Imported Upstream version 1.4upstream/1.4

1 files changed, 109 insertions, 0 deletions

diff --git a/src/sync/mutex.go b/src/sync/mutex.go
new file mode 100644
index 000000000..73b337702
--- /dev/null
+++ b/src/sync/mutex.go
@@ -0,0 +1,109 @@
+// 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 provides basic synchronization primitives such as mutual
+// exclusion locks.  Other than the Once and WaitGroup types, most are intended
+// for use by low-level library routines.  Higher-level synchronization is
+// better done via channels and communication.
+//
+// Values containing the types defined in this package should not be copied.
+package sync
+
+import (
+	"sync/atomic"
+	"unsafe"
+)
+
+// A Mutex is a mutual exclusion lock.
+// Mutexes can be created as part of other structures;
+// the zero value for a Mutex is an unlocked mutex.
+type Mutex struct {
+	state int32
+	sema  uint32
+}
+
+// A Locker represents an object that can be locked and unlocked.
+type Locker interface {
+	Lock()
+	Unlock()
+}
+
+const (
+	mutexLocked = 1 << iota // mutex is locked
+	mutexWoken
+	mutexWaiterShift = iota
+)
+
+// Lock locks m.
+// If the lock is already in use, the calling goroutine
+// blocks until the mutex is available.
+func (m *Mutex) Lock() {
+	// Fast path: grab unlocked mutex.
+	if atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked) {
+		if raceenabled {
+			raceAcquire(unsafe.Pointer(m))
+		}
+		return
+	}
+
+	awoke := false
+	for {
+		old := m.state
+		new := old | mutexLocked
+		if old&mutexLocked != 0 {
+			new = old + 1<<mutexWaiterShift
+		}
+		if awoke {
+			// The goroutine has been woken from sleep,
+			// so we need to reset the flag in either case.
+			new &^= mutexWoken
+		}
+		if atomic.CompareAndSwapInt32(&m.state, old, new) {
+			if old&mutexLocked == 0 {
+				break
+			}
+			runtime_Semacquire(&m.sema)
+			awoke = true
+		}
+	}
+
+	if raceenabled {
+		raceAcquire(unsafe.Pointer(m))
+	}
+}
+
+// Unlock unlocks m.
+// It is a run-time error if m is not locked on entry to Unlock.
+//
+// A locked Mutex is not associated with a particular goroutine.
+// It is allowed for one goroutine to lock a Mutex and then
+// arrange for another goroutine to unlock it.
+func (m *Mutex) Unlock() {
+	if raceenabled {
+		_ = m.state
+		raceRelease(unsafe.Pointer(m))
+	}
+
+	// Fast path: drop lock bit.
+	new := atomic.AddInt32(&m.state, -mutexLocked)
+	if (new+mutexLocked)&mutexLocked == 0 {
+		panic("sync: unlock of unlocked mutex")
+	}
+
+	old := new
+	for {
+		// If there are no waiters or a goroutine has already
+		// been woken or grabbed the lock, no need to wake anyone.
+		if old>>mutexWaiterShift == 0 || old&(mutexLocked|mutexWoken) != 0 {
+			return
+		}
+		// Grab the right to wake someone.
+		new = (old - 1<<mutexWaiterShift) | mutexWoken
+		if atomic.CompareAndSwapInt32(&m.state, old, new) {
+			runtime_Semrelease(&m.sema)
+			return
+		}
+		old = m.state
+	}
+}