Revert "Revert "cmd/compile,runtime: allocate defer records on the stack""

This reverts CL 180761

Reason for revert: Reinstate the stack-allocated defer CL.

There was nothing wrong with the CL proper, but stack allocation of defers exposed two other issues.

Issue #32477: Fix has been submitted as CL 181258.
Issue #32498: Possible fix is CL 181377 (not submitted yet).

Change-Id: I32b3365d5026600069291b068bbba6cb15295eb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/181378
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 18 insertions, 2 deletions

diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index efa007aa97..2c63724472 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -712,15 +712,31 @@ func scanstack(gp *g, gcw *gcWork) {
 
 	// Find additional pointers that point into the stack from the heap.
 	// Currently this includes defers and panics. See also function copystack.
+
+	// Find and trace all defer arguments.
 	tracebackdefers(gp, scanframe, nil)
+
+	// Find and trace other pointers in defer records.
 	for d := gp._defer; d != nil; d = d.link {
-		// tracebackdefers above does not scan the func value, which could
-		// be a stack allocated closure. See issue 30453.
 		if d.fn != nil {
+			// tracebackdefers above does not scan the func value, which could
+			// be a stack allocated closure. See issue 30453.
 			scanblock(uintptr(unsafe.Pointer(&d.fn)), sys.PtrSize, &oneptrmask[0], gcw, &state)
 		}
+		if d.link != nil {
+			// The link field of a stack-allocated defer record might point
+			// to a heap-allocated defer record. Keep that heap record live.
+			scanblock(uintptr(unsafe.Pointer(&d.link)), sys.PtrSize, &oneptrmask[0], gcw, &state)
+		}
+		// Retain defers records themselves.
+		// Defer records might not be reachable from the G through regular heap
+		// tracing because the defer linked list might weave between the stack and the heap.
+		if d.heap {
+			scanblock(uintptr(unsafe.Pointer(&d)), sys.PtrSize, &oneptrmask[0], gcw, &state)
+		}
 	}
 	if gp._panic != nil {
+		// Panics are always stack allocated.
 		state.putPtr(uintptr(unsafe.Pointer(gp._panic)))
 	}