119 lines
2.7 KiB
Go
119 lines
2.7 KiB
Go
// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package sync
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import (
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"sync/atomic"
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"unsafe"
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)
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// Cond implements a condition variable, a rendezvous point
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// for goroutines waiting for or announcing the occurrence
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// of an event.
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//
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// Each Cond has an associated Locker L (often a *Mutex or *RWMutex),
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// which must be held when changing the condition and
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// when calling the Wait method.
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//
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// A Cond can be created as part of other structures.
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// A Cond must not be copied after first use.
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type Cond struct {
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// L is held while observing or changing the condition
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L Locker
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sema syncSema
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waiters uint32 // number of waiters
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checker copyChecker
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}
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// NewCond returns a new Cond with Locker l.
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func NewCond(l Locker) *Cond {
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return &Cond{L: l}
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}
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// Wait atomically unlocks c.L and suspends execution
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// of the calling goroutine. After later resuming execution,
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// Wait locks c.L before returning. Unlike in other systems,
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// Wait cannot return unless awoken by Broadcast or Signal.
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//
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// Because c.L is not locked when Wait first resumes, the caller
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// typically cannot assume that the condition is true when
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// Wait returns. Instead, the caller should Wait in a loop:
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//
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// c.L.Lock()
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// for !condition() {
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// c.Wait()
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// }
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// ... make use of condition ...
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// c.L.Unlock()
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//
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func (c *Cond) Wait() {
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c.checker.check()
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if raceenabled {
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raceDisable()
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}
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atomic.AddUint32(&c.waiters, 1)
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if raceenabled {
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raceEnable()
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}
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c.L.Unlock()
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runtime_Syncsemacquire(&c.sema)
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c.L.Lock()
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}
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// Signal wakes one goroutine waiting on c, if there is any.
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//
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// It is allowed but not required for the caller to hold c.L
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// during the call.
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func (c *Cond) Signal() {
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c.signalImpl(false)
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}
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// Broadcast wakes all goroutines waiting on c.
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//
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// It is allowed but not required for the caller to hold c.L
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// during the call.
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func (c *Cond) Broadcast() {
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c.signalImpl(true)
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}
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func (c *Cond) signalImpl(all bool) {
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c.checker.check()
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if raceenabled {
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raceDisable()
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}
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for {
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old := atomic.LoadUint32(&c.waiters)
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if old == 0 {
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if raceenabled {
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raceEnable()
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}
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return
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}
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new := old - 1
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if all {
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new = 0
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}
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if atomic.CompareAndSwapUint32(&c.waiters, old, new) {
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if raceenabled {
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raceEnable()
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}
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runtime_Syncsemrelease(&c.sema, old-new)
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return
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}
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}
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}
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// copyChecker holds back pointer to itself to detect object copying.
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type copyChecker uintptr
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func (c *copyChecker) check() {
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if uintptr(*c) != uintptr(unsafe.Pointer(c)) &&
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!atomic.CompareAndSwapUintptr((*uintptr)(c), 0, uintptr(unsafe.Pointer(c))) &&
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uintptr(*c) != uintptr(unsafe.Pointer(c)) {
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panic("sync.Cond is copied")
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}
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}
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