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.
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.
10 // Values containing the types defined in this package should not be copied.
19 func throw(string) // provided by runtime
21 // A Mutex is a mutual exclusion lock.
22 // The zero value for a Mutex is an unlocked mutex.
24 // A Mutex must not be copied after first use.
30 // A Locker represents an object that can be locked and unlocked.
31 type Locker
interface {
37 mutexLocked
= 1 << iota // mutex is locked
40 mutexWaiterShift
= iota
44 // Mutex can be in 2 modes of operations: normal and starvation.
45 // In normal mode waiters are queued in FIFO order, but a woken up waiter
46 // does not own the mutex and competes with new arriving goroutines over
47 // the ownership. New arriving goroutines have an advantage -- they are
48 // already running on CPU and there can be lots of them, so a woken up
49 // waiter has good chances of losing. In such case it is queued at front
50 // of the wait queue. If a waiter fails to acquire the mutex for more than 1ms,
51 // it switches mutex to the starvation mode.
53 // In starvation mode ownership of the mutex is directly handed off from
54 // the unlocking goroutine to the waiter at the front of the queue.
55 // New arriving goroutines don't try to acquire the mutex even if it appears
56 // to be unlocked, and don't try to spin. Instead they queue themselves at
57 // the tail of the wait queue.
59 // If a waiter receives ownership of the mutex and sees that either
60 // (1) it is the last waiter in the queue, or (2) it waited for less than 1 ms,
61 // it switches mutex back to normal operation mode.
63 // Normal mode has considerably better performance as a goroutine can acquire
64 // a mutex several times in a row even if there are blocked waiters.
65 // Starvation mode is important to prevent pathological cases of tail latency.
66 starvationThresholdNs
= 1e6
70 // If the lock is already in use, the calling goroutine
71 // blocks until the mutex is available.
72 func (m
*Mutex
) Lock() {
73 // Fast path: grab unlocked mutex.
74 if atomic
.CompareAndSwapInt32(&m
.state
, 0, mutexLocked
) {
76 race
.Acquire(unsafe
.Pointer(m
))
81 var waitStartTime
int64
87 // Don't spin in starvation mode, ownership is handed off to waiters
88 // so we won't be able to acquire the mutex anyway.
89 if old
&(mutexLocked|mutexStarving
) == mutexLocked
&& runtime_canSpin(iter
) {
90 // Active spinning makes sense.
91 // Try to set mutexWoken flag to inform Unlock
92 // to not wake other blocked goroutines.
93 if !awoke
&& old
&mutexWoken
== 0 && old
>>mutexWaiterShift
!= 0 &&
94 atomic
.CompareAndSwapInt32(&m
.state
, old
, old|mutexWoken
) {
103 // Don't try to acquire starving mutex, new arriving goroutines must queue.
104 if old
&mutexStarving
== 0 {
107 if old
&(mutexLocked|mutexStarving
) != 0 {
108 new += 1 << mutexWaiterShift
110 // The current goroutine switches mutex to starvation mode.
111 // But if the mutex is currently unlocked, don't do the switch.
112 // Unlock expects that starving mutex has waiters, which will not
113 // be true in this case.
114 if starving
&& old
&mutexLocked
!= 0 {
118 // The goroutine has been woken from sleep,
119 // so we need to reset the flag in either case.
120 if new&mutexWoken
== 0 {
121 panic("sync: inconsistent mutex state")
125 if atomic
.CompareAndSwapInt32(&m
.state
, old
, new) {
126 if old
&(mutexLocked|mutexStarving
) == 0 {
127 break // locked the mutex with CAS
129 // If we were already waiting before, queue at the front of the queue.
130 queueLifo
:= waitStartTime
!= 0
131 if waitStartTime
== 0 {
132 waitStartTime
= runtime_nanotime()
134 runtime_SemacquireMutex(&m
.sema
, queueLifo
)
135 starving
= starving ||
runtime_nanotime()-waitStartTime
> starvationThresholdNs
137 if old
&mutexStarving
!= 0 {
138 // If this goroutine was woken and mutex is in starvation mode,
139 // ownership was handed off to us but mutex is in somewhat
140 // inconsistent state: mutexLocked is not set and we are still
141 // accounted as waiter. Fix that.
142 if old
&(mutexLocked|mutexWoken
) != 0 || old
>>mutexWaiterShift
== 0 {
143 panic("sync: inconsistent mutex state")
145 delta
:= int32(mutexLocked
- 1<<mutexWaiterShift
)
146 if !starving || old
>>mutexWaiterShift
== 1 {
147 // Exit starvation mode.
148 // Critical to do it here and consider wait time.
149 // Starvation mode is so inefficient, that two goroutines
150 // can go lock-step infinitely once they switch mutex
151 // to starvation mode.
152 delta
-= mutexStarving
154 atomic
.AddInt32(&m
.state
, delta
)
165 race
.Acquire(unsafe
.Pointer(m
))
170 // It is a run-time error if m is not locked on entry to Unlock.
172 // A locked Mutex is not associated with a particular goroutine.
173 // It is allowed for one goroutine to lock a Mutex and then
174 // arrange for another goroutine to unlock it.
175 func (m
*Mutex
) Unlock() {
178 race
.Release(unsafe
.Pointer(m
))
181 // Fast path: drop lock bit.
182 new := atomic
.AddInt32(&m
.state
, -mutexLocked
)
183 if (new+mutexLocked
)&mutexLocked
== 0 {
184 panic("sync: unlock of unlocked mutex")
186 if new&mutexStarving
== 0 {
189 // If there are no waiters or a goroutine has already
190 // been woken or grabbed the lock, no need to wake anyone.
191 // In starvation mode ownership is directly handed off from unlocking
192 // goroutine to the next waiter. We are not part of this chain,
193 // since we did not observe mutexStarving when we unlocked the mutex above.
194 // So get off the way.
195 if old
>>mutexWaiterShift
== 0 || old
&(mutexLocked|mutexWoken|mutexStarving
) != 0 {
198 // Grab the right to wake someone.
199 new = (old
- 1<<mutexWaiterShift
) | mutexWoken
200 if atomic
.CompareAndSwapInt32(&m
.state
, old
, new) {
201 runtime_Semrelease(&m
.sema
, false)
207 // Starving mode: handoff mutex ownership to the next waiter.
208 // Note: mutexLocked is not set, the waiter will set it after wakeup.
209 // But mutex is still considered locked if mutexStarving is set,
210 // so new coming goroutines won't acquire it.
211 runtime_Semrelease(&m
.sema
, true)