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// Copyright 2018 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.
// Garbage collector: stack objects and stack tracing
// See the design doc at https://docs.google.com/document/d/1un-Jn47yByHL7I0aVIP_uVCMxjdM5mpelJhiKlIqxkE/edit?usp=sharing
// Also see issue 22350.
// Stack tracing solves the problem of determining which parts of the
// stack are live and should be scanned. It runs as part of scanning
// a single goroutine stack.
//
// Normally determining which parts of the stack are live is easy to
// do statically, as user code has explicit references (reads and
// writes) to stack variables. The compiler can do a simple dataflow
// analysis to determine liveness of stack variables at every point in
// the code. See cmd/compile/internal/gc/plive.go for that analysis.
//
// However, when we take the address of a stack variable, determining
// whether that variable is still live is less clear. We can still
// look for static accesses, but accesses through a pointer to the
// variable are difficult in general to track statically. That pointer
// can be passed among functions on the stack, conditionally retained,
// etc.
//
// Instead, we will track pointers to stack variables dynamically.
// All pointers to stack-allocated variables will themselves be on the
// stack somewhere (or in associated locations, like defer records), so
// we can find them all efficiently.
//
// Stack tracing is organized as a mini garbage collection tracing
// pass. The objects in this garbage collection are all the variables
// on the stack whose address is taken, and which themselves contain a
// pointer. We call these variables "stack objects".
//
// We begin by determining all the stack objects on the stack and all
// the statically live pointers that may point into the stack. We then
// process each pointer to see if it points to a stack object. If it
// does, we scan that stack object. It may contain pointers into the
// heap, in which case those pointers are passed to the main garbage
// collection. It may also contain pointers into the stack, in which
// case we add them to our set of stack pointers.
//
// Once we're done processing all the pointers (including the ones we
// added during processing), we've found all the stack objects that
// are live. Any dead stack objects are not scanned and their contents
// will not keep heap objects live. Unlike the main garbage
// collection, we can't sweep the dead stack objects; they live on in
// a moribund state until the stack frame that contains them is
// popped.
//
// A stack can look like this:
//
// +----------+
// | foo() |
// | +------+ |
// | | A | | <---\
// | +------+ | |
// | | |
// | +------+ | |
// | | B | | |
// | +------+ | |
// | | |
// +----------+ |
// | bar() | |
// | +------+ | |
// | | C | | <-\ |
// | +----|-+ | | |
// | | | | |
// | +----v-+ | | |
// | | D ---------/
// | +------+ | |
// | | |
// +----------+ |
// | baz() | |
// | +------+ | |
// | | E -------/
// | +------+ |
// | ^ |
// | F: --/ |
// | |
// +----------+
//
// foo() calls bar() calls baz(). Each has a frame on the stack.
// foo() has stack objects A and B.
// bar() has stack objects C and D, with C pointing to D and D pointing to A.
// baz() has a stack object E pointing to C, and a local variable F pointing to E.
//
// Starting from the pointer in local variable F, we will eventually
// scan all of E, C, D, and A (in that order). B is never scanned
// because there is no live pointer to it. If B is also statically
// dead (meaning that foo() never accesses B again after it calls
// bar()), then B's pointers into the heap are not considered live.
package runtime
import (
"runtime/internal/sys"
"unsafe"
)
const stackTraceDebug = false
// Buffer for pointers found during stack tracing.
// Must be smaller than or equal to workbuf.
//
//go:notinheap
type stackWorkBuf struct {
stackWorkBufHdr
obj [(_WorkbufSize - unsafe.Sizeof(stackWorkBufHdr{})) / sys.PtrSize]uintptr
}
// Header declaration must come after the buf declaration above, because of issue #14620.
//
//go:notinheap
type stackWorkBufHdr struct {
workbufhdr
next *stackWorkBuf // linked list of workbufs
// Note: we could theoretically repurpose lfnode.next as this next pointer.
// It would save 1 word, but that probably isn't worth busting open
// the lfnode API.
}
// Buffer for stack objects found on a goroutine stack.
// Must be smaller than or equal to workbuf.
//
//go:notinheap
type stackObjectBuf struct {
stackObjectBufHdr
obj [(_WorkbufSize - unsafe.Sizeof(stackObjectBufHdr{})) / unsafe.Sizeof(stackObject{})]stackObject
}
//go:notinheap
type stackObjectBufHdr struct {
workbufhdr
next *stackObjectBuf
}
func init() {
if unsafe.Sizeof(stackWorkBuf{}) > unsafe.Sizeof(workbuf{}) {
panic("stackWorkBuf too big")
}
if unsafe.Sizeof(stackObjectBuf{}) > unsafe.Sizeof(workbuf{}) {
panic("stackObjectBuf too big")
}
}
// A stackObject represents a variable on the stack that has had
// its address taken.
//
//go:notinheap
type stackObject struct {
off uint32 // offset above stack.lo
size uint32 // size of object
typ *_type // type info (for ptr/nonptr bits). nil if object has been scanned.
left *stackObject // objects with lower addresses
right *stackObject // objects with higher addresses
}
// obj.typ = typ, but with no write barrier.
//go:nowritebarrier
func (obj *stackObject) setType(typ *_type) {
// Types of stack objects are always in read-only memory, not the heap.
// So not using a write barrier is ok.
*(*uintptr)(unsafe.Pointer(&obj.typ)) = uintptr(unsafe.Pointer(typ))
}
// A stackScanState keeps track of the state used during the GC walk
// of a goroutine.
//
//go:notinheap
type stackScanState struct {
cache pcvalueCache
// stack limits
stack stack
// buf contains the set of possible pointers to stack objects.
// Organized as a LIFO linked list of buffers.
// All buffers except possibly the head buffer are full.
buf *stackWorkBuf
freeBuf *stackWorkBuf // keep around one free buffer for allocation hysteresis
// list of stack objects
// Objects are in increasing address order.
head *stackObjectBuf
tail *stackObjectBuf
nobjs int
// root of binary tree for fast object lookup by address
// Initialized by buildIndex.
root *stackObject
}
// Add p as a potential pointer to a stack object.
// p must be a stack address.
func (s *stackScanState) putPtr(p uintptr) {
if p < s.stack.lo || p >= s.stack.hi {
throw("address not a stack address")
}
buf := s.buf
if buf == nil {
// Initial setup.
buf = (*stackWorkBuf)(unsafe.Pointer(getempty()))
buf.nobj = 0
buf.next = nil
s.buf = buf
} else if buf.nobj == len(buf.obj) {
if s.freeBuf != nil {
buf = s.freeBuf
s.freeBuf = nil
} else {
buf = (*stackWorkBuf)(unsafe.Pointer(getempty()))
}
buf.nobj = 0
buf.next = s.buf
s.buf = buf
}
buf.obj[buf.nobj] = p
buf.nobj++
}
// Remove and return a potential pointer to a stack object.
// Returns 0 if there are no more pointers available.
func (s *stackScanState) getPtr() uintptr {
buf := s.buf
if buf == nil {
// Never had any data.
return 0
}
if buf.nobj == 0 {
if s.freeBuf != nil {
// Free old freeBuf.
putempty((*workbuf)(unsafe.Pointer(s.freeBuf)))
}
// Move buf to the freeBuf.
s.freeBuf = buf
buf = buf.next
s.buf = buf
if buf == nil {
// No more data.
putempty((*workbuf)(unsafe.Pointer(s.freeBuf)))
s.freeBuf = nil
return 0
}
}
buf.nobj--
return buf.obj[buf.nobj]
}
// addObject adds a stack object at addr of type typ to the set of stack objects.
func (s *stackScanState) addObject(addr uintptr, typ *_type) {
x := s.tail
if x == nil {
// initial setup
x = (*stackObjectBuf)(unsafe.Pointer(getempty()))
x.next = nil
s.head = x
s.tail = x
}
if x.nobj > 0 && uint32(addr-s.stack.lo) < x.obj[x.nobj-1].off+x.obj[x.nobj-1].size {
throw("objects added out of order or overlapping")
}
if x.nobj == len(x.obj) {
// full buffer - allocate a new buffer, add to end of linked list
y := (*stackObjectBuf)(unsafe.Pointer(getempty()))
y.next = nil
x.next = y
s.tail = y
x = y
}
obj := &x.obj[x.nobj]
x.nobj++
obj.off = uint32(addr - s.stack.lo)
obj.size = uint32(typ.size)
obj.setType(typ)
// obj.left and obj.right will be initialized by buildIndex before use.
s.nobjs++
}
// buildIndex initializes s.root to a binary search tree.
// It should be called after all addObject calls but before
// any call of findObject.
func (s *stackScanState) buildIndex() {
s.root, _, _ = binarySearchTree(s.head, 0, s.nobjs)
}
// Build a binary search tree with the n objects in the list
// x.obj[idx], x.obj[idx+1], ..., x.next.obj[0], ...
// Returns the root of that tree, and the buf+idx of the nth object after x.obj[idx].
// (The first object that was not included in the binary search tree.)
// If n == 0, returns nil, x.
func binarySearchTree(x *stackObjectBuf, idx int, n int) (root *stackObject, restBuf *stackObjectBuf, restIdx int) {
if n == 0 {
return nil, x, idx
}
var left, right *stackObject
left, x, idx = binarySearchTree(x, idx, n/2)
root = &x.obj[idx]
idx++
if idx == len(x.obj) {
x = x.next
idx = 0
}
right, x, idx = binarySearchTree(x, idx, n-n/2-1)
root.left = left
root.right = right
return root, x, idx
}
// findObject returns the stack object containing address a, if any.
// Must have called buildIndex previously.
func (s *stackScanState) findObject(a uintptr) *stackObject {
off := uint32(a - s.stack.lo)
obj := s.root
for {
if obj == nil {
return nil
}
if off < obj.off {
obj = obj.left
continue
}
if off >= obj.off+obj.size {
obj = obj.right
continue
}
return obj
}
}