From adb0401dac41c81571722312d4586b2693f95aa6 Mon Sep 17 00:00:00 2001
From: Ian Lance Taylor <ian@gcc.gnu.org>
Date: Fri, 16 Sep 2011 15:47:21 +0000
Subject: Update Go library to r60.

From-SVN: r178910
---
 libgo/go/sync/waitgroup.go | 41 ++++++++++++++++++++++++++---------------
 1 file changed, 26 insertions(+), 15 deletions(-)

(limited to 'libgo/go/sync/waitgroup.go')

diff --git a/libgo/go/sync/waitgroup.go b/libgo/go/sync/waitgroup.go
index 05478c6..a4c9b7e 100644
--- a/libgo/go/sync/waitgroup.go
+++ b/libgo/go/sync/waitgroup.go
@@ -4,7 +4,10 @@
 
 package sync
 
-import "runtime"
+import (
+	"runtime"
+	"sync/atomic"
+)
 
 // A WaitGroup waits for a collection of goroutines to finish.
 // The main goroutine calls Add to set the number of
@@ -28,8 +31,8 @@ import "runtime"
 // 
 type WaitGroup struct {
 	m       Mutex
-	counter int
-	waiters int
+	counter int32
+	waiters int32
 	sema    *uint32
 }
 
@@ -48,19 +51,19 @@ type WaitGroup struct {
 // Add adds delta, which may be negative, to the WaitGroup counter.
 // If the counter becomes zero, all goroutines blocked on Wait() are released.
 func (wg *WaitGroup) Add(delta int) {
-	wg.m.Lock()
-	if delta < -wg.counter {
-		wg.m.Unlock()
+	v := atomic.AddInt32(&wg.counter, int32(delta))
+	if v < 0 {
 		panic("sync: negative WaitGroup count")
 	}
-	wg.counter += delta
-	if wg.counter == 0 && wg.waiters > 0 {
-		for i := 0; i < wg.waiters; i++ {
-			runtime.Semrelease(wg.sema)
-		}
-		wg.waiters = 0
-		wg.sema = nil
+	if v > 0 || atomic.LoadInt32(&wg.waiters) == 0 {
+		return
 	}
+	wg.m.Lock()
+	for i := int32(0); i < wg.waiters; i++ {
+		runtime.Semrelease(wg.sema)
+	}
+	wg.waiters = 0
+	wg.sema = nil
 	wg.m.Unlock()
 }
 
@@ -71,12 +74,20 @@ func (wg *WaitGroup) Done() {
 
 // Wait blocks until the WaitGroup counter is zero.
 func (wg *WaitGroup) Wait() {
+	if atomic.LoadInt32(&wg.counter) == 0 {
+		return
+	}
 	wg.m.Lock()
-	if wg.counter == 0 {
+	atomic.AddInt32(&wg.waiters, 1)
+	// This code is racing with the unlocked path in Add above.
+	// The code above modifies counter and then reads waiters.
+	// We must modify waiters and then read counter (the opposite order)
+	// to avoid missing an Add.
+	if atomic.LoadInt32(&wg.counter) == 0 {
+		atomic.AddInt32(&wg.waiters, -1)
 		wg.m.Unlock()
 		return
 	}
-	wg.waiters++
 	if wg.sema == nil {
 		wg.sema = new(uint32)
 	}
-- 
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