81 lines
2.3 KiB
Go
81 lines
2.3 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 "sync/atomic"
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// A WaitGroup waits for a collection of goroutines to finish.
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// The main goroutine calls Add to set the number of
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// goroutines to wait for. Then each of the goroutines
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// runs and calls Done when finished. At the same time,
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// Wait can be used to block until all goroutines have finished.
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type WaitGroup struct {
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m Mutex
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counter int32
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waiters int32
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sema *uint32
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}
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// WaitGroup creates a new semaphore each time the old semaphore
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// is released. This is to avoid the following race:
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//
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// G1: Add(1)
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// G1: go G2()
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// G1: Wait() // Context switch after Unlock() and before Semacquire().
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// G2: Done() // Release semaphore: sema == 1, waiters == 0. G1 doesn't run yet.
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// G3: Wait() // Finds counter == 0, waiters == 0, doesn't block.
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// G3: Add(1) // Makes counter == 1, waiters == 0.
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// G3: go G4()
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// G3: Wait() // G1 still hasn't run, G3 finds sema == 1, unblocked! Bug.
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// Add adds delta, which may be negative, to the WaitGroup counter.
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// If the counter becomes zero, all goroutines blocked on Wait() are released.
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// If the counter goes negative, Add panics.
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func (wg *WaitGroup) Add(delta int) {
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v := atomic.AddInt32(&wg.counter, int32(delta))
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if v < 0 {
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panic("sync: negative WaitGroup counter")
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}
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if v > 0 || atomic.LoadInt32(&wg.waiters) == 0 {
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return
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}
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wg.m.Lock()
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for i := int32(0); i < wg.waiters; i++ {
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runtime_Semrelease(wg.sema)
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}
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wg.waiters = 0
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wg.sema = nil
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wg.m.Unlock()
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}
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// Done decrements the WaitGroup counter.
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func (wg *WaitGroup) Done() {
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wg.Add(-1)
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}
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// Wait blocks until the WaitGroup counter is zero.
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func (wg *WaitGroup) Wait() {
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if atomic.LoadInt32(&wg.counter) == 0 {
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return
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}
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wg.m.Lock()
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atomic.AddInt32(&wg.waiters, 1)
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// This code is racing with the unlocked path in Add above.
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// The code above modifies counter and then reads waiters.
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// We must modify waiters and then read counter (the opposite order)
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// to avoid missing an Add.
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if atomic.LoadInt32(&wg.counter) == 0 {
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atomic.AddInt32(&wg.waiters, -1)
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wg.m.Unlock()
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return
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}
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if wg.sema == nil {
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wg.sema = new(uint32)
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}
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s := wg.sema
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wg.m.Unlock()
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runtime_Semacquire(s)
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}
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