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/10313
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 19 insertions, 4 deletions

diff --git a/src/runtime/stack1.go b/src/runtime/stack1.go
index e593b8a3a8..f77e87cdf9 100644
--- a/src/runtime/stack1.go
+++ b/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
 	}