runtime: make morestack less subtle

morestack writes the context pointer to gobuf.ctxt, but since
morestack is written in assembly (and has to be very careful with
state), it does *not* invoke the requisite write barrier for this
write. Instead, we patch this up later, in newstack, where we invoke
an explicit write barrier for ctxt.

This already requires some subtle reasoning, and it's going to get a
lot hairier with the hybrid barrier.

Fix this by simplifying the whole mechanism. Instead of writing
gobuf.ctxt in morestack, just pass the value of the context register
to newstack and let it write it to gobuf.ctxt. This is a normal Go
pointer write, so it gets the normal Go write barrier. No subtle
reasoning required.

Updates #17503.

Change-Id: Ia6bf8459bfefc6828f53682ade32c02412e4db63
Reviewed-on: https://go-review.googlesource.com/31550
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/asm_386.s

@@ -381,7 +381,7 @@ TEXT runtime·morestack(SB),NOSPLIT,$0-0
 	MOVL	SI, (g_sched+gobuf_g)(SI)
 	LEAL	4(SP), AX	// f's SP
 	MOVL	AX, (g_sched+gobuf_sp)(SI)
-	MOVL	DX, (g_sched+gobuf_ctxt)(SI)
+	// newstack will fill gobuf.ctxt.
 
 	// Call newstack on m->g0's stack.
 	MOVL	m_g0(BX), BP
@@ -389,8 +389,10 @@ TEXT runtime·morestack(SB),NOSPLIT,$0-0
 	MOVL	(g_sched+gobuf_sp)(BP), AX
 	MOVL	-4(AX), BX	// fault if CALL would, before smashing SP
 	MOVL	AX, SP
+	PUSHL	DX	// ctxt argument
 	CALL	runtime·newstack(SB)
 	MOVL	$0, 0x1003	// crash if newstack returns
+	POPL	DX	// keep balance check happy
 	RET
 
 TEXT runtime·morestack_noctxt(SB),NOSPLIT,$0-0

src/runtime/asm_amd64.s

@@ -358,15 +358,17 @@ TEXT runtime·morestack(SB),NOSPLIT,$0-0
 	MOVQ	SI, (g_sched+gobuf_g)(SI)
 	LEAQ	8(SP), AX // f's SP
 	MOVQ	AX, (g_sched+gobuf_sp)(SI)
-	MOVQ	DX, (g_sched+gobuf_ctxt)(SI)
 	MOVQ	BP, (g_sched+gobuf_bp)(SI)
+	// newstack will fill gobuf.ctxt.
 
 	// Call newstack on m->g0's stack.
 	MOVQ	m_g0(BX), BX
 	MOVQ	BX, g(CX)
 	MOVQ	(g_sched+gobuf_sp)(BX), SP
+	PUSHQ	DX	// ctxt argument
 	CALL	runtime·newstack(SB)
 	MOVQ	$0, 0x1003	// crash if newstack returns
+	POPQ	DX	// keep balance check happy
 	RET
 
 // morestack but not preserving ctxt.

src/runtime/asm_amd64p32.s

@@ -276,14 +276,16 @@ TEXT runtime·morestack(SB),NOSPLIT,$0-0
 	MOVL	SI, (g_sched+gobuf_g)(SI)
 	LEAL	8(SP), AX // f's SP
 	MOVL	AX, (g_sched+gobuf_sp)(SI)
-	MOVL	DX, (g_sched+gobuf_ctxt)(SI)
+	// newstack will fill gobuf.ctxt.
 
 	// Call newstack on m->g0's stack.
 	MOVL	m_g0(BX), BX
 	MOVL	BX, g(CX)
 	MOVL	(g_sched+gobuf_sp)(BX), SP
+	PUSHQ	DX	// ctxt argument
 	CALL	runtime·newstack(SB)
 	MOVL	$0, 0x1003	// crash if newstack returns
+	POPQ	DX	// keep balance check happy
 	RET
 
 // morestack trampolines

src/runtime/asm_arm.s

@@ -294,10 +294,10 @@ TEXT runtime·morestack(SB),NOSPLIT,$-4-0
 
 	// Called from f.
 	// Set g->sched to context in f.
-	MOVW	R7, (g_sched+gobuf_ctxt)(g)
 	MOVW	R13, (g_sched+gobuf_sp)(g)
 	MOVW	LR, (g_sched+gobuf_pc)(g)
 	MOVW	R3, (g_sched+gobuf_lr)(g)
+	// newstack will fill gobuf.ctxt.
 
 	// Called from f.
 	// Set m->morebuf to f's caller.
@@ -310,6 +310,9 @@ TEXT runtime·morestack(SB),NOSPLIT,$-4-0
 	MOVW	m_g0(R8), R0
 	BL	setg<>(SB)
 	MOVW	(g_sched+gobuf_sp)(g), R13
+	MOVW	$0, R0
+	MOVW.W	R0, -8(R13)	// create a call frame on g0
+	MOVW	R7, 4(R13)	// ctxt argument
 	BL	runtime·newstack(SB)
 
 	// Not reached, but make sure the return PC from the call to newstack

src/runtime/asm_arm64.s

@@ -269,11 +269,11 @@ TEXT runtime·morestack(SB),NOSPLIT,$-8-0
 
 	// Called from f.
 	// Set g->sched to context in f
-	MOVD	R26, (g_sched+gobuf_ctxt)(g)
 	MOVD	RSP, R0
 	MOVD	R0, (g_sched+gobuf_sp)(g)
 	MOVD	LR, (g_sched+gobuf_pc)(g)
 	MOVD	R3, (g_sched+gobuf_lr)(g)
+	// newstack will fill gobuf.ctxt.
 
 	// Called from f.
 	// Set m->morebuf to f's callers.
@@ -287,6 +287,8 @@ TEXT runtime·morestack(SB),NOSPLIT,$-8-0
 	BL	runtime·save_g(SB)
 	MOVD	(g_sched+gobuf_sp)(g), R0
 	MOVD	R0, RSP
+	MOVD.W	$0, -16(RSP)	// create a call frame on g0
+	MOVD	R26, 8(RSP)	// ctxt argument
 	BL	runtime·newstack(SB)
 
 	// Not reached, but make sure the return PC from the call to newstack

src/runtime/asm_mips64x.s

@@ -243,10 +243,10 @@ TEXT runtime·morestack(SB),NOSPLIT,$-8-0
 
 	// Called from f.
 	// Set g->sched to context in f.
-	MOVV	REGCTXT, (g_sched+gobuf_ctxt)(g)
 	MOVV	R29, (g_sched+gobuf_sp)(g)
 	MOVV	R31, (g_sched+gobuf_pc)(g)
 	MOVV	R3, (g_sched+gobuf_lr)(g)
+	// newstack will fill gobuf.ctxt.
 
 	// Called from f.
 	// Set m->morebuf to f's caller.
@@ -258,6 +258,10 @@ TEXT runtime·morestack(SB),NOSPLIT,$-8-0
 	MOVV	m_g0(R7), g
 	JAL	runtime·save_g(SB)
 	MOVV	(g_sched+gobuf_sp)(g), R29
+	// Create a stack frame on g0 to call newstack.
+	MOVV	R0, -16(R29)	// Zero saved LR in frame
+	ADDV	$-16, R29
+	MOVV	REGCTXT, 8(R29)	// ctxt argument
 	JAL	runtime·newstack(SB)
 
 	// Not reached, but make sure the return PC from the call to newstack

src/runtime/asm_ppc64x.s

@@ -297,11 +297,11 @@ TEXT runtime·morestack(SB),NOSPLIT|NOFRAME,$0-0
 
 	// Called from f.
 	// Set g->sched to context in f.
-	MOVD	R11, (g_sched+gobuf_ctxt)(g)
 	MOVD	R1, (g_sched+gobuf_sp)(g)
 	MOVD	LR, R8
 	MOVD	R8, (g_sched+gobuf_pc)(g)
 	MOVD	R5, (g_sched+gobuf_lr)(g)
+	// newstack will fill gobuf.ctxt.
 
 	// Called from f.
 	// Set m->morebuf to f's caller.
@@ -313,6 +313,8 @@ TEXT runtime·morestack(SB),NOSPLIT|NOFRAME,$0-0
 	MOVD	m_g0(R7), g
 	BL	runtime·save_g(SB)
 	MOVD	(g_sched+gobuf_sp)(g), R1
+	MOVDU   R0, -(FIXED_FRAME+8)(R1)	// create a call frame on g0
+	MOVD	R11, FIXED_FRAME+0(R1)	// ctxt argument
 	BL	runtime·newstack(SB)
 
 	// Not reached, but make sure the return PC from the call to newstack

src/runtime/asm_s390x.s

@@ -254,11 +254,11 @@ TEXT runtime·morestack(SB),NOSPLIT|NOFRAME,$0-0
 
 	// Called from f.
 	// Set g->sched to context in f.
-	MOVD	R12, (g_sched+gobuf_ctxt)(g)
 	MOVD	R15, (g_sched+gobuf_sp)(g)
 	MOVD	LR, R8
 	MOVD	R8, (g_sched+gobuf_pc)(g)
 	MOVD	R5, (g_sched+gobuf_lr)(g)
+	// newstack will fill gobuf.ctxt.
 
 	// Called from f.
 	// Set m->morebuf to f's caller.
@@ -270,6 +270,10 @@ TEXT runtime·morestack(SB),NOSPLIT|NOFRAME,$0-0
 	MOVD	m_g0(R7), g
 	BL	runtime·save_g(SB)
 	MOVD	(g_sched+gobuf_sp)(g), R15
+	// Create a stack frame on g0 to call newstack.
+	MOVD	$0, -16(R15)	// Zero saved LR in frame
+	SUB	$16, R15
+	MOVD	R12, 8(R15)	// ctxt argument
 	BL	runtime·newstack(SB)
 
 	// Not reached, but make sure the return PC from the call to newstack

src/runtime/stack.go

@@ -925,7 +925,10 @@ func round2(x int32) int32 {
 //
 // g->atomicstatus will be Grunning or Gscanrunning upon entry.
 // If the GC is trying to stop this g then it will set preemptscan to true.
-func newstack() {
+//
+// ctxt is the value of the context register on morestack. newstack
+// will write it to g.sched.ctxt.
+func newstack(ctxt unsafe.Pointer) {
 	thisg := getg()
 	// TODO: double check all gp. shouldn't be getg().
 	if thisg.m.morebuf.g.ptr().stackguard0 == stackFork {
@@ -937,8 +940,13 @@ func newstack() {
 		traceback(morebuf.pc, morebuf.sp, morebuf.lr, morebuf.g.ptr())
 		throw("runtime: wrong goroutine in newstack")
 	}
+
+	gp := thisg.m.curg
+	// Write ctxt to gp.sched. We do this here instead of in
+	// morestack so it has the necessary write barrier.
+	gp.sched.ctxt = ctxt
+
 	if thisg.m.curg.throwsplit {
-		gp := thisg.m.curg
 		// Update syscallsp, syscallpc in case traceback uses them.
 		morebuf := thisg.m.morebuf
 		gp.syscallsp = morebuf.sp
@@ -951,7 +959,6 @@ func newstack() {
 		throw("runtime: stack split at bad time")
 	}
 
-	gp := thisg.m.curg
 	morebuf := thisg.m.morebuf
 	thisg.m.morebuf.pc = 0
 	thisg.m.morebuf.lr = 0
@@ -1003,14 +1010,6 @@ func newstack() {
 		throw("runtime: split stack overflow")
 	}
 
-	if gp.sched.ctxt != nil {
-		// morestack wrote sched.ctxt on its way in here,
-		// without a write barrier. Run the write barrier now.
-		// It is not possible to be preempted between then
-		// and now, so it's okay.
-		writebarrierptr_nostore((*uintptr)(unsafe.Pointer(&gp.sched.ctxt)), uintptr(gp.sched.ctxt))
-	}
-
 	if preempt {
 		if gp == thisg.m.g0 {
 			throw("runtime: preempt g0")