runtime: fix conflict between lfstack and checkptr

lfstack does very unsafe things. In particular, it will not
work with nodes that live on the heap. In normal use by the runtime,
that is the case (it is only used for gc work bufs). But the lfstack
test does use heap objects. It goes through some hoops to prevent
premature deallocation, but those hoops are not enough to convince
-d=checkptr that everything is ok.

Instead, allocate the test objects outside the heap, like the runtime
does for all of its lfstack usage. Remove the lifetime workaround
from the test.

Reported in https://groups.google.com/g/golang-nuts/c/psjrUV2ZKyI

Change-Id: If611105eab6c823a4d6c105938ce145ed731781d
Reviewed-on: https://go-review.googlesource.com/c/go/+/448899
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>

src/runtime/export_test.go

@@ -69,6 +69,9 @@ func LFStackPush(head *uint64, node *LFNode) {
 func LFStackPop(head *uint64) *LFNode {
 	return (*LFNode)(unsafe.Pointer((*lfstack)(head).pop()))
 }
+func LFNodeValidate(node *LFNode) {
+	lfnodeValidate((*lfnode)(unsafe.Pointer(node)))
+}
 
 func Netpoll(delta int64) {
 	systemstack(func() {
@@ -1709,3 +1712,9 @@ func BlockUntilEmptyFinalizerQueue(timeout int64) bool {
 func FrameStartLine(f *Frame) int {
 	return f.startLine
 }
+
+// PersistentAlloc allocates some memory that lives outside the Go heap.
+// This memory will never be freed; use sparingly.
+func PersistentAlloc(n uintptr) unsafe.Pointer {
+	return persistentalloc(n, 0, &memstats.other_sys)
+}

src/runtime/lfstack.go

@@ -18,8 +18,7 @@ import (
 // This stack is intrusive. Nodes must embed lfnode as the first field.
 //
 // The stack does not keep GC-visible pointers to nodes, so the caller
-// is responsible for ensuring the nodes are not garbage collected
-// (typically by allocating them from manually-managed memory).
+// must ensure the nodes are allocated outside the Go heap.
 type lfstack uint64
 
 func (head *lfstack) push(node *lfnode) {
@@ -59,6 +58,9 @@ func (head *lfstack) empty() bool {
 // lfnodeValidate panics if node is not a valid address for use with
 // lfstack.push. This only needs to be called when node is allocated.
 func lfnodeValidate(node *lfnode) {
+	if base, _, _ := findObject(uintptr(unsafe.Pointer(node)), 0, 0); base != 0 {
+		throw("lfstack node allocated from the heap")
+	}
 	if lfstackUnpack(lfstackPack(node, ^uintptr(0))) != node {
 		printlock()
 		println("runtime: bad lfnode address", hex(uintptr(unsafe.Pointer(node))))

src/runtime/lfstack_test.go

@@ -16,6 +16,17 @@ type MyNode struct {
 	data int
 }
 
+// allocMyNode allocates nodes that are stored in an lfstack
+// outside the Go heap.
+// We require lfstack objects to live outside the heap so that
+// checkptr passes on the unsafe shenanigans used.
+func allocMyNode(data int) *MyNode {
+	n := (*MyNode)(PersistentAlloc(unsafe.Sizeof(MyNode{})))
+	LFNodeValidate(&n.LFNode)
+	n.data = data
+	return n
+}
+
 func fromMyNode(node *MyNode) *LFNode {
 	return (*LFNode)(unsafe.Pointer(node))
 }
@@ -30,22 +41,17 @@ func TestLFStack(t *testing.T) {
 	stack := new(uint64)
 	global = stack // force heap allocation
 
-	// Need to keep additional references to nodes, the stack is not all that type-safe.
-	var nodes []*MyNode
-
 	// Check the stack is initially empty.
 	if LFStackPop(stack) != nil {
 		t.Fatalf("stack is not empty")
 	}
 
 	// Push one element.
-	node := &MyNode{data: 42}
-	nodes = append(nodes, node)
+	node := allocMyNode(42)
 	LFStackPush(stack, fromMyNode(node))
 
 	// Push another.
-	node = &MyNode{data: 43}
-	nodes = append(nodes, node)
+	node = allocMyNode(43)
 	LFStackPush(stack, fromMyNode(node))
 
 	// Pop one element.
@@ -75,8 +81,6 @@ func TestLFStack(t *testing.T) {
 	}
 }
 
-var stress []*MyNode
-
 func TestLFStackStress(t *testing.T) {
 	const K = 100
 	P := 4 * GOMAXPROCS(-1)
@@ -86,15 +90,11 @@ func TestLFStackStress(t *testing.T) {
 	}
 	// Create 2 stacks.
 	stacks := [2]*uint64{new(uint64), new(uint64)}
-	// Need to keep additional references to nodes,
-	// the lock-free stack is not type-safe.
-	stress = nil
 	// Push K elements randomly onto the stacks.
 	sum := 0
 	for i := 0; i < K; i++ {
 		sum += i
-		node := &MyNode{data: i}
-		stress = append(stress, node)
+		node := allocMyNode(i)
 		LFStackPush(stacks[i%2], fromMyNode(node))
 	}
 	c := make(chan bool, P)
@@ -134,7 +134,4 @@ func TestLFStackStress(t *testing.T) {
 	if sum2 != sum {
 		t.Fatalf("Wrong sum %d/%d", sum2, sum)
 	}
-
-	// Let nodes be collected now.
-	stress = nil
 }