Currently the test fails as:
$ go test -v -cpu 1,1,1,1 runtime -test.run=TestStack
stack_test.go:1584: Stack inuse: want 4194304, got 18446744073709547520

Update #7468

LGTM=rsc
R=golang-codereviews, bradfitz
CC=golang-codereviews, khr, rsc
https://golang.org/cl/74010043