runtime: steal space for stack barrier tracking from stack

The stack barrier code will need a bookkeeping structure to keep track
of the overwritten return PCs. This commit introduces and allocates
this structure, but does not yet use the structure.

We don't want to allocate space for this structure during garbage
collection, so this commit allocates it along with the allocation of
the corresponding stack. However, we can't do a regular allocation in
newstack because mallocgc may itself grow the stack (which would lead
to a recursive allocation). Hence, this commit makes the bookkeeping
structure part of the stack allocation itself by stealing the
necessary space from the top of the stack allocation. Since the size
of this bookkeeping structure is logarithmic in the size of the stack,
this has minimal impact on stack behavior.

Change-Id: Ia14408be06aafa9ca4867f4e70bddb3fe0e96665
Reviewed-on: https://go-review.googlesource.com/10313Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mgc.go

@@ -125,6 +125,19 @@ const (
 	_RootSpans       = 3
 	_RootFlushCaches = 4
 	_RootCount       = 5
+
+	// firstStackBarrierOffset is the approximate byte offset at
+	// which to place the first stack barrier from the current SP.
+	// This is a lower bound on how much stack will have to be
+	// re-scanned during mark termination. Subsequent barriers are
+	// placed at firstStackBarrierOffset * 2^n offsets.
+	//
+	// For debugging, this can be set to 0, which will install a
+	// stack barrier at every frame. If you do this, you may also
+	// have to raise _StackMin, since the stack barrier
+	// bookkeeping will use a large amount of each stack.
+	firstStackBarrierOffset = 1024
+	debugStackBarrier       = false
 )
 
 // heapminimum is the minimum heap size at which to trigger GC.

src/runtime/mgcmark.go

@@ -399,6 +399,27 @@ func scanframeworker(frame *stkframe, unused unsafe.Pointer, gcw *gcWork) {
 	}
 }
 
+// gcMaxStackBarriers returns the maximum number of stack barriers
+// that can be installed in a stack of stackSize bytes.
+func gcMaxStackBarriers(stackSize int) (n int) {
+	if firstStackBarrierOffset == 0 {
+		// Special debugging case for inserting stack barriers
+		// at every frame. Steal half of the stack for the
+		// []stkbar. Technically, if the stack were to consist
+		// solely of return PCs we would need two thirds of
+		// the stack, but stealing that much breaks things and
+		// this doesn't happen in practice.
+		return stackSize / 2 / int(unsafe.Sizeof(stkbar{}))
+	}
+
+	offset := firstStackBarrierOffset
+	for offset < stackSize {
+		n++
+		offset *= 2
+	}
+	return n + 1
+}
+
 // TODO(austin): Can we consolidate the gcDrain* functions?
 
 // gcDrain scans objects in work buffers, blackening grey

src/runtime/proc1.go

@@ -2157,7 +2157,7 @@ func malg(stacksize int32) *g {
 	if stacksize >= 0 {
 		stacksize = round2(_StackSystem + stacksize)
 		systemstack(func() {
-			newg.stack = stackalloc(uint32(stacksize))
+			newg.stack, newg.stkbar = stackalloc(uint32(stacksize))
 		})
 		newg.stackguard0 = newg.stack.lo + _StackGuard
 		newg.stackguard1 = ^uintptr(0)
@@ -2285,6 +2285,8 @@ func gfput(_p_ *p, gp *g) {
 		gp.stack.lo = 0
 		gp.stack.hi = 0
 		gp.stackguard0 = 0
+		gp.stkbar = nil
+		gp.stkbarPos = 0
 	}
 
 	gp.schedlink.set(_p_.gfree)
@@ -2328,7 +2330,7 @@ retry:
 		if gp.stack.lo == 0 {
 			// Stack was deallocated in gfput.  Allocate a new one.
 			systemstack(func() {
-				gp.stack = stackalloc(_FixedStack)
+				gp.stack, gp.stkbar = stackalloc(_FixedStack)
 			})
 			gp.stackguard0 = gp.stack.lo + _StackGuard
 			gp.stackAlloc = _FixedStack

src/runtime/runtime2.go

@@ -202,6 +202,12 @@ type stack struct {
 	hi uintptr
 }
 
+// stkbar records the state of a G's stack barrier.
+type stkbar struct {
+	savedLRPtr uintptr // location overwritten by stack barrier PC
+	savedLRVal uintptr // value overwritten at savedLRPtr
+}
+
 type g struct {
 	// Stack parameters.
 	// stack describes the actual stack memory: [stack.lo, stack.hi).
@@ -220,6 +226,8 @@ type g struct {
 	sched        gobuf
 	syscallsp    uintptr        // if status==Gsyscall, syscallsp = sched.sp to use during gc
 	syscallpc    uintptr        // if status==Gsyscall, syscallpc = sched.pc to use during gc
+	stkbar       []stkbar       // stack barriers, from low to high
+	stkbarPos    uintptr        // index of lowest stack barrier not hit
 	param        unsafe.Pointer // passed parameter on wakeup
 	atomicstatus uint32
 	goid         int64

src/runtime/stack1.go

@@ -175,7 +175,7 @@ func stackcache_clear(c *mcache) {
 	unlock(&stackpoolmu)
 }
 
-func stackalloc(n uint32) stack {
+func stackalloc(n uint32) (stack, []stkbar) {
 	// Stackalloc must be called on scheduler stack, so that we
 	// never try to grow the stack during the code that stackalloc runs.
 	// Doing so would cause a deadlock (issue 1547).
@@ -190,12 +190,18 @@ func stackalloc(n uint32) stack {
 		print("stackalloc ", n, "\n")
 	}
 
+	// Compute the size of stack barrier array.
+	maxstkbar := gcMaxStackBarriers(int(n))
+	nstkbar := unsafe.Sizeof(stkbar{}) * uintptr(maxstkbar)
+
 	if debug.efence != 0 || stackFromSystem != 0 {
 		v := sysAlloc(round(uintptr(n), _PageSize), &memstats.stacks_sys)
 		if v == nil {
 			throw("out of memory (stackalloc)")
 		}
-		return stack{uintptr(v), uintptr(v) + uintptr(n)}
+		top := uintptr(n) - nstkbar
+		stkbarSlice := slice{add(v, top), 0, maxstkbar}
+		return stack{uintptr(v), uintptr(v) + top}, *(*[]stkbar)(unsafe.Pointer(&stkbarSlice))
 	}
 
 	// Small stacks are allocated with a fixed-size free-list allocator.
@@ -243,7 +249,9 @@ func stackalloc(n uint32) stack {
 	if stackDebug >= 1 {
 		print("  allocated ", v, "\n")
 	}
-	return stack{uintptr(v), uintptr(v) + uintptr(n)}
+	top := uintptr(n) - nstkbar
+	stkbarSlice := slice{add(v, top), 0, maxstkbar}
+	return stack{uintptr(v), uintptr(v) + top}, *(*[]stkbar)(unsafe.Pointer(&stkbarSlice))
 }
 
 func stackfree(stk stack, n uintptr) {
@@ -556,7 +564,7 @@ func copystack(gp *g, newsize uintptr) {
 	used := old.hi - gp.sched.sp
 
 	// allocate new stack
-	new := stackalloc(uint32(newsize))
+	new, newstkbar := stackalloc(uint32(newsize))
 	if stackPoisonCopy != 0 {
 		fillstack(new, 0xfd)
 	}
@@ -582,12 +590,17 @@ func copystack(gp *g, newsize uintptr) {
 	}
 	memmove(unsafe.Pointer(new.hi-used), unsafe.Pointer(old.hi-used), used)
 
+	// copy old stack barriers to new stack barrier array
+	newstkbar = newstkbar[:len(gp.stkbar)]
+	copy(newstkbar, gp.stkbar)
+
 	// Swap out old stack for new one
 	gp.stack = new
 	gp.stackguard0 = new.lo + _StackGuard // NOTE: might clobber a preempt request
 	gp.sched.sp = new.hi - used
 	oldsize := gp.stackAlloc
 	gp.stackAlloc = newsize
+	gp.stkbar = newstkbar
 
 	// free old stack
 	if stackPoisonCopy != 0 {
@@ -794,6 +807,8 @@ func shrinkstack(gp *g) {
 			stackfree(gp.stack, gp.stackAlloc)
 			gp.stack.lo = 0
 			gp.stack.hi = 0
+			gp.stkbar = nil
+			gp.stkbarPos = 0
 		}
 		return
 	}