diff --git a/src/cmd/gc/sys.go b/src/cmd/gc/sys.go
index 1f34e0c..775b83e 100644
--- a/src/cmd/gc/sys.go
+++ b/src/cmd/gc/sys.go
@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 
-package SYS	// rename to avoid redeclaration
+package SYS	// rename to avoid redeclaration errors
 
 export func	mal(int32) *any;
 export func	breakpoint();
@@ -92,3 +92,5 @@
 
 export func	symdat() (symtab *[]byte, pclntab *[]byte);
 
+export func	semacquire(sema *int32);
+export func	semrelease(sema *int32);
diff --git a/src/cmd/gc/sysimport.c b/src/cmd/gc/sysimport.c
index ba09bb9..3a3dbce 100644
--- a/src/cmd/gc/sysimport.c
+++ b/src/cmd/gc/sysimport.c
@@ -71,5 +71,7 @@
 	"export func sys.stringtorune (? string, ? int) (? int, ? int)\n"
 	"export func sys.exit (? int)\n"
 	"export func sys.symdat () (symtab *[]uint8, pclntab *[]uint8)\n"
+	"export func sys.semacquire (sema *int32)\n"
+	"export func sys.semrelease (sema *int32)\n"
 	"\n"
 	"$$\n";
diff --git a/src/lib/Makefile b/src/lib/Makefile
index 7b68551..a50bdf0 100644
--- a/src/lib/Makefile
+++ b/src/lib/Makefile
@@ -18,6 +18,7 @@
 	reflect\
 	regexp\
 	strconv\
+	sync\
 	tabwriter\
 	time\
 
@@ -95,4 +96,5 @@
 strconv.dirinstall: os.dirinstall utf8.install
 tabwriter.dirinstall: os.dirinstall io.dirinstall container/array.dirinstall
 time.dirinstall: once.install os.dirinstall
+sync.dirinstall:
 
diff --git a/src/lib/sync/Makefile b/src/lib/sync/Makefile
new file mode 100644
index 0000000..86aeeef
--- /dev/null
+++ b/src/lib/sync/Makefile
@@ -0,0 +1,56 @@
+# Copyright 2009 The Go Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+# DO NOT EDIT.  Automatically generated by gobuild.
+# gobuild -m >Makefile
+O=6
+GC=$(O)g
+CC=$(O)c -w
+AS=$(O)a
+AR=$(O)ar
+
+default: packages
+
+clean:
+	rm -f *.$O *.a $O.out
+
+test: packages
+	gotest
+
+coverage: packages
+	gotest
+	6cov -g `pwd` | grep -v '_test\.go:'
+
+%.$O: %.go
+	$(GC) $*.go
+
+%.$O: %.c
+	$(CC) $*.c
+
+%.$O: %.s
+	$(AS) $*.s
+
+O1=\
+	asm_$(GOARCH).$O\
+	mutex.$O\
+
+sync.a: a1
+
+a1:	$(O1)
+	$(AR) grc sync.a asm_$(GOARCH).$O mutex.$O
+	rm -f $(O1)
+
+newpkg: clean
+	$(AR) grc sync.a
+
+$(O1): newpkg
+
+nuke: clean
+	rm -f $(GOROOT)/pkg/sync.a
+
+packages: sync.a
+
+install: packages
+	cp sync.a $(GOROOT)/pkg/sync.a
+
diff --git a/src/lib/sync/asm_amd64.s b/src/lib/sync/asm_amd64.s
new file mode 100644
index 0000000..07389dd
--- /dev/null
+++ b/src/lib/sync/asm_amd64.s
@@ -0,0 +1,23 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// func cas(val *int32, old, new int32) bool
+// Atomically:
+//	if *val == old {
+//		*val = new;
+//		return true;
+//	}else
+//		return false;
+TEXT sync·cas(SB), 7, $0
+	MOVQ	8(SP), BX
+	MOVL	16(SP), AX
+	MOVL	20(SP), CX
+	LOCK
+	CMPXCHGL	CX, 0(BX)
+	JZ ok
+	MOVL	$0, 24(SP)
+	RET
+ok:
+	MOVL	$1, 24(SP)
+	RET
diff --git a/src/lib/sync/mutex.go b/src/lib/sync/mutex.go
new file mode 100644
index 0000000..accf55a
--- /dev/null
+++ b/src/lib/sync/mutex.go
@@ -0,0 +1,39 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sync
+
+package func cas(val *int32, old, new int32) bool
+
+export type Mutex struct {
+	key int32;
+	sema int32;
+}
+
+func xadd(val *int32, delta int32) (new int32) {
+	for {
+		v := *val;
+		if cas(val, v, v+delta) {
+			return v+delta;
+		}
+	}
+	panic("unreached")
+}
+
+func (m *Mutex) Lock() {
+	if xadd(&m.key, 1) == 1 {
+		// changed from 0 to 1; we hold lock
+		return;
+	}
+	sys.semacquire(&m.sema);
+}
+
+func (m *Mutex) Unlock() {
+	if xadd(&m.key, -1) == 0 {
+		// changed from 1 to 0; no contention
+		return;
+	}
+	sys.semrelease(&m.sema);
+}
+
diff --git a/src/lib/sync/mutex_test.go b/src/lib/sync/mutex_test.go
new file mode 100644
index 0000000..7a6dd18
--- /dev/null
+++ b/src/lib/sync/mutex_test.go
@@ -0,0 +1,53 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// GOMAXPROCS=10 gotest
+
+package sync
+
+import (
+	"sync";
+	"testing"
+)
+
+func HammerSemaphore(s *int32, cdone *chan bool) {
+	for i := 0; i < 1000; i++ {
+		sys.semacquire(s);
+		sys.semrelease(s);
+	}
+	cdone <- true;
+}
+
+export func TestSemaphore(t *testing.T) {
+	s := new(int32);
+	*s = 1;
+	c := new(chan bool);
+	for i := 0; i < 10; i++ {
+		go HammerSemaphore(s, c);
+	}
+	for i := 0; i < 10; i++ {
+		<-c;
+	}
+}
+
+
+func HammerMutex(m *Mutex, cdone *chan bool) {
+	for i := 0; i < 1000; i++ {
+		m.Lock();
+		m.Unlock();
+	}
+	cdone <- true;
+}
+
+export func TestMutex(t *testing.T) {
+	m := new(Mutex);
+	c := new(chan bool);
+	for i := 0; i < 10; i++ {
+		go HammerMutex(m, c);
+	}
+	for i := 0; i < 10; i++ {
+		<-c;
+	}
+}
+
diff --git a/src/run.bash b/src/run.bash
index 4bba1bf..64795b5 100755
--- a/src/run.bash
+++ b/src/run.bash
@@ -34,6 +34,12 @@
 
 (xcd lib; make test) || exit $?
 
+(xcd lib/sync;
+make clean;
+time make
+GOMAXPROCS=10 make test
+) || exit $?
+
 (xcd ../usr/gri/pretty
 make clean
 time make
diff --git a/src/runtime/Makefile b/src/runtime/Makefile
index da12c2c..000c889 100644
--- a/src/runtime/Makefile
+++ b/src/runtime/Makefile
@@ -25,6 +25,7 @@
 	print.$O\
 	rune.$O\
 	proc.$O\
+	sema.$O\
 	stack.$O\
 	string.$O\
 	symtab.$O\
diff --git a/src/runtime/sema.c b/src/runtime/sema.c
new file mode 100644
index 0000000..117b079
--- /dev/null
+++ b/src/runtime/sema.c
@@ -0,0 +1,178 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Semaphore implementation exposed to Go.
+// Intended use is provide a sleep and wakeup
+// primitive that can be used in the contended case
+// of other synchronization primitives.
+// Thus it targets the same goal as Linux's futex,
+// but it has much simpler semantics.
+//
+// That is, don't think of these as semaphores.
+// Think of them as a way to implement sleep and wakeup
+// such that every sleep is paired with a single wakeup,
+// even if, due to races, the wakeup happens before the sleep.
+//
+// See Mullender and Cox, ``Semaphores in Plan 9,''
+// http://swtch.com/semaphore.pdf
+
+#include "runtime.h"
+
+typedef struct Sema Sema;
+struct Sema
+{
+	uint32 *addr;
+	G *g;
+	Sema *prev;
+	Sema *next;
+};
+
+// TODO: For now, a linked list; maybe a hash table of linked lists later.
+static Sema *semfirst, *semlast;
+static Lock semlock;
+
+static void
+semqueue(uint32 *addr, Sema *s)
+{
+	s->addr = addr;
+	s->g = nil;
+
+	lock(&semlock);
+	s->prev = semlast;
+	s->next = nil;
+	if(semlast)
+		semlast->next = s;
+	else
+		semfirst = s;
+	semlast = s;
+	unlock(&semlock);
+}
+
+static void
+semdequeue(Sema *s)
+{
+	lock(&semlock);
+	if(s->next)
+		s->next->prev = s->prev;
+	else
+		semlast = s->prev;
+	if(s->prev)
+		s->prev->next = s->next;
+	else
+		semfirst = s->next;
+	s->prev = nil;
+	s->next = nil;
+	unlock(&semlock);
+}
+
+static void
+semwakeup(uint32 *addr)
+{
+	Sema *s;
+
+	lock(&semlock);
+	for(s=semfirst; s; s=s->next) {
+		if(s->addr == addr && s->g) {
+			ready(s->g);
+			s->g = nil;
+			break;
+		}
+	}
+	unlock(&semlock);
+}
+
+// Step 1 of sleep: make ourselves available for wakeup.
+// TODO(rsc): Maybe we can write a version without
+// locks by using cas on s->g.  Maybe not: I need to
+// think more about whether it would be correct.
+static void
+semsleep1(Sema *s)
+{
+	lock(&semlock);
+	s->g = g;
+	unlock(&semlock);
+}
+
+// Decided not to go through with it: undo step 1.
+static void
+semsleepundo1(Sema *s)
+{
+	lock(&semlock);
+	if(s->g != nil) {
+		s->g = nil;	// back ourselves out
+	} else {
+		// If s->g == nil already, semwakeup
+		// already readied us.  Since we never stopped
+		// running, readying us just set g->readyonstop.
+		// Clear it.
+		if(g->readyonstop == 0)
+			*(int32*)0x555 = 555;
+		g->readyonstop = 0;
+	}
+	unlock(&semlock);
+}
+
+// Step 2: wait for the wakeup.
+static void
+semsleep2(Sema *s)
+{
+	USED(s);
+	g->status = Gwaiting;
+	sys·gosched();
+}
+
+static int32
+cansemacquire(uint32 *addr)
+{
+	uint32 v;
+
+	while((v = *addr) > 0)
+		if(cas(addr, v, v-1))
+			return 1;
+	return 0;
+}
+
+// func sys.semacquire(addr *uint32)
+// For now has no return value.
+// Might return an ok (not interrupted) bool in the future?
+void
+sys·semacquire(uint32 *addr)
+{
+	Sema s;
+
+	// Easy case.
+	if(cansemacquire(addr))
+		return;
+
+	// Harder case:
+	//	queue
+	//	try semacquire one more time, sleep if failed
+	//	dequeue
+	//	wake up one more guy to avoid races (TODO(rsc): maybe unnecessary?)
+	semqueue(addr, &s);
+	for(;;) {
+		semsleep1(&s);
+		if(cansemacquire(addr)) {
+			semsleepundo1(&s);
+			break;
+		}
+		semsleep2(&s);
+	}
+	semdequeue(&s);
+	semwakeup(addr);
+}
+
+// func sys.semrelease(addr *uint32)
+void
+sys·semrelease(uint32 *addr)
+{
+	uint32 v;
+
+	for(;;) {
+		v = *addr;
+		if(cas(addr, v, v+1))
+			break;
+	}
+	semwakeup(addr);
+}