src/runtime/mgcstack.go - go - Git at Google

 // 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 (
 	"internal/goarch"
 	"internal/runtime/sys"
 	"unsafe"
 )

 const stackTraceDebug = false

 // Buffer for pointers found during stack tracing.
 // Must be smaller than or equal to workbuf.
 type stackWorkBuf struct {
 	_ sys.NotInHeap
 	stackWorkBufHdr
 	obj [(_WorkbufSize - unsafe.Sizeof(stackWorkBufHdr{})) / goarch.PtrSize]uintptr
 }

 // Header declaration must come after the buf declaration above, because of issue #14620.
 type stackWorkBufHdr struct {
 	_ sys.NotInHeap
 	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.
 type stackObjectBuf struct {
 	_ sys.NotInHeap
 	stackObjectBufHdr
 	obj [(_WorkbufSize - unsafe.Sizeof(stackObjectBufHdr{})) / unsafe.Sizeof(stackObject{})]stackObject
 }

 type stackObjectBufHdr struct {
 	_ sys.NotInHeap
 	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.
 type stackObject struct {
 	_     sys.NotInHeap
 	off   uint32             // offset above stack.lo
 	size  uint32             // size of object
 	r     *stackObjectRecord // info of the object (for ptr/nonptr bits). nil if object has been scanned.
 	left  *stackObject       // objects with lower addresses
 	right *stackObject       // objects with higher addresses
 }

 // obj.r = r, but with no write barrier.
 //
 //go:nowritebarrier
 func (obj *stackObject) setRecord(r *stackObjectRecord) {
 	// 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.r)) = uintptr(unsafe.Pointer(r))
 }

 // A stackScanState keeps track of the state used during the GC walk
 // of a goroutine.
 type stackScanState struct {
 	// stack limits
 	stack stack

 	// conservative indicates that the next frame must be scanned conservatively.
 	// This applies only to the innermost frame at an async safe-point.
 	conservative bool

 	// 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

 	// cbuf contains conservative pointers to stack objects. If
 	// all pointers to a stack object are obtained via
 	// conservative scanning, then the stack object may be dead
 	// and may contain dead pointers, so it must be scanned
 	// defensively.
 	cbuf *stackWorkBuf

 	// 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, conservative bool) {
 	if p < s.stack.lo || p >= s.stack.hi {
 		throw("address not a stack address")
 	}
 	head := &s.buf
 	if conservative {
 		head = &s.cbuf
 	}
 	buf := *head
 	if buf == nil {
 		// Initial setup.
 		buf = (*stackWorkBuf)(unsafe.Pointer(getempty()))
 		buf.nobj = 0
 		buf.next = nil
 		*head = 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 = *head
 		*head = 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.
 //
 // This prefers non-conservative pointers so we scan stack objects
 // precisely if there are any non-conservative pointers to them.
 func (s *stackScanState) getPtr() (p uintptr, conservative bool) {
 	for _, head := range []**stackWorkBuf{&s.buf, &s.cbuf} {
 		buf := *head
 		if buf == nil {
 			// Never had any data.
 			continue
 		}
 		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
 			*head = buf
 			if buf == nil {
 				// No more data in this list.
 				continue
 			}
 		}
 		buf.nobj--
 		return buf.obj[buf.nobj], head == &s.cbuf
 	}
 	// No more data in either list.
 	if s.freeBuf != nil {
 		putempty((*workbuf)(unsafe.Pointer(s.freeBuf)))
 		s.freeBuf = nil
 	}
 	return 0, false
 }

 // addObject adds a stack object at addr of type typ to the set of stack objects.
 func (s *stackScanState) addObject(addr uintptr, r *stackObjectRecord) {
 	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(r.size)
 	obj.setRecord(r)
 	// 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
 	}
 }
	// 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 (
	"internal/goarch"
	"internal/runtime/sys"
	"unsafe"
	)

	const stackTraceDebug = false

	// Buffer for pointers found during stack tracing.
	// Must be smaller than or equal to workbuf.
	type stackWorkBuf struct {
	_ sys.NotInHeap
	stackWorkBufHdr
	obj [(_WorkbufSize - unsafe.Sizeof(stackWorkBufHdr{})) / goarch.PtrSize]uintptr
	}

	// Header declaration must come after the buf declaration above, because of issue #14620.
	type stackWorkBufHdr struct {
	_ sys.NotInHeap
	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.
	type stackObjectBuf struct {
	_ sys.NotInHeap
	stackObjectBufHdr
	obj [(_WorkbufSize - unsafe.Sizeof(stackObjectBufHdr{})) / unsafe.Sizeof(stackObject{})]stackObject
	}

	type stackObjectBufHdr struct {
	_ sys.NotInHeap
	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.
	type stackObject struct {
	_ sys.NotInHeap
	off uint32 // offset above stack.lo
	size uint32 // size of object
	r *stackObjectRecord // info of the object (for ptr/nonptr bits). nil if object has been scanned.
	left *stackObject // objects with lower addresses
	right *stackObject // objects with higher addresses
	}

	// obj.r = r, but with no write barrier.
	//
	//go:nowritebarrier
	func (obj stackObject) setRecord(r stackObjectRecord) {
	// 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.r)) = uintptr(unsafe.Pointer(r))
	}

	// A stackScanState keeps track of the state used during the GC walk
	// of a goroutine.
	type stackScanState struct {
	// stack limits
	stack stack

	// conservative indicates that the next frame must be scanned conservatively.
	// This applies only to the innermost frame at an async safe-point.
	conservative bool

	// 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

	// cbuf contains conservative pointers to stack objects. If
	// all pointers to a stack object are obtained via
	// conservative scanning, then the stack object may be dead
	// and may contain dead pointers, so it must be scanned
	// defensively.
	cbuf *stackWorkBuf

	// 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, conservative bool) {
	if p < s.stack.lo \|\| p >= s.stack.hi {
	throw("address not a stack address")
	}
	head := &s.buf
	if conservative {
	head = &s.cbuf
	}
	buf := *head
	if buf == nil {
	// Initial setup.
	buf = (*stackWorkBuf)(unsafe.Pointer(getempty()))
	buf.nobj = 0
	buf.next = nil
	*head = 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 = *head
	*head = 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.
	//
	// This prefers non-conservative pointers so we scan stack objects
	// precisely if there are any non-conservative pointers to them.
	func (s *stackScanState) getPtr() (p uintptr, conservative bool) {
	for _, head := range []**stackWorkBuf{&s.buf, &s.cbuf} {
	buf := *head
	if buf == nil {
	// Never had any data.
	continue
	}
	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
	*head = buf
	if buf == nil {
	// No more data in this list.
	continue
	}
	}
	buf.nobj--
	return buf.obj[buf.nobj], head == &s.cbuf
	}
	// No more data in either list.
	if s.freeBuf != nil {
	putempty((*workbuf)(unsafe.Pointer(s.freeBuf)))
	s.freeBuf = nil
	}
	return 0, false
	}

	// addObject adds a stack object at addr of type typ to the set of stack objects.
	func (s stackScanState) addObject(addr uintptr, r stackObjectRecord) {
	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(r.size)
	obj.setRecord(r)
	// 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
	}
	}