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Diffstat (limited to '')
| -rw-r--r-- | src/runtime/mgcstack.go | 348 |
1 files changed, 348 insertions, 0 deletions
diff --git a/src/runtime/mgcstack.go b/src/runtime/mgcstack.go new file mode 100644 index 0000000..f4a83f5 --- /dev/null +++ b/src/runtime/mgcstack.go @@ -0,0 +1,348 @@ +// 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 { + // 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 + } +} |