWAIT(name, tag)			\
+do {									\
+	struct eventhandler_list *_el;					\
+									\
+	if ((_el = eventhandler_find_list(#name)) != NULL)		\
+		eventhandler_deregister_nowait(_el, tag);		\
+} while(0)
+
 eventhandler_tag eventhandler_register(struct eventhandler_list *list, 
 	    const char *name, void *func, void *arg, int priority);
 void	eventhandler_deregister(struct eventhandler_list *list,
+	    eventhandler_tag tag);
+void	eventhandler_deregister_nowait(struct eventhandler_list *list,
 	    eventhandler_tag tag);
 struct eventhandler_list *eventhandler_find_list(const char *name);
 void	eventhandler_prune_list(struct eventhandler_list *list);