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>

8 files changed, 142 insertions, 46 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)))
 	}
 
diff --git a/src/runtime/panic.go b/src/runtime/panic.go
index f39a4bc0a2..ce26eb540d 100644
--- a/src/runtime/panic.go
+++ b/src/runtime/panic.go
@@ -228,6 +228,46 @@ func deferproc(siz int32, fn *funcval) { // arguments of fn follow fn
 	// been set and must not be clobbered.
 }
 
+// deferprocStack queues a new deferred function with a defer record on the stack.
+// The defer record must have its siz and fn fields initialized.
+// All other fields can contain junk.
+// The defer record must be immediately followed in memory by
+// the arguments of the defer.
+// Nosplit because the arguments on the stack won't be scanned
+// until the defer record is spliced into the gp._defer list.
+//go:nosplit
+func deferprocStack(d *_defer) {
+	gp := getg()
+	if gp.m.curg != gp {
+		// go code on the system stack can't defer
+		throw("defer on system stack")
+	}
+	// siz and fn are already set.
+	// The other fields are junk on entry to deferprocStack and
+	// are initialized here.
+	d.started = false
+	d.heap = false
+	d.sp = getcallersp()
+	d.pc = getcallerpc()
+	// The lines below implement:
+	//   d.panic = nil
+	//   d.link = gp._defer
+	//   gp._defer = d
+	// But without write barriers. The first two are writes to
+	// the stack so they don't need a write barrier, and furthermore
+	// are to uninitialized memory, so they must not use a write barrier.
+	// The third write does not require a write barrier because we
+	// explicitly mark all the defer structures, so we don't need to
+	// keep track of pointers to them with a write barrier.
+	*(*uintptr)(unsafe.Pointer(&d._panic)) = 0
+	*(*uintptr)(unsafe.Pointer(&d.link)) = uintptr(unsafe.Pointer(gp._defer))
+	*(*uintptr)(unsafe.Pointer(&gp._defer)) = uintptr(unsafe.Pointer(d))
+
+	return0()
+	// No code can go here - the C return register has
+	// been set and must not be clobbered.
+}
+
 // Small malloc size classes >= 16 are the multiples of 16: 16, 32, 48, 64, 80, 96, 112, 128, 144, ...
 // Each P holds a pool for defers with small arg sizes.
 // Assign defer allocations to pools by rounding to 16, to match malloc size classes.
@@ -349,6 +389,7 @@ func newdefer(siz int32) *_defer {
 		}
 	}
 	d.siz = siz
+	d.heap = true
 	d.link = gp._defer
 	gp._defer = d
 	return d
@@ -368,6 +409,9 @@ func freedefer(d *_defer) {
 	if d.fn != nil {
 		freedeferfn()
 	}
+	if !d.heap {
+		return
+	}
 	sc := deferclass(uintptr(d.siz))
 	if sc >= uintptr(len(p{}.deferpool)) {
 		return
diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index bc5b48222b..16c02cd1ed 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -775,9 +775,16 @@ func extendRandom(r []byte, n int) {
 
 // A _defer holds an entry on the list of deferred calls.
 // If you add a field here, add code to clear it in freedefer.
+// This struct must match the code in cmd/compile/internal/gc/reflect.go:deferstruct
+// and cmd/compile/internal/gc/ssa.go:(*state).call.
+// Some defers will be allocated on the stack and some on the heap.
+// All defers are logically part of the stack, so write barriers to
+// initialize them are not required. All defers must be manually scanned,
+// and for heap defers, marked.
 type _defer struct {
-	siz     int32
+	siz     int32 // includes both arguments and results
 	started bool
+	heap    bool
 	sp      uintptr // sp at time of defer
 	pc      uintptr
 	fn      *funcval
diff --git a/src/runtime/stack.go b/src/runtime/stack.go
index 22a0053fdb..7ae3eeef83 100644
--- a/src/runtime/stack.go
+++ b/src/runtime/stack.go
@@ -719,16 +719,21 @@ func adjustctxt(gp *g, adjinfo *adjustinfo) {
 }
 
 func adjustdefers(gp *g, adjinfo *adjustinfo) {
-	// Adjust defer argument blocks the same way we adjust active stack frames.
-	tracebackdefers(gp, adjustframe, noescape(unsafe.Pointer(adjinfo)))
-
 	// Adjust pointers in the Defer structs.
-	// Defer structs themselves are never on the stack.
+	// We need to do this first because we need to adjust the
+	// defer.link fields so we always work on the new stack.
+	adjustpointer(adjinfo, unsafe.Pointer(&gp._defer))
 	for d := gp._defer; d != nil; d = d.link {
 		adjustpointer(adjinfo, unsafe.Pointer(&d.fn))
 		adjustpointer(adjinfo, unsafe.Pointer(&d.sp))
 		adjustpointer(adjinfo, unsafe.Pointer(&d._panic))
+		adjustpointer(adjinfo, unsafe.Pointer(&d.link))
 	}
+
+	// Adjust defer argument blocks the same way we adjust active stack frames.
+	// Note: this code is after the loop above, so that if a defer record is
+	// stack allocated, we work on the copy in the new stack.
+	tracebackdefers(gp, adjustframe, noescape(unsafe.Pointer(adjinfo)))
 }
 
 func adjustpanics(gp *g, adjinfo *adjustinfo) {
diff --git a/src/runtime/stack_test.go b/src/runtime/stack_test.go
index df73b3a1d5..143d3a99a0 100644
--- a/src/runtime/stack_test.go
+++ b/src/runtime/stack_test.go
@@ -799,3 +799,58 @@ func TestDeferLiveness(t *testing.T) {
 		t.Errorf("output:\n%s\n\nwant no output", output)
 	}
 }
+
+func TestDeferHeapAndStack(t *testing.T) {
+	P := 4     // processors
+	N := 10000 //iterations
+	D := 200   // stack depth
+
+	if testing.Short() {
+		P /= 2
+		N /= 10
+		D /= 10
+	}
+	c := make(chan bool)
+	for p := 0; p < P; p++ {
+		go func() {
+			for i := 0; i < N; i++ {
+				if deferHeapAndStack(D) != 2*D {
+					panic("bad result")
+				}
+			}
+			c <- true
+		}()
+	}
+	for p := 0; p < P; p++ {
+		<-c
+	}
+}
+
+// deferHeapAndStack(n) computes 2*n
+func deferHeapAndStack(n int) (r int) {
+	if n == 0 {
+		return 0
+	}
+	if n%2 == 0 {
+		// heap-allocated defers
+		for i := 0; i < 2; i++ {
+			defer func() {
+				r++
+			}()
+		}
+	} else {
+		// stack-allocated defers
+		defer func() {
+			r++
+		}()
+		defer func() {
+			r++
+		}()
+	}
+	r = deferHeapAndStack(n - 1)
+	escapeMe(new([1024]byte)) // force some GCs
+	return
+}
+
+// Pass a value to escapeMe to force it to escape.
+var escapeMe = func(x interface{}) {}
diff --git a/src/runtime/stubs.go b/src/runtime/stubs.go
index 18e64dd5f7..26aaf2224d 100644
--- a/src/runtime/stubs.go
+++ b/src/runtime/stubs.go
@@ -248,9 +248,6 @@ func getclosureptr() uintptr
 //go:noescape
 func asmcgocall(fn, arg unsafe.Pointer) int32
 
-// argp used in Defer structs when there is no argp.
-const _NoArgs = ^uintptr(0)
-
 func morestack()
 func morestack_noctxt()
 func rt0_go()
diff --git a/src/runtime/syscall_windows.go b/src/runtime/syscall_windows.go
index 36ad7511af..722a73d108 100644
--- a/src/runtime/syscall_windows.go
+++ b/src/runtime/syscall_windows.go
@@ -112,7 +112,6 @@ const _LOAD_LIBRARY_SEARCH_SYSTEM32 = 0x00000800
 //go:nosplit
 func syscall_loadsystemlibrary(filename *uint16, absoluteFilepath *uint16) (handle, err uintptr) {
 	lockOSThread()
-	defer unlockOSThread()
 	c := &getg().m.syscall
 
 	if useLoadLibraryEx {
@@ -135,6 +134,7 @@ func syscall_loadsystemlibrary(filename *uint16, absoluteFilepath *uint16) (hand
 	if handle == 0 {
 		err = c.err
 	}
+	unlockOSThread() // not defer'd after the lockOSThread above to save stack frame size.
 	return
 }
 
diff --git a/src/runtime/traceback.go b/src/runtime/traceback.go
index d817018501..ef48c9fa1f 100644
--- a/src/runtime/traceback.go
+++ b/src/runtime/traceback.go
@@ -148,11 +148,6 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 	waspanic := false
 	cgoCtxt := gp.cgoCtxt
 	printing := pcbuf == nil && callback == nil
-	_defer := gp._defer
-
-	for _defer != nil && _defer.sp == _NoArgs {
-		_defer = _defer.link
-	}
 
 	// If the PC is zero, it's likely a nil function call.
 	// Start in the caller's frame.
@@ -319,15 +314,14 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 		// In the latter case, use a deferreturn call site as the continuation pc.
 		frame.continpc = frame.pc
 		if waspanic {
-			// We match up defers with frames using the SP.
-			// However, if the function has an empty stack
-			// frame, then it's possible (on LR machines)
-			// for multiple call frames to have the same
-			// SP. But, since a function with no frame
-			// can't push a defer, the defer can't belong
-			// to that frame.
-			if _defer != nil && _defer.sp == frame.sp && frame.sp != frame.fp {
+			if frame.fn.deferreturn != 0 {
 				frame.continpc = frame.fn.entry + uintptr(frame.fn.deferreturn) + 1
+				// Note: this may perhaps keep return variables alive longer than
+				// strictly necessary, as we are using "function has a defer statement"
+				// as a proxy for "function actually deferred something". It seems
+				// to be a minor drawback. (We used to actually look through the
+				// gp._defer for a defer corresponding to this function, but that
+				// is hard to do with defer records on the stack during a stack copy.)
 				// Note: the +1 is to offset the -1 that
 				// stack.go:getStackMap does to back up a return
 				// address make sure the pc is in the CALL instruction.
@@ -336,11 +330,6 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 			}
 		}
 
-		// Unwind our local defer stack past this frame.
-		for _defer != nil && ((_defer.sp == frame.sp && frame.sp != frame.fp) || _defer.sp == _NoArgs) {
-			_defer = _defer.link
-		}
-
 		if callback != nil {
 			if !callback((*stkframe)(noescape(unsafe.Pointer(&frame))), v) {
 				return n
@@ -510,13 +499,6 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 		n = nprint
 	}
 
-	// If callback != nil, we're being called to gather stack information during
-	// garbage collection or stack growth. In that context, require that we used
-	// up the entire defer stack. If not, then there is a bug somewhere and the
-	// garbage collection or stack growth may not have seen the correct picture
-	// of the stack. Crash now instead of silently executing the garbage collection
-	// or stack copy incorrectly and setting up for a mysterious crash later.
-	//
 	// Note that panic != nil is okay here: there can be leftover panics,
 	// because the defers on the panic stack do not nest in frame order as
 	// they do on the defer stack. If you have:
@@ -557,16 +539,6 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 	// At other times, such as when gathering a stack for a profiling signal
 	// or when printing a traceback during a crash, everything may not be
 	// stopped nicely, and the stack walk may not be able to complete.
-	// It's okay in those situations not to use up the entire defer stack:
-	// incomplete information then is still better than nothing.
-	if callback != nil && n < max && _defer != nil {
-		print("runtime: g", gp.goid, ": leftover defer sp=", hex(_defer.sp), " pc=", hex(_defer.pc), "\n")
-		for _defer = gp._defer; _defer != nil; _defer = _defer.link {
-			print("\tdefer ", _defer, " sp=", hex(_defer.sp), " pc=", hex(_defer.pc), "\n")
-		}
-		throw("traceback has leftover defers")
-	}
-
 	if callback != nil && n < max && frame.sp != gp.stktopsp {
 		print("runtime: g", gp.goid, ": frame.sp=", hex(frame.sp), " top=", hex(gp.stktopsp), "\n")
 		print("\tstack=[", hex(gp.stack.lo), "-", hex(gp.stack.hi), "] n=", n, " max=", max, "\n")