runtime: fix stackCache=0 debug mode

Setting stackCache to 0 to disable stack caches for debugging hasn't
worked for a long time. It causes stackalloc to fall back to full span
allocation, round sub-page stacks down to 0 pages, and blow up.

Fix this debug mode so it disables the per-P caches, but continues to
use the global stack pools for small stacks, which correctly handle
sub-page stacks. While we're here, rename stackCache to stackNoCache
so it acts like the rest of the stack allocator debug modes where "0"
is the right default value.

Fixes #17291.

Change-Id: If401c41cee3448513cbd7bb2e9334a8efab257a7
Reviewed-on: https://go-review.googlesource.com/43637Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/stack.go

@@ -121,8 +121,7 @@ const (
 	stackFromSystem  = 0 // allocate stacks from system memory instead of the heap
 	stackFaultOnFree = 0 // old stacks are mapped noaccess to detect use after free
 	stackPoisonCopy  = 0 // fill stack that should not be accessed with garbage, to detect bad dereferences during copy
-
-	stackCache = 1
+	stackNoCache     = 0 // disable per-P small stack caches
 
 	// check the BP links during traceback.
 	debugCheckBP = false
@@ -349,7 +348,7 @@ func stackalloc(n uint32) stack {
 	// If we need a stack of a bigger size, we fall back on allocating
 	// a dedicated span.
 	var v unsafe.Pointer
-	if stackCache != 0 && n < _FixedStack<<_NumStackOrders && n < _StackCacheSize {
+	if n < _FixedStack<<_NumStackOrders && n < _StackCacheSize {
 		order := uint8(0)
 		n2 := n
 		for n2 > _FixedStack {
@@ -358,7 +357,7 @@ func stackalloc(n uint32) stack {
 		}
 		var x gclinkptr
 		c := thisg.m.mcache
-		if c == nil || thisg.m.preemptoff != "" || thisg.m.helpgc != 0 {
+		if stackNoCache != 0 || c == nil || thisg.m.preemptoff != "" || thisg.m.helpgc != 0 {
 			// c == nil can happen in the guts of exitsyscall or
 			// procresize. Just get a stack from the global pool.
 			// Also don't touch stackcache during gc
@@ -443,7 +442,7 @@ func stackfree(stk stack) {
 	if msanenabled {
 		msanfree(v, n)
 	}
-	if stackCache != 0 && n < _FixedStack<<_NumStackOrders && n < _StackCacheSize {
+	if n < _FixedStack<<_NumStackOrders && n < _StackCacheSize {
 		order := uint8(0)
 		n2 := n
 		for n2 > _FixedStack {
@@ -452,7 +451,7 @@ func stackfree(stk stack) {
 		}
 		x := gclinkptr(v)
 		c := gp.m.mcache
-		if c == nil || gp.m.preemptoff != "" || gp.m.helpgc != 0 {
+		if stackNoCache != 0 || c == nil || gp.m.preemptoff != "" || gp.m.helpgc != 0 {
 			lock(&stackpoolmu)
 			stackpoolfree(x, order)
 			unlock(&stackpoolmu)