diff --git a/ChangeLog b/ChangeLog
index 26dfcf9..f16a4ac 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,6 +1,6 @@
 2018-11-09  NIIBE Yutaka  <gniibe@fsij.org>
 
-	* eventflag.c (eventflag_wait_all): New.
+	* eventflag.c (eventflag_set_mask): New.
 
 2018-10-02  NIIBE Yutaka  <gniibe@fsij.org>
 
diff --git a/eventflag.c b/eventflag.c
index 2691713..69916ab 100644
--- a/eventflag.c
+++ b/eventflag.c
@@ -36,17 +36,29 @@ void
 eventflag_init (struct eventflag *ev)
 {
   ev->flags = 0;
+  ev->mask = ~0;
   chopstx_cond_init (&ev->cond);
   chopstx_mutex_init (&ev->mutex);
 }
 
 
+void
+eventflag_set_mask (struct eventflag *ev, eventmask_t m)
+{
+  chopstx_mutex_lock (&ev->mutex);
+  ev->mask = m;
+  if ((ev->flags & ev->mask))
+    chopstx_cond_signal (&ev->cond);
+  chopstx_mutex_unlock (&ev->mutex);
+}
+
+
 static int
 eventflag_check (void *arg)
 {
   struct eventflag *ev = arg;
 
-  return ev->flags != 0;
+  return (ev->flags & ev->mask) != 0;
 }
 
 
@@ -69,7 +81,7 @@ eventflag_get (struct eventflag *ev)
   eventmask_t m;
 
   chopstx_mutex_lock (&ev->mutex);
-  n = __builtin_ffs (ev->flags);
+  n = __builtin_ffs ((ev->flags & ev->mask));
   if (n)
     {
       m = (1 << (n - 1));
@@ -90,10 +102,10 @@ eventflag_wait (struct eventflag *ev)
   eventmask_t m;
 
   chopstx_mutex_lock (&ev->mutex);
-  if (!ev->flags)
+  while (!(ev->flags & ev->mask))
     chopstx_cond_wait (&ev->cond, &ev->mutex);
 
-  n = __builtin_ffs (ev->flags);
+  n = __builtin_ffs ((ev->flags & ev->mask));
   if (n) /* Always n > 0 when waked up, but make sure no bad things.  */
     {
       m = (1 << (n - 1));
@@ -107,17 +119,6 @@ eventflag_wait (struct eventflag *ev)
 }
 
 
-void
-eventflag_wait_all (struct eventflag *ev, eventmask_t m)
-{
-  chopstx_mutex_lock (&ev->mutex);
-  while ((ev->flags & m) != m)
-    chopstx_cond_wait (&ev->cond, &ev->mutex);
-  ev->flags &= ~m;
-  chopstx_mutex_unlock (&ev->mutex);
-}
-
-
 eventmask_t
 eventflag_wait_timeout (struct eventflag *ev, uint32_t usec)
 {
@@ -135,6 +136,7 @@ eventflag_signal (struct eventflag *ev, eventmask_t m)
 {
   chopstx_mutex_lock (&ev->mutex);
   ev->flags |= m;
-  chopstx_cond_signal (&ev->cond);
+  if ((ev->flags & ev->mask))
+    chopstx_cond_signal (&ev->cond);
   chopstx_mutex_unlock (&ev->mutex);
 }
diff --git a/eventflag.h b/eventflag.h
index f1bc074..7a9c145 100644
--- a/eventflag.h
+++ b/eventflag.h
@@ -2,13 +2,14 @@ typedef uint32_t eventmask_t;
 
 struct eventflag {
   eventmask_t flags;
+  eventmask_t mask;
   chopstx_mutex_t mutex;
   chopstx_cond_t cond;
 };
 
 void eventflag_init (struct eventflag *ev);
+void eventflag_set_mask (struct eventflag *ev, eventmask_t m);
 eventmask_t eventflag_wait (struct eventflag *ev);
-void eventflag_wait_all (struct eventflag *ev, eventmask_t m);
 eventmask_t eventflag_wait_timeout (struct eventflag *ev, uint32_t usec);
 void eventflag_signal (struct eventflag *ev, eventmask_t m);