114 lines
3.1 KiB
Go
114 lines
3.1 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 "runtime"
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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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type Cond struct {
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L Locker // held while observing or changing the condition
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m Mutex // held to avoid internal races
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// We must be careful to make sure that when Signal
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// releases a semaphore, the corresponding acquire is
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// executed by a goroutine that was already waiting at
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// the time of the call to Signal, not one that arrived later.
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// To ensure this, we segment waiting goroutines into
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// generations punctuated by calls to Signal. Each call to
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// Signal begins another generation if there are no goroutines
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// left in older generations for it to wake. Because of this
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// optimization (only begin another generation if there
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// are no older goroutines left), we only need to keep track
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// of the two most recent generations, which we call old
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// and new.
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oldWaiters int // number of waiters in old generation...
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oldSema *uint32 // ... waiting on this semaphore
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newWaiters int // number of waiters in new generation...
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newSema *uint32 // ... waiting on this semaphore
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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.
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//
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// Because 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.m.Lock()
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if c.newSema == nil {
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c.newSema = new(uint32)
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}
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s := c.newSema
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c.newWaiters++
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c.m.Unlock()
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c.L.Unlock()
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runtime.Semacquire(s)
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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.m.Lock()
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if c.oldWaiters == 0 && c.newWaiters > 0 {
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// Retire old generation; rename new to old.
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c.oldWaiters = c.newWaiters
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c.oldSema = c.newSema
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c.newWaiters = 0
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c.newSema = nil
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}
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if c.oldWaiters > 0 {
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c.oldWaiters--
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runtime.Semrelease(c.oldSema)
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}
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c.m.Unlock()
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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.m.Lock()
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// Wake both generations.
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if c.oldWaiters > 0 {
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for i := 0; i < c.oldWaiters; i++ {
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runtime.Semrelease(c.oldSema)
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}
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c.oldWaiters = 0
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}
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if c.newWaiters > 0 {
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for i := 0; i < c.newWaiters; i++ {
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runtime.Semrelease(c.newSema)
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}
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c.newWaiters = 0
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c.newSema = nil
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}
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c.m.Unlock()
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}
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