sync: enable Pool when using race detector

Disabled by https://golang.org/cl/53020044 due to false positives.
Reenable and model properly.

Fixes #17306.

Change-Id: I28405ddfcd17f58cf1427c300273212729154359
Reviewed-on: https://go-review.googlesource.com/31589
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/runtime/race/testdata/pool_test.go

@@ -0,0 +1,47 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package race_test
+
+import (
+	"sync"
+	"testing"
+	"time"
+)
+
+func TestRacePool(t *testing.T) {
+	// Pool randomly drops the argument on the floor during Put.
+	// Repeat so that at least one iteration gets reuse.
+	for i := 0; i < 10; i++ {
+		c := make(chan int)
+		p := &sync.Pool{New: func() interface{} { return make([]byte, 10) }}
+		x := p.Get().([]byte)
+		x[0] = 1
+		p.Put(x)
+		go func() {
+			y := p.Get().([]byte)
+			y[0] = 2
+			c <- 1
+		}()
+		x[0] = 3
+		<-c
+	}
+}
+
+func TestNoRacePool(t *testing.T) {
+	for i := 0; i < 10; i++ {
+		p := &sync.Pool{New: func() interface{} { return make([]byte, 10) }}
+		x := p.Get().([]byte)
+		x[0] = 1
+		p.Put(x)
+		go func() {
+			y := p.Get().([]byte)
+			y[0] = 2
+			p.Put(y)
+		}()
+		time.Sleep(100 * time.Millisecond)
+		x = p.Get().([]byte)
+		x[0] = 3
+	}
+}

src/runtime/stubs.go

@@ -96,6 +96,9 @@ var hashLoad = loadFactor
 // in asm_*.s
 func fastrand() uint32
 
+//go:linkname sync_fastrand sync.fastrand
+func sync_fastrand() uint32 { return fastrand() }
+
 // in asm_*.s
 //go:noescape
 func memequal(a, b unsafe.Pointer, size uintptr) bool

src/sync/pool.go

@@ -61,29 +61,49 @@ type poolLocal struct {
 	pad     [128]byte     // Prevents false sharing.
 }
 
+// from runtime
+func fastrand() uint32
+
+var poolRaceHash [128]uint64
+
+// poolRaceAddr returns an address to use as the synchronization point
+// for race detector logic. We don't use the actual pointer stored in x
+// directly, for fear of conflicting with other synchronization on that address.
+// Instead, we hash the pointer to get an index into poolRaceHash.
+// See discussion on golang.org/cl/31589.
+func poolRaceAddr(x interface{}) unsafe.Pointer {
+	ptr := uintptr((*[2]unsafe.Pointer)(unsafe.Pointer(&x))[1])
+	h := uint32((uint64(uint32(ptr)) * 0x85ebca6b) >> 16)
+	return unsafe.Pointer(&poolRaceHash[h%uint32(len(poolRaceHash))])
+}
+
 // Put adds x to the pool.
 func (p *Pool) Put(x interface{}) {
-	if race.Enabled {
-		// Under race detector the Pool degenerates into no-op.
-		// It's conforming, simple and does not introduce excessive
-		// happens-before edges between unrelated goroutines.
-		return
-	}
 	if x == nil {
 		return
 	}
+	if race.Enabled {
+		if fastrand()%4 == 0 {
+			// Randomly drop x on floor.
+			return
+		}
+		race.ReleaseMerge(poolRaceAddr(x))
+		race.Disable()
+	}
 	l := p.pin()
 	if l.private == nil {
 		l.private = x
 		x = nil
 	}
 	runtime_procUnpin()
-	if x == nil {
-		return
-	}
+	if x != nil {
 		l.Lock()
 		l.shared = append(l.shared, x)
 		l.Unlock()
+	}
+	if race.Enabled {
+		race.Enable()
+	}
 }
 
 // Get selects an arbitrary item from the Pool, removes it from the
@@ -96,18 +116,13 @@ func (p *Pool) Put(x interface{}) {
 // the result of calling p.New.
 func (p *Pool) Get() interface{} {
 	if race.Enabled {
-		if p.New != nil {
-			return p.New()
-		}
-		return nil
+		race.Disable()
 	}
 	l := p.pin()
 	x := l.private
 	l.private = nil
 	runtime_procUnpin()
-	if x != nil {
-		return x
-	}
+	if x == nil {
 		l.Lock()
 		last := len(l.shared) - 1
 		if last >= 0 {
@@ -115,10 +130,20 @@ func (p *Pool) Get() interface{} {
 			l.shared = l.shared[:last]
 		}
 		l.Unlock()
-	if x != nil {
-		return x
+		if x == nil {
+			x = p.getSlow()
 		}
-	return p.getSlow()
+	}
+	if race.Enabled {
+		race.Enable()
+		if x != nil {
+			race.Acquire(poolRaceAddr(x))
+		}
+	}
+	if x == nil && p.New != nil {
+		x = p.New()
+	}
+	return x
 }
 
 func (p *Pool) getSlow() (x interface{}) {
@@ -140,10 +165,6 @@ func (p *Pool) getSlow() (x interface{}) {
 		}
 		l.Unlock()
 	}
-
-	if x == nil && p.New != nil {
-		x = p.New()
-	}
 	return x
 }