// 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
    }
}

type Mutex struct {
        key  int32
        sema uint32
}

func (m *Mutex) Lock() {
if atomic.AddInt32(&m.key, 1) == 1 {
// changed from 0 to 1; we hold lock
return
        }
        runtime_Semacquire(&m.sema)
}

func (m *Mutex) Unlock() {
switch v := atomic.AddInt32(&m.key, -1); {
case v == 0:
// changed from 1 to 0; no contention
return
case v == -1:
// changed from 0 to -1: wasn't locked
// (or there are 4 billion goroutines waiting)
                panic("sync: unlock of unlocked mutex")
        }
        runtime_Semrelease(&m.sema)
}