Source file src/pkg/sync/mutex.go

     1	// Copyright 2009 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	// Package sync provides basic synchronization primitives such as mutual
     6	// exclusion locks.  Other than the Once and WaitGroup types, most are intended
     7	// for use by low-level library routines.  Higher-level synchronization is
     8	// better done via channels and communication.
     9	package sync
    10	
    11	import (
    12		"runtime"
    13		"sync/atomic"
    14	)
    15	
    16	// A Mutex is a mutual exclusion lock.
    17	// Mutexes can be created as part of other structures;
    18	// the zero value for a Mutex is an unlocked mutex.
    19	type Mutex struct {
    20		state int32
    21		sema  uint32
    22	}
    23	
    24	// A Locker represents an object that can be locked and unlocked.
    25	type Locker interface {
    26		Lock()
    27		Unlock()
    28	}
    29	
    30	const (
    31		mutexLocked = 1 << iota // mutex is locked
    32		mutexWoken
    33		mutexWaiterShift = iota
    34	)
    35	
    36	// Lock locks m.
    37	// If the lock is already in use, the calling goroutine
    38	// blocks until the mutex is available.
    39	func (m *Mutex) Lock() {
    40		// Fast path: grab unlocked mutex.
    41		if atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked) {
    42			return
    43		}
    44	
    45		awoke := false
    46		for {
    47			old := m.state
    48			new := old | mutexLocked
    49			if old&mutexLocked != 0 {
    50				new = old + 1<<mutexWaiterShift
    51			}
    52			if awoke {
    53				// The goroutine has been woken from sleep,
    54				// so we need to reset the flag in either case.
    55				new &^= mutexWoken
    56			}
    57			if atomic.CompareAndSwapInt32(&m.state, old, new) {
    58				if old&mutexLocked == 0 {
    59					break
    60				}
    61				runtime.Semacquire(&m.sema)
    62				awoke = true
    63			}
    64		}
    65	}
    66	
    67	// Unlock unlocks m.
    68	// It is a run-time error if m is not locked on entry to Unlock.
    69	//
    70	// A locked Mutex is not associated with a particular goroutine.
    71	// It is allowed for one goroutine to lock a Mutex and then
    72	// arrange for another goroutine to unlock it.
    73	func (m *Mutex) Unlock() {
    74		// Fast path: drop lock bit.
    75		new := atomic.AddInt32(&m.state, -mutexLocked)
    76		if (new+mutexLocked)&mutexLocked == 0 {
    77			panic("sync: unlock of unlocked mutex")
    78		}
    79	
    80		old := new
    81		for {
    82			// If there are no waiters or a goroutine has already
    83			// been woken or grabbed the lock, no need to wake anyone.
    84			if old>>mutexWaiterShift == 0 || old&(mutexLocked|mutexWoken) != 0 {
    85				return
    86			}
    87			// Grab the right to wake someone.
    88			new = (old - 1<<mutexWaiterShift) | mutexWoken
    89			if atomic.CompareAndSwapInt32(&m.state, old, new) {
    90				runtime.Semrelease(&m.sema)
    91				return
    92			}
    93			old = m.state
    94		}
    95	}
The Go Programming Language

Source file src/pkg/sync/mutex.go
