waitgroup.go

     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 only guarantee that 64-bit fields are 32-bit aligned.
    26  	// For this reason on 32 bit architectures we need to check in state()
    27  	// if state1 is aligned or not, and dynamically "swap" the field order if
    28  	// needed.
    29  	state1 uint64
    30  	state2 uint32
    31  }
    32  
    33  // state returns pointers to the state and sema fields stored within wg.state*.
    34  func (wg *WaitGroup) state() (statep *uint64, semap *uint32) {
    35  	if unsafe.Alignof(wg.state1) == 8 || uintptr(unsafe.Pointer(&wg.state1))%8 == 0 {
    36  		// state1 is 64-bit aligned: nothing to do.
    37  		return &wg.state1, &wg.state2
    38  	} else {
    39  		// state1 is 32-bit aligned but not 64-bit aligned: this means that
    40  		// (&state1)+4 is 64-bit aligned.
    41  		state := (*[3]uint32)(unsafe.Pointer(&wg.state1))
    42  		return (*uint64)(unsafe.Pointer(&state[1])), &state[0]
    43  	}
    44  }
    45  
    46  // Add adds delta, which may be negative, to the WaitGroup counter.
    47  // If the counter becomes zero, all goroutines blocked on Wait are released.
    48  // If the counter goes negative, Add panics.
    49  //
    50  // Note that calls with a positive delta that occur when the counter is zero
    51  // must happen before a Wait. Calls with a negative delta, or calls with a
    52  // positive delta that start when the counter is greater than zero, may happen
    53  // at any time.
    54  // Typically this means the calls to Add should execute before the statement
    55  // creating the goroutine or other event to be waited for.
    56  // If a WaitGroup is reused to wait for several independent sets of events,
    57  // new Add calls must happen after all previous Wait calls have returned.
    58  // See the WaitGroup example.
    59  func (wg *WaitGroup) Add(delta int) {
    60  	statep, semap := wg.state()
    61  	if race.Enabled {
    62  		_ = *statep // trigger nil deref early
    63  		if delta < 0 {
    64  			// Synchronize decrements with Wait.
    65  			race.ReleaseMerge(unsafe.Pointer(wg))
    66  		}
    67  		race.Disable()
    68  		defer race.Enable()
    69  	}
    70  	state := atomic.AddUint64(statep, uint64(delta)<<32)
    71  	v := int32(state >> 32)
    72  	w := uint32(state)
    73  	if race.Enabled && delta > 0 && v == int32(delta) {
    74  		// The first increment must be synchronized with Wait.
    75  		// Need to model this as a read, because there can be
    76  		// several concurrent wg.counter transitions from 0.
    77  		race.Read(unsafe.Pointer(semap))
    78  	}
    79  	if v < 0 {
    80  		panic("sync: negative WaitGroup counter")
    81  	}
    82  	if w != 0 && delta > 0 && v == int32(delta) {
    83  		panic("sync: WaitGroup misuse: Add called concurrently with Wait")
    84  	}
    85  	if v > 0 || w == 0 {
    86  		return
    87  	}
    88  	// This goroutine has set counter to 0 when waiters > 0.
    89  	// Now there can't be concurrent mutations of state:
    90  	// - Adds must not happen concurrently with Wait,
    91  	// - Wait does not increment waiters if it sees counter == 0.
    92  	// Still do a cheap sanity check to detect WaitGroup misuse.
    93  	if *statep != state {
    94  		panic("sync: WaitGroup misuse: Add called concurrently with Wait")
    95  	}
    96  	// Reset waiters count to 0.
    97  	*statep = 0
    98  	for ; w != 0; w-- {
    99  		runtime_Semrelease(semap, false, 0)
   100  	}
   101  }
   102  
   103  // Done decrements the WaitGroup counter by one.
   104  func (wg *WaitGroup) Done() {
   105  	wg.Add(-1)
   106  }
   107  
   108  // Wait blocks until the WaitGroup counter is zero.
   109  func (wg *WaitGroup) Wait() {
   110  	statep, semap := wg.state()
   111  	if race.Enabled {
   112  		_ = *statep // trigger nil deref early
   113  		race.Disable()
   114  	}
   115  	for {
   116  		state := atomic.LoadUint64(statep)
   117  		v := int32(state >> 32)
   118  		w := uint32(state)
   119  		if v == 0 {
   120  			// Counter is 0, no need to wait.
   121  			if race.Enabled {
   122  				race.Enable()
   123  				race.Acquire(unsafe.Pointer(wg))
   124  			}
   125  			return
   126  		}
   127  		// Increment waiters count.
   128  		if atomic.CompareAndSwapUint64(statep, state, state+1) {
   129  			if race.Enabled && w == 0 {
   130  				// Wait must be synchronized with the first Add.
   131  				// Need to model this is as a write to race with the read in Add.
   132  				// As a consequence, can do the write only for the first waiter,
   133  				// otherwise concurrent Waits will race with each other.
   134  				race.Write(unsafe.Pointer(semap))
   135  			}
   136  			runtime_Semacquire(semap)
   137  			if *statep != 0 {
   138  				panic("sync: WaitGroup is reused before previous Wait has returned")
   139  			}
   140  			if race.Enabled {
   141  				race.Enable()
   142  				race.Acquire(unsafe.Pointer(wg))
   143  			}
   144  			return
   145  		}
   146  	}
   147  }
   148
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