diff --git a/src/runtime/rt1_amd64_darwin.c b/src/runtime/rt1_amd64_darwin.c
index 02f03cdf720..2456bd78887 100644
--- a/src/runtime/rt1_amd64_darwin.c
+++ b/src/runtime/rt1_amd64_darwin.c
@@ -262,7 +262,7 @@ xadd(uint32 volatile *val, int32 delta)
 // releases the lock by decrementing l->key, l->key will
 // be >0, so it will increment the semaphore to wake up
 // one of the others.  This is the same algorithm used
-// in Plan 9's user-space locks.
+// in Plan 9's user-level locks.
 //
 // Note that semaphores are never destroyed (the kernel
 // will clean up when the process exits).  We assume for now
@@ -287,6 +287,25 @@ unlock(Lock *l)
 }
 
 
+// User-level semaphore implementation:
+// try to do the operations in user space on u,
+// but when it's time to block, fall back on the kernel semaphore k.
+// This is the same algorithm used in Plan 9.
+void
+usemacquire(Usema *s)
+{
+	if((int32)xadd(&s->u, -1) < 0)
+		semacquire(s->k);
+}
+
+void
+usemrelease(Usema *s)
+{
+	if((int32)xadd(&s->u, 1) <= 0)
+		semrelease(s->k);
+}
+
+
 // Event notifications.
 void
 noteclear(Note *n)
@@ -297,19 +316,19 @@ noteclear(Note *n)
 void
 notesleep(Note *n)
 {
-	if(n->sema == 0)
-		initsema(&n->sema);
+	if(n->sema.k == 0)
+		initsema(&n->sema.k);
 	while(!n->wakeup)
-		semacquire(n->sema);
+		usemacquire(&n->sema);
 }
 
 void
 notewakeup(Note *n)
 {
-	if(n->sema == 0)
-		initsema(&n->sema);
+	if(n->sema.k == 0)
+		initsema(&n->sema.k);
 	n->wakeup = 1;
-	semrelease(n->sema);
+	usemrelease(&n->sema);
 }
 
 
diff --git a/src/runtime/runtime.h b/src/runtime/runtime.h
index 3d439ca4524..411b6046bfa 100644
--- a/src/runtime/runtime.h
+++ b/src/runtime/runtime.h
@@ -43,6 +43,7 @@ typedef	struct	Alg		Alg;
 typedef	struct	Lock		Lock;
 typedef	union	Note	Note;
 typedef	struct	Mem		Mem;
+typedef	struct	Usema	Usema;
 
 /*
  * per cpu declaration
@@ -77,6 +78,11 @@ struct	Lock
 	uint32	key;
 	uint32	sema;	// for OS X
 };
+struct	Usema
+{
+	uint32	u;
+	uint32	k;
+};
 union	Note
 {
 	struct {	// Linux
@@ -84,7 +90,7 @@ union	Note
 	};
 	struct {	// OS X
 		int32	wakeup;
-		uint32	sema;
+		Usema	sema;
 	};
 };
 struct String