runtime: add always-preempt maymorestack hook

This adds a maymorestack hook that forces a preemption at every
possible cooperative preemption point. This would have helped us catch
several recent preemption-related bugs earlier, including golang#47302,
 golang#47304, and golang#47441.

For golang#48297.

Change-Id: Ib82c973589c8a7223900e1842913b8591938fb9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/359796
Trust: Austin Clements <[email protected]>
Run-TryBot: Austin Clements <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: David Chase <[email protected]>

src/runtime/debug.go

@@ -61,3 +61,37 @@ func NumGoroutine() int {
 func debug_modinfo() string {
 	return modinfo
 }
+
+// mayMoreStackPreempt is a maymorestack hook that forces a preemption
+// at every possible cooperative preemption point.
+//
+// This is valuable to apply to the runtime, which can be sensitive to
+// preemption points. To apply this to all preemption points in the
+// runtime and runtime-like code, use the following in bash or zsh:
+//
+//   X=(-{gc,asm}flags={runtime/...,reflect,sync}=-d=maymorestack=runtime.mayMoreStackPreempt) GOFLAGS=${X[@]}
+//
+// This must be deeply nosplit because it is called from a function
+// prologue before the stack is set up and because the compiler will
+// call it from any splittable prologue (leading to infinite
+// recursion).
+//
+// Ideally it should also use very little stack because the linker
+// doesn't currently account for this in nosplit stack depth checking.
+//
+//go:nosplit
+//
+// Ensure mayMoreStackPreempt can be called for all ABIs.
+//
+//go:linkname mayMoreStackPreempt
+func mayMoreStackPreempt() {
+	// Don't do anything on the g0 or gsignal stack.
+	g := getg()
+	if g == g.m.g0 || g == g.m.gsignal {
+		return
+	}
+	// Force a preemption, unless the stack is already poisoned.
+	if g.stackguard0 < stackPoisonMin {
+		g.stackguard0 = stackPreempt
+	}
+}

src/runtime/stack.go

@@ -144,6 +144,9 @@ const (
 	// Force a stack movement. Used for debugging.
 	// 0xfffffeed in hex.
 	stackForceMove = uintptrMask & -275
+
+	// stackPoisonMin is the lowest allowed stack poison value.
+	stackPoisonMin = uintptrMask & -4096
 )
 
 // Global pool of spans that have free stacks.