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/43637
Reviewed-by: Keith Randall <khr@golang.org>

diff --git a/src/runtime/stack.go b/src/runtime/stack.go
index 7860cb183e31d65b7d7766755d055554684e4f44..e1c70c3158608cf8f75ffbaee42bc4496bae0004 100644 (file)
--- a/src/runtime/stack.go
+++ b/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)