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Source file src/sync/waitgroup.go

Documentation: sync

     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, and the other 4 as storage
    27  	// for the sema.
    28  	state1 [3]uint32
    29  }
    30  
    31  // state returns pointers to the state and sema fields stored within wg.state1.
    32  func (wg *WaitGroup) state() (statep *uint64, semap *uint32) {
    33  	if uintptr(unsafe.Pointer(&wg.state1))%8 == 0 {
    34  		return (*uint64)(unsafe.Pointer(&wg.state1)), &wg.state1[2]
    35  	} else {
    36  		return (*uint64)(unsafe.Pointer(&wg.state1[1])), &wg.state1[0]
    37  	}
    38  }
    39  
    40  // Add adds delta, which may be negative, to the WaitGroup counter.
    41  // If the counter becomes zero, all goroutines blocked on Wait are released.
    42  // If the counter goes negative, Add panics.
    43  //
    44  // Note that calls with a positive delta that occur when the counter is zero
    45  // must happen before a Wait. Calls with a negative delta, or calls with a
    46  // positive delta that start when the counter is greater than zero, may happen
    47  // at any time.
    48  // Typically this means the calls to Add should execute before the statement
    49  // creating the goroutine or other event to be waited for.
    50  // If a WaitGroup is reused to wait for several independent sets of events,
    51  // new Add calls must happen after all previous Wait calls have returned.
    52  // See the WaitGroup example.
    53  func (wg *WaitGroup) Add(delta int) {
    54  	statep, semap := wg.state()
    55  	if race.Enabled {
    56  		_ = *statep // trigger nil deref early
    57  		if delta < 0 {
    58  			// Synchronize decrements with Wait.
    59  			race.ReleaseMerge(unsafe.Pointer(wg))
    60  		}
    61  		race.Disable()
    62  		defer race.Enable()
    63  	}
    64  	state := atomic.AddUint64(statep, uint64(delta)<<32)
    65  	v := int32(state >> 32)
    66  	w := uint32(state)
    67  	if race.Enabled && delta > 0 && v == int32(delta) {
    68  		// The first increment must be synchronized with Wait.
    69  		// Need to model this as a read, because there can be
    70  		// several concurrent wg.counter transitions from 0.
    71  		race.Read(unsafe.Pointer(semap))
    72  	}
    73  	if v < 0 {
    74  		panic("sync: negative WaitGroup counter")
    75  	}
    76  	if w != 0 && delta > 0 && v == int32(delta) {
    77  		panic("sync: WaitGroup misuse: Add called concurrently with Wait")
    78  	}
    79  	if v > 0 || w == 0 {
    80  		return
    81  	}
    82  	// This goroutine has set counter to 0 when waiters > 0.
    83  	// Now there can't be concurrent mutations of state:
    84  	// - Adds must not happen concurrently with Wait,
    85  	// - Wait does not increment waiters if it sees counter == 0.
    86  	// Still do a cheap sanity check to detect WaitGroup misuse.
    87  	if *statep != state {
    88  		panic("sync: WaitGroup misuse: Add called concurrently with Wait")
    89  	}
    90  	// Reset waiters count to 0.
    91  	*statep = 0
    92  	for ; w != 0; w-- {
    93  		runtime_Semrelease(semap, false)
    94  	}
    95  }
    96  
    97  // Done decrements the WaitGroup counter by one.
    98  func (wg *WaitGroup) Done() {
    99  	wg.Add(-1)
   100  }
   101  
   102  // Wait blocks until the WaitGroup counter is zero.
   103  func (wg *WaitGroup) Wait() {
   104  	statep, semap := wg.state()
   105  	if race.Enabled {
   106  		_ = *statep // trigger nil deref early
   107  		race.Disable()
   108  	}
   109  	for {
   110  		state := atomic.LoadUint64(statep)
   111  		v := int32(state >> 32)
   112  		w := uint32(state)
   113  		if v == 0 {
   114  			// Counter is 0, no need to wait.
   115  			if race.Enabled {
   116  				race.Enable()
   117  				race.Acquire(unsafe.Pointer(wg))
   118  			}
   119  			return
   120  		}
   121  		// Increment waiters count.
   122  		if atomic.CompareAndSwapUint64(statep, state, state+1) {
   123  			if race.Enabled && w == 0 {
   124  				// Wait must be synchronized with the first Add.
   125  				// Need to model this is as a write to race with the read in Add.
   126  				// As a consequence, can do the write only for the first waiter,
   127  				// otherwise concurrent Waits will race with each other.
   128  				race.Write(unsafe.Pointer(semap))
   129  			}
   130  			runtime_Semacquire(semap)
   131  			if *statep != 0 {
   132  				panic("sync: WaitGroup is reused before previous Wait has returned")
   133  			}
   134  			if race.Enabled {
   135  				race.Enable()
   136  				race.Acquire(unsafe.Pointer(wg))
   137  			}
   138  			return
   139  		}
   140  	}
   141  }
   142  

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