Adding upstream version 1.21.8.upstream/1.21.8

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 350 insertions, 0 deletions

diff --git a/src/runtime/mgcstack.go b/src/runtime/mgcstack.go
new file mode 100644
index 0000000..6b55220
--- /dev/null
+++ b/src/runtime/mgcstack.go
@@ -0,0 +1,350 @@
+// 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"
+	"runtime/internal/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 {
+	cache pcvalueCache
+
+	// 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
+	}
+}