1 // Copyright 2011 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 6 7 import ( 8 "internal/race" 9 "sync/atomic" 10 "unsafe" 11 ) 12 13 // A WaitGroup waits for a collection of goroutines to finish. 14 // The main goroutine calls Add to set the number of 15 // goroutines to wait for. Then each of the goroutines 16 // runs and calls Done when finished. At the same time, 17 // Wait can be used to block until all goroutines have finished. 18 // 19 // A WaitGroup must not be copied after first use. 20 type WaitGroup struct { 21 noCopy noCopy 22 23 // 64-bit value: high 32 bits are counter, low 32 bits are waiter count. 24 // 64-bit atomic operations require 64-bit alignment, but 32-bit 25 // compilers do not ensure it. So we allocate 12 bytes and then use 26 // the aligned 8 bytes in them as state. 27 state1 [12]byte 28 sema uint32 29 } 30 31 func (wg *WaitGroup) state() *uint64 { 32 if uintptr(unsafe.Pointer(&wg.state1))%8 == 0 { 33 return (*uint64)(unsafe.Pointer(&wg.state1)) 34 } else { 35 return (*uint64)(unsafe.Pointer(&wg.state1[4])) 36 } 37 } 38 39 // Add adds delta, which may be negative, to the WaitGroup counter. 40 // If the counter becomes zero, all goroutines blocked on Wait are released. 41 // If the counter goes negative, Add panics. 42 // 43 // Note that calls with a positive delta that occur when the counter is zero 44 // must happen before a Wait. Calls with a negative delta, or calls with a 45 // positive delta that start when the counter is greater than zero, may happen 46 // at any time. 47 // Typically this means the calls to Add should execute before the statement 48 // creating the goroutine or other event to be waited for. 49 // If a WaitGroup is reused to wait for several independent sets of events, 50 // new Add calls must happen after all previous Wait calls have returned. 51 // See the WaitGroup example. 52 func (wg *WaitGroup) Add(delta int) { 53 statep := wg.state() 54 if race.Enabled { 55 _ = *statep // trigger nil deref early 56 if delta < 0 { 57 // Synchronize decrements with Wait. 58 race.ReleaseMerge(unsafe.Pointer(wg)) 59 } 60 race.Disable() 61 defer race.Enable() 62 } 63 state := atomic.AddUint64(statep, uint64(delta)<<32) 64 v := int32(state >> 32) 65 w := uint32(state) 66 if race.Enabled && delta > 0 && v == int32(delta) { 67 // The first increment must be synchronized with Wait. 68 // Need to model this as a read, because there can be 69 // several concurrent wg.counter transitions from 0. 70 race.Read(unsafe.Pointer(&wg.sema)) 71 } 72 if v < 0 { 73 panic("sync: negative WaitGroup counter") 74 } 75 if w != 0 && delta > 0 && v == int32(delta) { 76 panic("sync: WaitGroup misuse: Add called concurrently with Wait") 77 } 78 if v > 0 || w == 0 { 79 return 80 } 81 // This goroutine has set counter to 0 when waiters > 0. 82 // Now there can't be concurrent mutations of state: 83 // - Adds must not happen concurrently with Wait, 84 // - Wait does not increment waiters if it sees counter == 0. 85 // Still do a cheap sanity check to detect WaitGroup misuse. 86 if *statep != state { 87 panic("sync: WaitGroup misuse: Add called concurrently with Wait") 88 } 89 // Reset waiters count to 0. 90 *statep = 0 91 for ; w != 0; w-- { 92 runtime_Semrelease(&wg.sema, false) 93 } 94 } 95 96 // Done decrements the WaitGroup counter by one. 97 func (wg *WaitGroup) Done() { 98 wg.Add(-1) 99 } 100 101 // Wait blocks until the WaitGroup counter is zero. 102 func (wg *WaitGroup) Wait() { 103 statep := wg.state() 104 if race.Enabled { 105 _ = *statep // trigger nil deref early 106 race.Disable() 107 } 108 for { 109 state := atomic.LoadUint64(statep) 110 v := int32(state >> 32) 111 w := uint32(state) 112 if v == 0 { 113 // Counter is 0, no need to wait. 114 if race.Enabled { 115 race.Enable() 116 race.Acquire(unsafe.Pointer(wg)) 117 } 118 return 119 } 120 // Increment waiters count. 121 if atomic.CompareAndSwapUint64(statep, state, state+1) { 122 if race.Enabled && w == 0 { 123 // Wait must be synchronized with the first Add. 124 // Need to model this is as a write to race with the read in Add. 125 // As a consequence, can do the write only for the first waiter, 126 // otherwise concurrent Waits will race with each other. 127 race.Write(unsafe.Pointer(&wg.sema)) 128 } 129 runtime_Semacquire(&wg.sema) 130 if *statep != 0 { 131 panic("sync: WaitGroup is reused before previous Wait has returned") 132 } 133 if race.Enabled { 134 race.Enable() 135 race.Acquire(unsafe.Pointer(wg)) 136 } 137 return 138 } 139 } 140 } 141
