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Source file src/runtime/mgcstack.go

Documentation: runtime

     1  // Copyright 2018 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Garbage collector: stack objects and stack tracing
     6  // See the design doc at https://docs.google.com/document/d/1un-Jn47yByHL7I0aVIP_uVCMxjdM5mpelJhiKlIqxkE/edit?usp=sharing
     7  // Also see issue 22350.
     8  
     9  // Stack tracing solves the problem of determining which parts of the
    10  // stack are live and should be scanned. It runs as part of scanning
    11  // a single goroutine stack.
    12  //
    13  // Normally determining which parts of the stack are live is easy to
    14  // do statically, as user code has explicit references (reads and
    15  // writes) to stack variables. The compiler can do a simple dataflow
    16  // analysis to determine liveness of stack variables at every point in
    17  // the code. See cmd/compile/internal/gc/plive.go for that analysis.
    18  //
    19  // However, when we take the address of a stack variable, determining
    20  // whether that variable is still live is less clear. We can still
    21  // look for static accesses, but accesses through a pointer to the
    22  // variable are difficult in general to track statically. That pointer
    23  // can be passed among functions on the stack, conditionally retained,
    24  // etc.
    25  //
    26  // Instead, we will track pointers to stack variables dynamically.
    27  // All pointers to stack-allocated variables will themselves be on the
    28  // stack somewhere (or in associated locations, like defer records), so
    29  // we can find them all efficiently.
    30  //
    31  // Stack tracing is organized as a mini garbage collection tracing
    32  // pass. The objects in this garbage collection are all the variables
    33  // on the stack whose address is taken, and which themselves contain a
    34  // pointer. We call these variables "stack objects".
    35  //
    36  // We begin by determining all the stack objects on the stack and all
    37  // the statically live pointers that may point into the stack. We then
    38  // process each pointer to see if it points to a stack object. If it
    39  // does, we scan that stack object. It may contain pointers into the
    40  // heap, in which case those pointers are passed to the main garbage
    41  // collection. It may also contain pointers into the stack, in which
    42  // case we add them to our set of stack pointers.
    43  //
    44  // Once we're done processing all the pointers (including the ones we
    45  // added during processing), we've found all the stack objects that
    46  // are live. Any dead stack objects are not scanned and their contents
    47  // will not keep heap objects live. Unlike the main garbage
    48  // collection, we can't sweep the dead stack objects; they live on in
    49  // a moribund state until the stack frame that contains them is
    50  // popped.
    51  //
    52  // A stack can look like this:
    53  //
    54  // +----------+
    55  // | foo()    |
    56  // | +------+ |
    57  // | |  A   | | <---\
    58  // | +------+ |     |
    59  // |          |     |
    60  // | +------+ |     |
    61  // | |  B   | |     |
    62  // | +------+ |     |
    63  // |          |     |
    64  // +----------+     |
    65  // | bar()    |     |
    66  // | +------+ |     |
    67  // | |  C   | | <-\ |
    68  // | +----|-+ |   | |
    69  // |      |   |   | |
    70  // | +----v-+ |   | |
    71  // | |  D  ---------/
    72  // | +------+ |   |
    73  // |          |   |
    74  // +----------+   |
    75  // | baz()    |   |
    76  // | +------+ |   |
    77  // | |  E  -------/
    78  // | +------+ |
    79  // |      ^   |
    80  // | F: --/   |
    81  // |          |
    82  // +----------+
    83  //
    84  // foo() calls bar() calls baz(). Each has a frame on the stack.
    85  // foo() has stack objects A and B.
    86  // bar() has stack objects C and D, with C pointing to D and D pointing to A.
    87  // baz() has a stack object E pointing to C, and a local variable F pointing to E.
    88  //
    89  // Starting from the pointer in local variable F, we will eventually
    90  // scan all of E, C, D, and A (in that order). B is never scanned
    91  // because there is no live pointer to it. If B is also statically
    92  // dead (meaning that foo() never accesses B again after it calls
    93  // bar()), then B's pointers into the heap are not considered live.
    94  
    95  package runtime
    96  
    97  import (
    98  	"runtime/internal/sys"
    99  	"unsafe"
   100  )
   101  
   102  const stackTraceDebug = false
   103  
   104  // Buffer for pointers found during stack tracing.
   105  // Must be smaller than or equal to workbuf.
   106  //
   107  //go:notinheap
   108  type stackWorkBuf struct {
   109  	stackWorkBufHdr
   110  	obj [(_WorkbufSize - unsafe.Sizeof(stackWorkBufHdr{})) / sys.PtrSize]uintptr
   111  }
   112  
   113  // Header declaration must come after the buf declaration above, because of issue #14620.
   114  //
   115  //go:notinheap
   116  type stackWorkBufHdr struct {
   117  	workbufhdr
   118  	next *stackWorkBuf // linked list of workbufs
   119  	// Note: we could theoretically repurpose lfnode.next as this next pointer.
   120  	// It would save 1 word, but that probably isn't worth busting open
   121  	// the lfnode API.
   122  }
   123  
   124  // Buffer for stack objects found on a goroutine stack.
   125  // Must be smaller than or equal to workbuf.
   126  //
   127  //go:notinheap
   128  type stackObjectBuf struct {
   129  	stackObjectBufHdr
   130  	obj [(_WorkbufSize - unsafe.Sizeof(stackObjectBufHdr{})) / unsafe.Sizeof(stackObject{})]stackObject
   131  }
   132  
   133  //go:notinheap
   134  type stackObjectBufHdr struct {
   135  	workbufhdr
   136  	next *stackObjectBuf
   137  }
   138  
   139  func init() {
   140  	if unsafe.Sizeof(stackWorkBuf{}) > unsafe.Sizeof(workbuf{}) {
   141  		panic("stackWorkBuf too big")
   142  	}
   143  	if unsafe.Sizeof(stackObjectBuf{}) > unsafe.Sizeof(workbuf{}) {
   144  		panic("stackObjectBuf too big")
   145  	}
   146  }
   147  
   148  // A stackObject represents a variable on the stack that has had
   149  // its address taken.
   150  //
   151  //go:notinheap
   152  type stackObject struct {
   153  	off   uint32       // offset above stack.lo
   154  	size  uint32       // size of object
   155  	typ   *_type       // type info (for ptr/nonptr bits). nil if object has been scanned.
   156  	left  *stackObject // objects with lower addresses
   157  	right *stackObject // objects with higher addresses
   158  }
   159  
   160  // obj.typ = typ, but with no write barrier.
   161  //go:nowritebarrier
   162  func (obj *stackObject) setType(typ *_type) {
   163  	// Types of stack objects are always in read-only memory, not the heap.
   164  	// So not using a write barrier is ok.
   165  	*(*uintptr)(unsafe.Pointer(&obj.typ)) = uintptr(unsafe.Pointer(typ))
   166  }
   167  
   168  // A stackScanState keeps track of the state used during the GC walk
   169  // of a goroutine.
   170  type stackScanState struct {
   171  	cache pcvalueCache
   172  
   173  	// stack limits
   174  	stack stack
   175  
   176  	// conservative indicates that the next frame must be scanned conservatively.
   177  	// This applies only to the innermost frame at an async safe-point.
   178  	conservative bool
   179  
   180  	// buf contains the set of possible pointers to stack objects.
   181  	// Organized as a LIFO linked list of buffers.
   182  	// All buffers except possibly the head buffer are full.
   183  	buf     *stackWorkBuf
   184  	freeBuf *stackWorkBuf // keep around one free buffer for allocation hysteresis
   185  
   186  	// cbuf contains conservative pointers to stack objects. If
   187  	// all pointers to a stack object are obtained via
   188  	// conservative scanning, then the stack object may be dead
   189  	// and may contain dead pointers, so it must be scanned
   190  	// defensively.
   191  	cbuf *stackWorkBuf
   192  
   193  	// list of stack objects
   194  	// Objects are in increasing address order.
   195  	head  *stackObjectBuf
   196  	tail  *stackObjectBuf
   197  	nobjs int
   198  
   199  	// root of binary tree for fast object lookup by address
   200  	// Initialized by buildIndex.
   201  	root *stackObject
   202  }
   203  
   204  // Add p as a potential pointer to a stack object.
   205  // p must be a stack address.
   206  func (s *stackScanState) putPtr(p uintptr, conservative bool) {
   207  	if p < s.stack.lo || p >= s.stack.hi {
   208  		throw("address not a stack address")
   209  	}
   210  	head := &s.buf
   211  	if conservative {
   212  		head = &s.cbuf
   213  	}
   214  	buf := *head
   215  	if buf == nil {
   216  		// Initial setup.
   217  		buf = (*stackWorkBuf)(unsafe.Pointer(getempty()))
   218  		buf.nobj = 0
   219  		buf.next = nil
   220  		*head = buf
   221  	} else if buf.nobj == len(buf.obj) {
   222  		if s.freeBuf != nil {
   223  			buf = s.freeBuf
   224  			s.freeBuf = nil
   225  		} else {
   226  			buf = (*stackWorkBuf)(unsafe.Pointer(getempty()))
   227  		}
   228  		buf.nobj = 0
   229  		buf.next = *head
   230  		*head = buf
   231  	}
   232  	buf.obj[buf.nobj] = p
   233  	buf.nobj++
   234  }
   235  
   236  // Remove and return a potential pointer to a stack object.
   237  // Returns 0 if there are no more pointers available.
   238  //
   239  // This prefers non-conservative pointers so we scan stack objects
   240  // precisely if there are any non-conservative pointers to them.
   241  func (s *stackScanState) getPtr() (p uintptr, conservative bool) {
   242  	for _, head := range []**stackWorkBuf{&s.buf, &s.cbuf} {
   243  		buf := *head
   244  		if buf == nil {
   245  			// Never had any data.
   246  			continue
   247  		}
   248  		if buf.nobj == 0 {
   249  			if s.freeBuf != nil {
   250  				// Free old freeBuf.
   251  				putempty((*workbuf)(unsafe.Pointer(s.freeBuf)))
   252  			}
   253  			// Move buf to the freeBuf.
   254  			s.freeBuf = buf
   255  			buf = buf.next
   256  			*head = buf
   257  			if buf == nil {
   258  				// No more data in this list.
   259  				continue
   260  			}
   261  		}
   262  		buf.nobj--
   263  		return buf.obj[buf.nobj], head == &s.cbuf
   264  	}
   265  	// No more data in either list.
   266  	if s.freeBuf != nil {
   267  		putempty((*workbuf)(unsafe.Pointer(s.freeBuf)))
   268  		s.freeBuf = nil
   269  	}
   270  	return 0, false
   271  }
   272  
   273  // addObject adds a stack object at addr of type typ to the set of stack objects.
   274  func (s *stackScanState) addObject(addr uintptr, typ *_type) {
   275  	x := s.tail
   276  	if x == nil {
   277  		// initial setup
   278  		x = (*stackObjectBuf)(unsafe.Pointer(getempty()))
   279  		x.next = nil
   280  		s.head = x
   281  		s.tail = x
   282  	}
   283  	if x.nobj > 0 && uint32(addr-s.stack.lo) < x.obj[x.nobj-1].off+x.obj[x.nobj-1].size {
   284  		throw("objects added out of order or overlapping")
   285  	}
   286  	if x.nobj == len(x.obj) {
   287  		// full buffer - allocate a new buffer, add to end of linked list
   288  		y := (*stackObjectBuf)(unsafe.Pointer(getempty()))
   289  		y.next = nil
   290  		x.next = y
   291  		s.tail = y
   292  		x = y
   293  	}
   294  	obj := &x.obj[x.nobj]
   295  	x.nobj++
   296  	obj.off = uint32(addr - s.stack.lo)
   297  	obj.size = uint32(typ.size)
   298  	obj.setType(typ)
   299  	// obj.left and obj.right will be initialized by buildIndex before use.
   300  	s.nobjs++
   301  }
   302  
   303  // buildIndex initializes s.root to a binary search tree.
   304  // It should be called after all addObject calls but before
   305  // any call of findObject.
   306  func (s *stackScanState) buildIndex() {
   307  	s.root, _, _ = binarySearchTree(s.head, 0, s.nobjs)
   308  }
   309  
   310  // Build a binary search tree with the n objects in the list
   311  // x.obj[idx], x.obj[idx+1], ..., x.next.obj[0], ...
   312  // Returns the root of that tree, and the buf+idx of the nth object after x.obj[idx].
   313  // (The first object that was not included in the binary search tree.)
   314  // If n == 0, returns nil, x.
   315  func binarySearchTree(x *stackObjectBuf, idx int, n int) (root *stackObject, restBuf *stackObjectBuf, restIdx int) {
   316  	if n == 0 {
   317  		return nil, x, idx
   318  	}
   319  	var left, right *stackObject
   320  	left, x, idx = binarySearchTree(x, idx, n/2)
   321  	root = &x.obj[idx]
   322  	idx++
   323  	if idx == len(x.obj) {
   324  		x = x.next
   325  		idx = 0
   326  	}
   327  	right, x, idx = binarySearchTree(x, idx, n-n/2-1)
   328  	root.left = left
   329  	root.right = right
   330  	return root, x, idx
   331  }
   332  
   333  // findObject returns the stack object containing address a, if any.
   334  // Must have called buildIndex previously.
   335  func (s *stackScanState) findObject(a uintptr) *stackObject {
   336  	off := uint32(a - s.stack.lo)
   337  	obj := s.root
   338  	for {
   339  		if obj == nil {
   340  			return nil
   341  		}
   342  		if off < obj.off {
   343  			obj = obj.left
   344  			continue
   345  		}
   346  		if off >= obj.off+obj.size {
   347  			obj = obj.right
   348  			continue
   349  		}
   350  		return obj
   351  	}
   352  }
   353  

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