runtime: remove stack barriers

Now that we don't rescan stacks, stack barriers are unnecessary. This
removes all of the code and structures supporting them as well as
tests that were specifically for stack barriers.

Updates #17503.

Change-Id: Ia29221730e0f2bbe7beab4fa757f31a032d9690c
Reviewed-on: https://go-review.googlesource.com/36620
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/asm_386.s

@@ -415,22 +415,6 @@ TEXT runtime·morestack_noctxt(SB),NOSPLIT,$0-0
 	MOVL	$0, DX
 	JMP runtime·morestack(SB)
 
-TEXT runtime·stackBarrier(SB),NOSPLIT,$0
-	// We came here via a RET to an overwritten return PC.
-	// AX may be live. Other registers are available.
-
-	// Get the original return PC, g.stkbar[g.stkbarPos].savedLRVal.
-	get_tls(CX)
-	MOVL	g(CX), CX
-	MOVL	(g_stkbar+slice_array)(CX), DX
-	MOVL	g_stkbarPos(CX), BX
-	IMULL	$stkbar__size, BX	// Too big for SIB.
-	MOVL	stkbar_savedLRVal(DX)(BX*1), BX
-	// Record that this stack barrier was hit.
-	ADDL	$1, g_stkbarPos(CX)
-	// Jump to the original return PC.
-	JMP	BX
-
 // reflectcall: call a function with the given argument list
 // func call(argtype *_type, f *FuncVal, arg *byte, argsize, retoffset uint32).
 // we don't have variable-sized frames, so we use a small number
@@ -812,28 +796,14 @@ TEXT runtime·stackcheck(SB), NOSPLIT, $0-0
 TEXT runtime·getcallerpc(SB),NOSPLIT,$4-8
 	MOVL	argp+0(FP),AX		// addr of first arg
 	MOVL	-4(AX),AX		// get calling pc
-	CMPL	AX, runtime·stackBarrierPC(SB)
-	JNE	nobar
-	// Get original return PC.
-	CALL	runtime·nextBarrierPC(SB)
-	MOVL	0(SP), AX
-nobar:
 	MOVL	AX, ret+4(FP)
 	RET
 
 TEXT runtime·setcallerpc(SB),NOSPLIT,$4-8
 	MOVL	argp+0(FP),AX		// addr of first arg
 	MOVL	pc+4(FP), BX
-	MOVL	-4(AX), DX
-	CMPL	DX, runtime·stackBarrierPC(SB)
-	JEQ	setbar
 	MOVL	BX, -4(AX)		// set calling pc
 	RET
-setbar:
-	// Set the stack barrier return PC.
-	MOVL	BX, 0(SP)
-	CALL	runtime·setNextBarrierPC(SB)
-	RET
 
 // func cputicks() int64
 TEXT runtime·cputicks(SB),NOSPLIT,$0-8

src/runtime/asm_amd64.s

@@ -405,28 +405,6 @@ TEXT runtime·morestack_noctxt(SB),NOSPLIT,$0
 	MOVL	$0, DX
 	JMP	runtime·morestack(SB)
 
-TEXT runtime·stackBarrier(SB),NOSPLIT,$0
-	// We came here via a RET to an overwritten return PC.
-	// AX may be live. Other registers are available.
-
-	// Get the original return PC, g.stkbar[g.stkbarPos].savedLRVal.
-	get_tls(CX)
-	MOVQ	g(CX), CX
-	MOVQ	(g_stkbar+slice_array)(CX), DX
-	MOVQ	g_stkbarPos(CX), BX
-	IMULQ	$stkbar__size, BX	// Too big for SIB.
-	MOVQ	stkbar_savedLRPtr(DX)(BX*1), R8
-	MOVQ	stkbar_savedLRVal(DX)(BX*1), BX
-	// Assert that we're popping the right saved LR.
-	ADDQ	$8, R8
-	CMPQ	R8, SP
-	JEQ	2(PC)
-	MOVL	$0, 0
-	// Record that this stack barrier was hit.
-	ADDQ	$1, g_stkbarPos(CX)
-	// Jump to the original return PC.
-	JMP	BX
-
 // reflectcall: call a function with the given argument list
 // func call(argtype *_type, f *FuncVal, arg *byte, argsize, retoffset uint32).
 // we don't have variable-sized frames, so we use a small number
@@ -841,28 +819,14 @@ TEXT runtime·stackcheck(SB), NOSPLIT, $0-0
 TEXT runtime·getcallerpc(SB),NOSPLIT,$8-16
 	MOVQ	argp+0(FP),AX		// addr of first arg
 	MOVQ	-8(AX),AX		// get calling pc
-	CMPQ	AX, runtime·stackBarrierPC(SB)
-	JNE	nobar
-	// Get original return PC.
-	CALL	runtime·nextBarrierPC(SB)
-	MOVQ	0(SP), AX
-nobar:
 	MOVQ	AX, ret+8(FP)
 	RET
 
 TEXT runtime·setcallerpc(SB),NOSPLIT,$8-16
 	MOVQ	argp+0(FP),AX		// addr of first arg
 	MOVQ	pc+8(FP), BX
-	MOVQ	-8(AX), CX
-	CMPQ	CX, runtime·stackBarrierPC(SB)
-	JEQ	setbar
 	MOVQ	BX, -8(AX)		// set calling pc
 	RET
-setbar:
-	// Set the stack barrier return PC.
-	MOVQ	BX, 0(SP)
-	CALL	runtime·setNextBarrierPC(SB)
-	RET
 
 // func cputicks() int64
 TEXT runtime·cputicks(SB),NOSPLIT,$0-0

src/runtime/asm_amd64p32.s

@@ -309,23 +309,6 @@ TEXT runtime·morestack_noctxt(SB),NOSPLIT,$0
 	MOVL	$0, DX
 	JMP	runtime·morestack(SB)
 
-TEXT runtime·stackBarrier(SB),NOSPLIT,$0
-	// We came here via a RET to an overwritten return PC.
-	// AX may be live. Other registers are available.
-
-	// Get the original return PC, g.stkbar[g.stkbarPos].savedLRVal.
-	get_tls(CX)
-	MOVL	g(CX), CX
-	MOVL	(g_stkbar+slice_array)(CX), DX
-	MOVL	g_stkbarPos(CX), BX
-	IMULL	$stkbar__size, BX	// Too big for SIB.
-	ADDL	DX, BX
-	MOVL	stkbar_savedLRVal(BX), BX
-	// Record that this stack barrier was hit.
-	ADDL	$1, g_stkbarPos(CX)
-	// Jump to the original return PC.
-	JMP	BX
-
 // reflectcall: call a function with the given argument list
 // func call(argtype *_type, f *FuncVal, arg *byte, argsize, retoffset uint32).
 // we don't have variable-sized frames, so we use a small number
@@ -521,28 +504,14 @@ TEXT runtime·memclrNoHeapPointers(SB),NOSPLIT,$0-8
 TEXT runtime·getcallerpc(SB),NOSPLIT,$8-12
 	MOVL	argp+0(FP),AX		// addr of first arg
 	MOVL	-8(AX),AX		// get calling pc
-	CMPL	AX, runtime·stackBarrierPC(SB)
-	JNE	nobar
-	// Get original return PC.
-	CALL	runtime·nextBarrierPC(SB)
-	MOVL	0(SP), AX
-nobar:
 	MOVL	AX, ret+8(FP)
 	RET
 
 TEXT runtime·setcallerpc(SB),NOSPLIT,$8-8
 	MOVL	argp+0(FP),AX		// addr of first arg
 	MOVL	pc+4(FP), BX		// pc to set
-	MOVL	-8(AX), CX
-	CMPL	CX, runtime·stackBarrierPC(SB)
-	JEQ	setbar
 	MOVQ	BX, -8(AX)		// set calling pc
 	RET
-setbar:
-	// Set the stack barrier return PC.
-	MOVL	BX, 0(SP)
-	CALL	runtime·setNextBarrierPC(SB)
-	RET
 
 // int64 runtime·cputicks(void)
 TEXT runtime·cputicks(SB),NOSPLIT,$0-0

src/runtime/asm_arm.s

@@ -340,23 +340,6 @@ TEXT runtime·morestack_noctxt(SB),NOSPLIT,$-4-0
 	MOVW	$0, R7
 	B runtime·morestack(SB)
 
-TEXT runtime·stackBarrier(SB),NOSPLIT,$0
-	// We came here via a RET to an overwritten LR.
-	// R0 may be live. Other registers are available.
-
-	// Get the original return PC, g.stkbar[g.stkbarPos].savedLRVal.
-	MOVW	(g_stkbar+slice_array)(g), R4
-	MOVW	g_stkbarPos(g), R5
-	MOVW	$stkbar__size, R6
-	MUL	R5, R6
-	ADD	R4, R6
-	MOVW	stkbar_savedLRVal(R6), R6
-	// Record that this stack barrier was hit.
-	ADD	$1, R5
-	MOVW	R5, g_stkbarPos(g)
-	// Jump to the original return PC.
-	B	(R6)
-
 // reflectcall: call a function with the given argument list
 // func call(argtype *_type, f *FuncVal, arg *byte, argsize, retoffset uint32).
 // we don't have variable-sized frames, so we use a small number
@@ -696,29 +679,13 @@ TEXT setg<>(SB),NOSPLIT,$-4-0
 
 TEXT runtime·getcallerpc(SB),NOSPLIT,$4-8
 	MOVW	8(R13), R0		// LR saved by caller
-	MOVW	runtime·stackBarrierPC(SB), R1
-	CMP	R0, R1
-	BNE	nobar
-	// Get original return PC.
-	BL	runtime·nextBarrierPC(SB)
-	MOVW	4(R13), R0
-nobar:
 	MOVW	R0, ret+4(FP)
 	RET
 
 TEXT runtime·setcallerpc(SB),NOSPLIT,$4-8
 	MOVW	pc+4(FP), R0
-	MOVW	8(R13), R1
-	MOVW	runtime·stackBarrierPC(SB), R2
-	CMP	R1, R2
-	BEQ	setbar
 	MOVW	R0, 8(R13)		// set LR in caller
 	RET
-setbar:
-	// Set the stack barrier return PC.
-	MOVW	R0, 4(R13)
-	BL	runtime·setNextBarrierPC(SB)
-	RET
 
 TEXT runtime·emptyfunc(SB),0,$0-0
 	RET

src/runtime/asm_arm64.s

@@ -315,23 +315,6 @@ TEXT runtime·morestack_noctxt(SB),NOSPLIT,$-4-0
 	MOVW	$0, R26
 	B runtime·morestack(SB)
 
-TEXT runtime·stackBarrier(SB),NOSPLIT,$0
-	// We came here via a RET to an overwritten LR.
-	// R0 may be live (see return0). Other registers are available.
-
-	// Get the original return PC, g.stkbar[g.stkbarPos].savedLRVal.
-	MOVD	(g_stkbar+slice_array)(g), R4
-	MOVD	g_stkbarPos(g), R5
-	MOVD	$stkbar__size, R6
-	MUL	R5, R6
-	ADD	R4, R6
-	MOVD	stkbar_savedLRVal(R6), R6
-	// Record that this stack barrier was hit.
-	ADD	$1, R5
-	MOVD	R5, g_stkbarPos(g)
-	// Jump to the original return PC.
-	B	(R6)
-
 // reflectcall: call a function with the given argument list
 // func call(argtype *_type, f *FuncVal, arg *byte, argsize, retoffset uint32).
 // we don't have variable-sized frames, so we use a small number
@@ -723,29 +706,13 @@ TEXT setg_gcc<>(SB),NOSPLIT,$8
 
 TEXT runtime·getcallerpc(SB),NOSPLIT,$8-16
 	MOVD	16(RSP), R0		// LR saved by caller
-	MOVD	runtime·stackBarrierPC(SB), R1
-	CMP	R0, R1
-	BNE	nobar
-	// Get original return PC.
-	BL	runtime·nextBarrierPC(SB)
-	MOVD	8(RSP), R0
-nobar:
 	MOVD	R0, ret+8(FP)
 	RET
 
 TEXT runtime·setcallerpc(SB),NOSPLIT,$8-16
 	MOVD	pc+8(FP), R0
-	MOVD	16(RSP), R1
-	MOVD	runtime·stackBarrierPC(SB), R2
-	CMP	R1, R2
-	BEQ	setbar
 	MOVD	R0, 16(RSP)		// set LR in caller
 	RET
-setbar:
-	// Set the stack barrier return PC.
-	MOVD	R0, 8(RSP)
-	BL	runtime·setNextBarrierPC(SB)
-	RET
 
 TEXT runtime·abort(SB),NOSPLIT,$-8-0
 	B	(ZR)

src/runtime/asm_mips64x.s

@@ -286,24 +286,6 @@ TEXT runtime·morestack_noctxt(SB),NOSPLIT,$-8-0
 	MOVV	R0, REGCTXT
 	JMP	runtime·morestack(SB)
 
-TEXT runtime·stackBarrier(SB),NOSPLIT,$0
-	// We came here via a RET to an overwritten LR.
-	// R1 may be live. Other registers are available.
-
-	// Get the original return PC, g.stkbar[g.stkbarPos].savedLRVal.
-	MOVV	(g_stkbar+slice_array)(g), R2
-	MOVV	g_stkbarPos(g), R3
-	MOVV	$stkbar__size, R4
-	MULVU	R3, R4
-	MOVV	LO, R4
-	ADDV	R2, R4
-	MOVV	stkbar_savedLRVal(R4), R4
-	// Record that this stack barrier was hit.
-	ADDV	$1, R3
-	MOVV	R3, g_stkbarPos(g)
-	// Jump to the original return PC.
-	JMP	(R4)
-
 // reflectcall: call a function with the given argument list
 // func call(argtype *_type, f *FuncVal, arg *byte, argsize, retoffset uint32).
 // we don't have variable-sized frames, so we use a small number
@@ -636,27 +618,13 @@ TEXT setg_gcc<>(SB),NOSPLIT,$0-0
 
 TEXT runtime·getcallerpc(SB),NOSPLIT,$8-16
 	MOVV	16(R29), R1		// LR saved by caller
-	MOVV	runtime·stackBarrierPC(SB), R2
-	BNE	R1, R2, nobar
-	// Get original return PC.
-	JAL	runtime·nextBarrierPC(SB)
-	MOVV	8(R29), R1
-nobar:
 	MOVV	R1, ret+8(FP)
 	RET
 
 TEXT runtime·setcallerpc(SB),NOSPLIT,$8-16
 	MOVV	pc+8(FP), R1
-	MOVV	16(R29), R2
-	MOVV	runtime·stackBarrierPC(SB), R3
-	BEQ	R2, R3, setbar
 	MOVV	R1, 16(R29)		// set LR in caller
 	RET
-setbar:
-	// Set the stack barrier return PC.
-	MOVV	R1, 8(R29)
-	JAL	runtime·setNextBarrierPC(SB)
-	RET
 
 TEXT runtime·abort(SB),NOSPLIT,$-8-0
 	MOVW	(R0), R0

src/runtime/asm_mipsx.s

@@ -287,22 +287,6 @@ TEXT runtime·morestack_noctxt(SB),NOSPLIT,$0-0
 	MOVW	R0, REGCTXT
 	JMP	runtime·morestack(SB)
 
-TEXT runtime·stackBarrier(SB),NOSPLIT,$0
-	// We came here via a RET to an overwritten LR.
-	// R1 may be live. Other registers are available.
-
-	// Get the original return PC, g.stkbar[g.stkbarPos].savedLRVal.
-	MOVW	(g_stkbar+slice_array)(g), R2
-	MOVW	g_stkbarPos(g), R3
-	MOVW	$stkbar__size, R4
-	MULU	R3, R4
-	MOVW	LO, R4
-	ADDU	R2, R4
-	MOVW	stkbar_savedLRVal(R4), R4
-	ADDU	$1, R3
-	MOVW	R3, g_stkbarPos(g)	// Record that this stack barrier was hit.
-	JMP	(R4)	// Jump to the original return PC.
-
 // reflectcall: call a function with the given argument list
 // func call(argtype *_type, f *FuncVal, arg *byte, argsize, retoffset uint32).
 // we don't have variable-sized frames, so we use a small number
@@ -637,25 +621,13 @@ TEXT setg_gcc<>(SB),NOSPLIT,$0
 
 TEXT runtime·getcallerpc(SB),NOSPLIT,$4-8
 	MOVW	8(R29), R1	// LR saved by caller
-	MOVW	runtime·stackBarrierPC(SB), R2
-	BNE	R1, R2, nobar
-	JAL	runtime·nextBarrierPC(SB)	// Get original return PC.
-	MOVW	4(R29), R1
-nobar:
 	MOVW	R1, ret+4(FP)
 	RET
 
 TEXT runtime·setcallerpc(SB),NOSPLIT,$4-8
 	MOVW	pc+4(FP), R1
-	MOVW	8(R29), R2
-	MOVW	runtime·stackBarrierPC(SB), R3
-	BEQ	R2, R3, setbar
 	MOVW	R1, 8(R29)	// set LR in caller
 	RET
-setbar:
-	MOVW	R1, 4(R29)
-	JAL	runtime·setNextBarrierPC(SB)	// Set the stack barrier return PC.
-	RET
 
 TEXT runtime·abort(SB),NOSPLIT,$0-0
 	UNDEF

src/runtime/asm_ppc64x.s

@@ -341,24 +341,6 @@ TEXT runtime·morestack_noctxt(SB),NOSPLIT|NOFRAME,$0-0
 	MOVD	R0, R11
 	BR	runtime·morestack(SB)
 
-TEXT runtime·stackBarrier(SB),NOSPLIT,$0
-	// We came here via a RET to an overwritten LR.
-	// R3 may be live. Other registers are available.
-
-	// Get the original return PC, g.stkbar[g.stkbarPos].savedLRVal.
-	MOVD	(g_stkbar+slice_array)(g), R4
-	MOVD	g_stkbarPos(g), R5
-	MOVD	$stkbar__size, R6
-	MULLD	R5, R6
-	ADD	R4, R6
-	MOVD	stkbar_savedLRVal(R6), R6
-	// Record that this stack barrier was hit.
-	ADD	$1, R5
-	MOVD	R5, g_stkbarPos(g)
-	// Jump to the original return PC.
-	MOVD	R6, CTR
-	BR	(CTR)
-
 // reflectcall: call a function with the given argument list
 // func call(argtype *_type, f *FuncVal, arg *byte, argsize, retoffset uint32).
 // we don't have variable-sized frames, so we use a small number
@@ -734,29 +716,13 @@ TEXT setg_gcc<>(SB),NOSPLIT|NOFRAME,$0-0
 
 TEXT runtime·getcallerpc(SB),NOSPLIT,$8-16
 	MOVD	FIXED_FRAME+8(R1), R3		// LR saved by caller
-	MOVD	runtime·stackBarrierPC(SB), R4
-	CMP	R3, R4
-	BNE	nobar
-	// Get original return PC.
-	BL	runtime·nextBarrierPC(SB)
-	MOVD	FIXED_FRAME+0(R1), R3
-nobar:
 	MOVD	R3, ret+8(FP)
 	RET
 
 TEXT runtime·setcallerpc(SB),NOSPLIT,$8-16
 	MOVD	pc+8(FP), R3
-	MOVD	FIXED_FRAME+8(R1), R4
-	MOVD	runtime·stackBarrierPC(SB), R5
-	CMP	R4, R5
-	BEQ	setbar
 	MOVD	R3, FIXED_FRAME+8(R1)		// set LR in caller
 	RET
-setbar:
-	// Set the stack barrier return PC.
-	MOVD	R3, FIXED_FRAME+0(R1)
-	BL	runtime·setNextBarrierPC(SB)
-	RET
 
 TEXT runtime·abort(SB),NOSPLIT|NOFRAME,$0-0
 	MOVW	(R0), R0

src/runtime/asm_s390x.s

@@ -298,23 +298,6 @@ TEXT runtime·morestack_noctxt(SB),NOSPLIT|NOFRAME,$0-0
 	MOVD	$0, R12
 	BR	runtime·morestack(SB)
 
-TEXT runtime·stackBarrier(SB),NOSPLIT,$0
-	// We came here via a RET to an overwritten LR.
-	// R3 may be live. Other registers are available.
-
-	// Get the original return PC, g.stkbar[g.stkbarPos].savedLRVal.
-	MOVD	(g_stkbar+slice_array)(g), R4
-	MOVD	g_stkbarPos(g), R5
-	MOVD	$stkbar__size, R6
-	MULLD	R5, R6
-	ADD	R4, R6
-	MOVD	stkbar_savedLRVal(R6), R6
-	// Record that this stack barrier was hit.
-	ADD	$1, R5
-	MOVD	R5, g_stkbarPos(g)
-	// Jump to the original return PC.
-	BR	(R6)
-
 // reflectcall: call a function with the given argument list
 // func call(argtype *_type, f *FuncVal, arg *byte, argsize, retoffset uint32).
 // we don't have variable-sized frames, so we use a small number
@@ -675,27 +658,13 @@ TEXT setg_gcc<>(SB),NOSPLIT|NOFRAME,$0-0
 
 TEXT runtime·getcallerpc(SB),NOSPLIT,$8-16
 	MOVD	16(R15), R3		// LR saved by caller
-	MOVD	runtime·stackBarrierPC(SB), R4
-	CMPBNE	R3, R4, nobar
-	// Get original return PC.
-	BL	runtime·nextBarrierPC(SB)
-	MOVD	8(R15), R3
-nobar:
 	MOVD	R3, ret+8(FP)
 	RET
 
 TEXT runtime·setcallerpc(SB),NOSPLIT,$8-16
 	MOVD	pc+8(FP), R3
-	MOVD	16(R15), R4
-	MOVD	runtime·stackBarrierPC(SB), R5
-	CMPBEQ	R4, R5, setbar
 	MOVD	R3, 16(R15)		// set LR in caller
 	RET
-setbar:
-	// Set the stack barrier return PC.
-	MOVD	R3, 8(R15)
-	BL	runtime·setNextBarrierPC(SB)
-	RET
 
 TEXT runtime·abort(SB),NOSPLIT|NOFRAME,$0-0
 	MOVW	(R0), R0

src/runtime/extern.go

@@ -50,13 +50,6 @@ It is a comma-separated list of name=val pairs setting these named variables:
 	gcshrinkstackoff: setting gcshrinkstackoff=1 disables moving goroutines
 	onto smaller stacks. In this mode, a goroutine's stack can only grow.
 
-	gcstackbarrieroff: setting gcstackbarrieroff=1 disables the use of stack barriers
-	that allow the garbage collector to avoid repeating a stack scan during the
-	mark termination phase.
-
-	gcstackbarrierall: setting gcstackbarrierall=1 installs stack barriers
-	in every stack frame, rather than in exponentially-spaced frames.
-
 	gcrescanstacks: setting gcrescanstacks=1 enables stack
 	re-scanning during the STW mark termination phase. This is
 	helpful for debugging if objects are being prematurely

src/runtime/mbitmap.go

@@ -573,29 +573,9 @@ func bulkBarrierPreWrite(dst, src, size uintptr) {
 		return
 	}
 	if !inheap(dst) {
-		// If dst is on the stack and in a higher frame than the
-		// caller, we either need to execute write barriers on
-		// it (which is what happens for normal stack writes
-		// through pointers to higher frames), or we need to
-		// force the mark termination stack scan to scan the
-		// frame containing dst.
-		//
-		// Executing write barriers on dst is complicated in the
-		// general case because we either need to unwind the
-		// stack to get the stack map, or we need the type's
-		// bitmap, which may be a GC program.
-		//
-		// Hence, we opt for forcing the re-scan to scan the
-		// frame containing dst, which we can do by simply
-		// unwinding the stack barriers between the current SP
-		// and dst's frame.
 		gp := getg().m.curg
 		if gp != nil && gp.stack.lo <= dst && dst < gp.stack.hi {
-			// Run on the system stack to give it more
-			// stack space.
-			systemstack(func() {
-				gcUnwindBarriers(gp, dst)
-			})
+			// Destination is our own stack. No need for barriers.
 			return
 		}
 

src/runtime/mgc.go

@@ -1016,18 +1016,7 @@ func gcStart(mode gcMode, forceTrigger bool) {
 		// the time we start the world and begin
 		// scanning.
 		//
-		// It's necessary to enable write barriers
-		// during the scan phase for several reasons:
-		//
-		// They must be enabled for writes to higher
-		// stack frames before we scan stacks and
-		// install stack barriers because this is how
-		// we track writes to inactive stack frames.
-		// (Alternatively, we could not install stack
-		// barriers over frame boundaries with
-		// up-pointers).
-		//
-		// They must be enabled before assists are
+		// Write barriers must be enabled before assists are
 		// enabled because they must be enabled before
 		// any non-leaf heap objects are marked. Since
 		// allocations are blocked until assists can
@@ -1328,13 +1317,6 @@ func gcMarkTermination() {
 	// Free stack spans. This must be done between GC cycles.
 	systemstack(freeStackSpans)
 
-	// Best-effort remove stack barriers so they don't get in the
-	// way of things like GDB and perf.
-	lock(&allglock)
-	myallgs := allgs
-	unlock(&allglock)
-	gcTryRemoveAllStackBarriers(myallgs)
-
 	// Print gctrace before dropping worldsema. As soon as we drop
 	// worldsema another cycle could start and smash the stats
 	// we're trying to print.

src/runtime/mgcmark.go

@@ -721,10 +721,6 @@ func gcFlushBgCredit(scanWork int64) {
 
 // scanstack scans gp's stack, greying all pointers found on the stack.
 //
-// During mark phase, it also installs stack barriers while traversing
-// gp's stack. During mark termination, it stops scanning when it
-// reaches an unhit stack barrier.
-//
 // scanstack is marked go:systemstack because it must not be preempted
 // while using a workbuf.
 //
@@ -767,86 +763,14 @@ func scanstack(gp *g, gcw *gcWork) {
 		shrinkstack(gp)
 	}
 
-	// Prepare for stack barrier insertion/removal.
-	var sp, barrierOffset, nextBarrier uintptr
-	if gp.syscallsp != 0 {
-		sp = gp.syscallsp
-	} else {
-		sp = gp.sched.sp
-	}
-	gcLockStackBarriers(gp) // Not necessary during mark term, but harmless.
-	switch gcphase {
-	case _GCmark:
-		// Install stack barriers during stack scan.
-		barrierOffset = uintptr(firstStackBarrierOffset)
-		nextBarrier = sp + barrierOffset
-
-		if debug.gcstackbarrieroff > 0 {
-			nextBarrier = ^uintptr(0)
-		}
-
-		// Remove any existing stack barriers before we
-		// install new ones.
-		gcRemoveStackBarriers(gp)
-
-	case _GCmarktermination:
-		if !work.markrootDone {
-			// This is a STW GC. There may be stale stack
-			// barriers from an earlier cycle since we
-			// never passed through mark phase.
-			gcRemoveStackBarriers(gp)
-		}
-
-		if int(gp.stkbarPos) == len(gp.stkbar) {
-			// gp hit all of the stack barriers (or there
-			// were none). Re-scan the whole stack.
-			nextBarrier = ^uintptr(0)
-		} else {
-			// Only re-scan up to the lowest un-hit
-			// barrier. Any frames above this have not
-			// executed since the concurrent scan of gp and
-			// any writes through up-pointers to above
-			// this barrier had write barriers.
-			nextBarrier = gp.stkbar[gp.stkbarPos].savedLRPtr
-			if debugStackBarrier {
-				print("rescan below ", hex(nextBarrier), " in [", hex(sp), ",", hex(gp.stack.hi), ") goid=", gp.goid, "\n")
-			}
-		}
-
-	default:
-		throw("scanstack in wrong phase")
-	}
-
 	// Scan the stack.
 	var cache pcvalueCache
-	n := 0
 	scanframe := func(frame *stkframe, unused unsafe.Pointer) bool {
 		scanframeworker(frame, &cache, gcw)
-
-		if frame.fp > nextBarrier {
-			// We skip installing a barrier on bottom-most
-			// frame because on LR machines this LR is not
-			// on the stack.
-			if gcphase == _GCmark && n != 0 {
-				if gcInstallStackBarrier(gp, frame) {
-					barrierOffset *= 2
-					nextBarrier = sp + barrierOffset
-				}
-			} else if gcphase == _GCmarktermination {
-				// We just scanned a frame containing
-				// a return to a stack barrier. Since
-				// this frame never returned, we can
-				// stop scanning.
-				return false
-			}
-		}
-		n++
-
 		return true
 	}
 	gentraceback(^uintptr(0), ^uintptr(0), 0, gp, 0, nil, 0x7fffffff, scanframe, nil, 0)
 	tracebackdefers(gp, scanframe, nil)
-	gcUnlockStackBarriers(gp)
 	gp.gcscanvalid = true
 }
 

src/runtime/mstkbar.go

-// Copyright 2015 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// Garbage collector: stack barriers
-//
-// Stack barriers enable the garbage collector to determine how much
-// of a gorountine stack has changed between when a stack is scanned
-// during the concurrent scan phase and when it is re-scanned during
-// the stop-the-world mark termination phase. Mark termination only
-// needs to re-scan the changed part, so for deep stacks this can
-// significantly reduce GC pause time compared to the alternative of
-// re-scanning whole stacks. The deeper the stacks, the more stack
-// barriers help.
-//
-// When stacks are scanned during the concurrent scan phase, the stack
-// scan installs stack barriers by selecting stack frames and
-// overwriting the saved return PCs (or link registers) of these
-// frames with the PC of a "stack barrier trampoline". Later, when a
-// selected frame returns, it "returns" to this trampoline instead of
-// returning to its actual caller. The trampoline records that the
-// stack has unwound past this frame and jumps to the original return
-// PC recorded when the stack barrier was installed. Mark termination
-// re-scans only as far as the first frame that hasn't hit a stack
-// barrier and then removes and un-hit stack barriers.
-//
-// This scheme is very lightweight. No special code is required in the
-// mutator to record stack unwinding and the trampoline is only a few
-// assembly instructions.
-//
-// Book-keeping
-// ------------
-//
-// The primary cost of stack barriers is book-keeping: the runtime has
-// to record the locations of all stack barriers and the original
-// return PCs in order to return to the correct caller when a stack
-// barrier is hit and so it can remove un-hit stack barriers. In order
-// to minimize this cost, the Go runtime places stack barriers in
-// exponentially-spaced frames, starting 1K past the current frame.
-// The book-keeping structure hence grows logarithmically with the
-// size of the stack and mark termination re-scans at most twice as
-// much stack as necessary.
-//
-// The runtime reserves space for this book-keeping structure at the
-// top of the stack allocation itself (just above the outermost
-// frame). This is necessary because the regular memory allocator can
-// itself grow the stack, and hence can't be used when allocating
-// stack-related structures.
-//
-// For debugging, the runtime also supports installing stack barriers
-// at every frame. However, this requires significantly more
-// book-keeping space.
-//
-// Correctness
-// -----------
-//
-// The runtime and the compiler cooperate to ensure that all objects
-// reachable from the stack as of mark termination are marked.
-// Anything unchanged since the concurrent scan phase will be marked
-// because it is marked by the concurrent scan. After the concurrent
-// scan, there are three possible classes of stack modifications that
-// must be tracked:
-//
-// 1) Mutator writes below the lowest un-hit stack barrier. This
-// includes all writes performed by an executing function to its own
-// stack frame. This part of the stack will be re-scanned by mark
-// termination, which will mark any objects made reachable from
-// modifications to this part of the stack.
-//
-// 2) Mutator writes above the lowest un-hit stack barrier. It's
-// possible for a mutator to modify the stack above the lowest un-hit
-// stack barrier if a higher frame has passed down a pointer to a
-// stack variable in its frame. This is called an "up-pointer". The
-// compiler ensures that writes through up-pointers have an
-// accompanying write barrier (it simply doesn't distinguish between
-// writes through up-pointers and writes through heap pointers). This
-// write barrier marks any object made reachable from modifications to
-// this part of the stack.
-//
-// 3) Runtime writes to the stack. Various runtime operations such as
-// sends to unbuffered channels can write to arbitrary parts of the
-// stack, including above the lowest un-hit stack barrier. We solve
-// this in two ways. In many cases, the runtime can perform an
-// explicit write barrier operation like in case 2. However, in the
-// case of bulk memory move (typedmemmove), the runtime doesn't
-// necessary have ready access to a pointer bitmap for the memory
-// being copied, so it simply unwinds any stack barriers below the
-// destination.
-//
-// Gotchas
-// -------
-//
-// Anything that inspects or manipulates the stack potentially needs
-// to understand stack barriers. The most obvious case is that
-// gentraceback needs to use the original return PC when it encounters
-// the stack barrier trampoline. Anything that unwinds the stack such
-// as panic/recover must unwind stack barriers in tandem with
-// unwinding the stack.
-//
-// Stack barriers require that any goroutine whose stack has been
-// scanned must execute write barriers. Go solves this by simply
-// enabling write barriers globally during the concurrent scan phase.
-// However, traditionally, write barriers are not enabled during this
-// phase.
-//
-// Synchronization
-// ---------------
-//
-// For the most part, accessing and modifying stack barriers is
-// synchronized around GC safe points. Installing stack barriers
-// forces the G to a safe point, while all other operations that
-// modify stack barriers run on the G and prevent it from reaching a
-// safe point.
-//
-// Subtlety arises when a G may be tracebacked when *not* at a safe
-// point. This happens during sigprof. For this, each G has a "stack
-// barrier lock" (see gcLockStackBarriers, gcUnlockStackBarriers).
-// Operations that manipulate stack barriers acquire this lock, while
-// sigprof tries to acquire it and simply skips the traceback if it
-// can't acquire it. There is one exception for performance and
-// complexity reasons: hitting a stack barrier manipulates the stack
-// barrier list without acquiring the stack barrier lock. For this,
-// gentraceback performs a special fix up if the traceback starts in
-// the stack barrier function.
-
-package runtime
-
-import (
-	"runtime/internal/atomic"
-	"runtime/internal/sys"
-	"unsafe"
-)
-
-const debugStackBarrier = false
-
-// firstStackBarrierOffset is the approximate byte offset at
-// which to place the first stack barrier from the current SP.
-// This is a lower bound on how much stack will have to be
-// re-scanned during mark termination. Subsequent barriers are
-// placed at firstStackBarrierOffset * 2^n offsets.
-//
-// For debugging, this can be set to 0, which will install a
-// stack barrier at every frame. If you do this, you may also
-// have to raise _StackMin, since the stack barrier
-// bookkeeping will use a large amount of each stack.
-var firstStackBarrierOffset = 1024
-
-// gcMaxStackBarriers returns the maximum number of stack barriers
-// that can be installed in a stack of stackSize bytes.
-func gcMaxStackBarriers(stackSize int) (n int) {
-	if debug.gcstackbarrieroff > 0 {
-		return 0
-	}
-
-	if firstStackBarrierOffset == 0 {
-		// Special debugging case for inserting stack barriers
-		// at every frame. Steal half of the stack for the
-		// []stkbar. Technically, if the stack were to consist
-		// solely of return PCs we would need two thirds of
-		// the stack, but stealing that much breaks things and
-		// this doesn't happen in practice.
-		return stackSize / 2 / int(unsafe.Sizeof(stkbar{}))
-	}
-
-	offset := firstStackBarrierOffset
-	for offset < stackSize {
-		n++
-		offset *= 2
-	}
-	return n + 1
-}
-
-// gcInstallStackBarrier installs a stack barrier over the return PC of frame.
-//go:nowritebarrier
-func gcInstallStackBarrier(gp *g, frame *stkframe) bool {
-	if frame.lr == 0 {
-		if debugStackBarrier {
-			print("not installing stack barrier with no LR, goid=", gp.goid, "\n")
-		}
-		return false
-	}
-
-	if frame.fn.entry == cgocallback_gofuncPC {
-		// cgocallback_gofunc doesn't return to its LR;
-		// instead, its return path puts LR in g.sched.pc and
-		// switches back to the system stack on which
-		// cgocallback_gofunc was originally called. We can't
-		// have a stack barrier in g.sched.pc, so don't
-		// install one in this frame.
-		if debugStackBarrier {
-			print("not installing stack barrier over LR of cgocallback_gofunc, goid=", gp.goid, "\n")
-		}
-		return false
-	}
-
-	// Save the return PC and overwrite it with stackBarrier.
-	var lrUintptr uintptr
-	if usesLR {
-		lrUintptr = frame.sp
-	} else {
-		lrUintptr = frame.fp - sys.RegSize
-	}
-	lrPtr := (*sys.Uintreg)(unsafe.Pointer(lrUintptr))
-	if debugStackBarrier {
-		print("install stack barrier at ", hex(lrUintptr), " over ", hex(*lrPtr), ", goid=", gp.goid, "\n")
-		if uintptr(*lrPtr) != frame.lr {
-			print("frame.lr=", hex(frame.lr))
-			throw("frame.lr differs from stack LR")
-		}
-	}
-
-	gp.stkbar = gp.stkbar[:len(gp.stkbar)+1]
-	stkbar := &gp.stkbar[len(gp.stkbar)-1]
-	stkbar.savedLRPtr = lrUintptr
-	stkbar.savedLRVal = uintptr(*lrPtr)
-	*lrPtr = sys.Uintreg(stackBarrierPC)
-	return true
-}
-
-// gcRemoveStackBarriers removes all stack barriers installed in gp's stack.
-//
-// gp's stack barriers must be locked.
-//
-//go:nowritebarrier
-func gcRemoveStackBarriers(gp *g) {
-	if debugStackBarrier && gp.stkbarPos != 0 {
-		print("hit ", gp.stkbarPos, " stack barriers, goid=", gp.goid, "\n")
-	}
-
-	// Remove stack barriers that we didn't hit.
-	for _, stkbar := range gp.stkbar[gp.stkbarPos:] {
-		gcRemoveStackBarrier(gp, stkbar)
-	}
-
-	// Clear recorded stack barriers so copystack doesn't try to
-	// adjust them.
-	gp.stkbarPos = 0
-	gp.stkbar = gp.stkbar[:0]
-}
-
-// gcRemoveStackBarrier removes a single stack barrier. It is the
-// inverse operation of gcInstallStackBarrier.
-//
-// This is nosplit to ensure gp's stack does not move.
-//
-//go:nowritebarrier
-//go:nosplit
-func gcRemoveStackBarrier(gp *g, stkbar stkbar) {
-	if debugStackBarrier {
-		print("remove stack barrier at ", hex(stkbar.savedLRPtr), " with ", hex(stkbar.savedLRVal), ", goid=", gp.goid, "\n")
-	}
-	lrPtr := (*sys.Uintreg)(unsafe.Pointer(stkbar.savedLRPtr))
-	if val := *lrPtr; val != sys.Uintreg(stackBarrierPC) {
-		printlock()
-		print("at *", hex(stkbar.savedLRPtr), " expected stack barrier PC ", hex(stackBarrierPC), ", found ", hex(val), ", goid=", gp.goid, "\n")
-		print("gp.stkbar=")
-		gcPrintStkbars(gp, -1)
-		print(", gp.stack=[", hex(gp.stack.lo), ",", hex(gp.stack.hi), ")\n")
-		throw("stack barrier lost")
-	}
-	*lrPtr = sys.Uintreg(stkbar.savedLRVal)
-}
-
-// gcTryRemoveAllStackBarriers tries to remove stack barriers from all
-// Gs in gps. It is best-effort and efficient. If it can't remove
-// barriers from a G immediately, it will simply skip it.
-func gcTryRemoveAllStackBarriers(gps []*g) {
-	for _, gp := range gps {
-	retry:
-		for {
-			switch s := readgstatus(gp); s {
-			default:
-				break retry
-
-			case _Grunnable, _Gsyscall, _Gwaiting:
-				if !castogscanstatus(gp, s, s|_Gscan) {
-					continue
-				}
-				gcLockStackBarriers(gp)
-				gcRemoveStackBarriers(gp)
-				gcUnlockStackBarriers(gp)
-				restartg(gp)
-				break retry
-			}
-		}
-	}
-}
-
-// gcPrintStkbars prints the stack barriers of gp for debugging. It
-// places a "@@@" marker at gp.stkbarPos. If marker >= 0, it will also
-// place a "==>" marker before the marker'th entry.
-func gcPrintStkbars(gp *g, marker int) {
-	print("[")
-	for i, s := range gp.stkbar {
-		if i > 0 {
-			print(" ")
-		}
-		if i == int(gp.stkbarPos) {
-			print("@@@ ")
-		}
-		if i == marker {
-			print("==> ")
-		}
-		print("*", hex(s.savedLRPtr), "=", hex(s.savedLRVal))
-	}
-	if int(gp.stkbarPos) == len(gp.stkbar) {
-		print(" @@@")
-	}
-	if marker == len(gp.stkbar) {
-		print(" ==>")
-	}
-	print("]")
-}
-
-// gcUnwindBarriers marks all stack barriers up the frame containing
-// sp as hit and removes them. This is used during stack unwinding for
-// panic/recover and by heapBitsBulkBarrier to force stack re-scanning
-// when its destination is on the stack.
-//
-// This is nosplit to ensure gp's stack does not move.
-//
-//go:nosplit
-func gcUnwindBarriers(gp *g, sp uintptr) {
-	gcLockStackBarriers(gp)
-	// On LR machines, if there is a stack barrier on the return
-	// from the frame containing sp, this will mark it as hit even
-	// though it isn't, but it's okay to be conservative.
-	before := gp.stkbarPos
-	for int(gp.stkbarPos) < len(gp.stkbar) && gp.stkbar[gp.stkbarPos].savedLRPtr < sp {
-		gcRemoveStackBarrier(gp, gp.stkbar[gp.stkbarPos])
-		gp.stkbarPos++
-	}
-	gcUnlockStackBarriers(gp)
-	if debugStackBarrier && gp.stkbarPos != before {
-		print("skip barriers below ", hex(sp), " in goid=", gp.goid, ": ")
-		// We skipped barriers between the "==>" marker
-		// (before) and the "@@@" marker (gp.stkbarPos).
-		gcPrintStkbars(gp, int(before))
-		print("\n")
-	}
-}
-
-// nextBarrierPC returns the original return PC of the next stack barrier.
-// Used by getcallerpc, so it must be nosplit.
-//go:nosplit
-func nextBarrierPC() uintptr {
-	gp := getg()
-	return gp.stkbar[gp.stkbarPos].savedLRVal
-}
-
-// setNextBarrierPC sets the return PC of the next stack barrier.
-// Used by setcallerpc, so it must be nosplit.
-//go:nosplit
-func setNextBarrierPC(pc uintptr) {
-	gp := getg()
-	gcLockStackBarriers(gp)
-	gp.stkbar[gp.stkbarPos].savedLRVal = pc
-	gcUnlockStackBarriers(gp)
-}
-
-// gcLockStackBarriers synchronizes with tracebacks of gp's stack
-// during sigprof for installation or removal of stack barriers. It
-// blocks until any current sigprof is done tracebacking gp's stack
-// and then disallows profiling tracebacks of gp's stack.
-//
-// This is necessary because a sigprof during barrier installation or
-// removal could observe inconsistencies between the stkbar array and
-// the stack itself and crash.
-//
-//go:nosplit
-func gcLockStackBarriers(gp *g) {
-	// Disable preemption so scanstack cannot run while the caller
-	// is manipulating the stack barriers.
-	acquirem()
-	for !atomic.Cas(&gp.stackLock, 0, 1) {
-		osyield()
-	}
-}
-
-//go:nosplit
-func gcTryLockStackBarriers(gp *g) bool {
-	mp := acquirem()
-	result := atomic.Cas(&gp.stackLock, 0, 1)
-	if !result {
-		releasem(mp)
-	}
-	return result
-}
-
-func gcUnlockStackBarriers(gp *g) {
-	atomic.Store(&gp.stackLock, 0)
-	releasem(getg().m)
-}

src/runtime/panic.go

@@ -617,7 +617,6 @@ func recovery(gp *g) {
 	// Make the deferproc for this d return again,
 	// this time returning 1.  The calling function will
 	// jump to the standard return epilogue.
-	gcUnwindBarriers(gp, sp)
 	gp.sched.sp = sp
 	gp.sched.pc = pc
 	gp.sched.lr = 0

src/runtime/pprof/pprof_test.go

@@ -8,12 +8,9 @@ package pprof_test
 
 import (
 	"bytes"
-	"compress/gzip"
 	"fmt"
 	"internal/pprof/profile"
 	"internal/testenv"
-	"io"
-	"io/ioutil"
 	"math/big"
 	"os"
 	"os/exec"
@@ -124,8 +121,7 @@ func testCPUProfile(t *testing.T, need []string, f func(dur time.Duration)) {
 
 	const maxDuration = 5 * time.Second
 	// If we're running a long test, start with a long duration
-	// because some of the tests (e.g., TestStackBarrierProfiling)
-	// are trying to make sure something *doesn't* happen.
+	// for tests that try to make sure something *doesn't* happen.
 	duration := 5 * time.Second
 	if testing.Short() {
 		duration = 200 * time.Millisecond
@@ -187,10 +183,6 @@ func profileOk(t *testing.T, need []string, prof bytes.Buffer, duration time.Dur
 					have[i] += count
 				}
 			}
-			if strings.Contains(f.Name(), "stackBarrier") {
-				// The runtime should have unwound this.
-				t.Fatalf("profile includes stackBarrier")
-			}
 		}
 	})
 	t.Logf("total %d CPU profile samples collected", samples)
@@ -350,111 +342,6 @@ func TestMathBigDivide(t *testing.T) {
 	})
 }
 
-func slurpString(r io.Reader) string {
-	slurp, _ := ioutil.ReadAll(r)
-	return string(slurp)
-}
-
-func getLinuxKernelConfig() string {
-	if f, err := os.Open("/proc/config"); err == nil {
-		defer f.Close()
-		return slurpString(f)
-	}
-	if f, err := os.Open("/proc/config.gz"); err == nil {
-		defer f.Close()
-		r, err := gzip.NewReader(f)
-		if err != nil {
-			return ""
-		}
-		return slurpString(r)
-	}
-	if f, err := os.Open("/boot/config"); err == nil {
-		defer f.Close()
-		return slurpString(f)
-	}
-	uname, _ := exec.Command("uname", "-r").Output()
-	if len(uname) > 0 {
-		if f, err := os.Open("/boot/config-" + strings.TrimSpace(string(uname))); err == nil {
-			defer f.Close()
-			return slurpString(f)
-		}
-	}
-	return ""
-}
-
-func haveLinuxHiresTimers() bool {
-	config := getLinuxKernelConfig()
-	return strings.Contains(config, "CONFIG_HIGH_RES_TIMERS=y")
-}
-
-func TestStackBarrierProfiling(t *testing.T) {
-	if (runtime.GOOS == "linux" && runtime.GOARCH == "arm") ||
-		runtime.GOOS == "openbsd" ||
-		runtime.GOOS == "solaris" ||
-		runtime.GOOS == "dragonfly" ||
-		runtime.GOOS == "freebsd" {
-		// This test currently triggers a large number of
-		// usleep(100)s. These kernels/arches have poor
-		// resolution timers, so this gives up a whole
-		// scheduling quantum. On Linux and the BSDs (and
-		// probably Solaris), profiling signals are only
-		// generated when a process completes a whole
-		// scheduling quantum, so this test often gets zero
-		// profiling signals and fails.
-		t.Skipf("low resolution timers inhibit profiling signals (golang.org/issue/13405)")
-		return
-	}
-
-	if runtime.GOOS == "linux" && strings.HasPrefix(runtime.GOARCH, "mips") {
-		if !haveLinuxHiresTimers() {
-			t.Skipf("low resolution timers inhibit profiling signals (golang.org/issue/13405, golang.org/issue/17936)")
-		}
-	}
-
-	if !strings.Contains(os.Getenv("GODEBUG"), "gcstackbarrierall=1") {
-		// Re-execute this test with constant GC and stack
-		// barriers at every frame.
-		testenv.MustHaveExec(t)
-		if runtime.GOARCH == "ppc64" || runtime.GOARCH == "ppc64le" {
-			t.Skip("gcstackbarrierall doesn't work on ppc64")
-		}
-		args := []string{"-test.run=TestStackBarrierProfiling"}
-		if testing.Short() {
-			args = append(args, "-test.short")
-		}
-		cmd := exec.Command(os.Args[0], args...)
-		cmd.Env = append([]string{"GODEBUG=gcstackbarrierall=1", "GOGC=1", "GOTRACEBACK=system"}, os.Environ()...)
-		if out, err := cmd.CombinedOutput(); err != nil {
-			t.Fatalf("subprocess failed with %v:\n%s", err, out)
-		}
-		return
-	}
-
-	testCPUProfile(t, nil, func(duration time.Duration) {
-		// In long mode, we're likely to get one or two
-		// samples in stackBarrier.
-		t := time.After(duration)
-		for {
-			deepStack(1000)
-			select {
-			case <-t:
-				return
-			default:
-			}
-		}
-	})
-}
-
-var x []byte
-
-func deepStack(depth int) int {
-	if depth == 0 {
-		return 0
-	}
-	x = make([]byte, 1024)
-	return deepStack(depth-1) + 1
-}
-
 // Operating systems that are expected to fail the tests. See issue 13841.
 var badOS = map[string]bool{
 	"darwin":    true,

src/runtime/proc.go

@@ -816,8 +816,6 @@ func scang(gp *g, gcw *gcWork) {
 	// Nothing is racing with us now, but gcscandone might be set to true left over
 	// from an earlier round of stack scanning (we scan twice per GC).
 	// We use gcscandone to record whether the scan has been done during this round.
-	// It is important that the scan happens exactly once: if called twice,
-	// the installation of stack barriers will detect the double scan and die.
 
 	gp.gcscandone = false
 
@@ -2810,7 +2808,7 @@ func malg(stacksize int32) *g {
 	if stacksize >= 0 {
 		stacksize = round2(_StackSystem + stacksize)
 		systemstack(func() {
-			newg.stack, newg.stkbar = stackalloc(uint32(stacksize))
+			newg.stack = stackalloc(uint32(stacksize))
 		})
 		newg.stackguard0 = newg.stack.lo + _StackGuard
 		newg.stackguard1 = ^uintptr(0)
@@ -2959,12 +2957,6 @@ func gfput(_p_ *p, gp *g) {
 		gp.stack.lo = 0
 		gp.stack.hi = 0
 		gp.stackguard0 = 0
-		gp.stkbar = nil
-		gp.stkbarPos = 0
-	} else {
-		// Reset stack barriers.
-		gp.stkbar = gp.stkbar[:0]
-		gp.stkbarPos = 0
 	}
 
 	gp.schedlink.set(_p_.gfree)
@@ -3021,7 +3013,7 @@ retry:
 		if gp.stack.lo == 0 {
 			// Stack was deallocated in gfput. Allocate a new one.
 			systemstack(func() {
-				gp.stack, gp.stkbar = stackalloc(_FixedStack)
+				gp.stack = stackalloc(_FixedStack)
 			})
 			gp.stackguard0 = gp.stack.lo + _StackGuard
 			gp.stackAlloc = _FixedStack
@@ -3240,7 +3232,6 @@ func sigprof(pc, sp, lr uintptr, gp *g, mp *m) {
 		traceback = false
 	}
 	var stk [maxCPUProfStack]uintptr
-	var haveStackLock *g
 	n := 0
 	if mp.ncgo > 0 && mp.curg != nil && mp.curg.syscallpc != 0 && mp.curg.syscallsp != 0 {
 		cgoOff := 0
@@ -3258,26 +3249,9 @@ func sigprof(pc, sp, lr uintptr, gp *g, mp *m) {
 		}
 
 		// Collect Go stack that leads to the cgo call.
-		if gcTryLockStackBarriers(mp.curg) {
-			haveStackLock = mp.curg
-			n = gentraceback(mp.curg.syscallpc, mp.curg.syscallsp, 0, mp.curg, 0, &stk[cgoOff], len(stk)-cgoOff, nil, nil, 0)
-		}
+		n = gentraceback(mp.curg.syscallpc, mp.curg.syscallsp, 0, mp.curg, 0, &stk[cgoOff], len(stk)-cgoOff, nil, nil, 0)
 	} else if traceback {
-		var flags uint = _TraceTrap
-		if gp.m.curg != nil && gcTryLockStackBarriers(gp.m.curg) {
-			// It's safe to traceback the user stack.
-			haveStackLock = gp.m.curg
-			flags |= _TraceJumpStack
-		}
-		// Traceback is safe if we're on the system stack (if
-		// necessary, flags will stop it before switching to
-		// the user stack), or if we locked the user stack.
-		if gp != gp.m.curg || haveStackLock != nil {
-			n = gentraceback(pc, sp, lr, gp, 0, &stk[0], len(stk), nil, nil, flags)
-		}
-	}
-	if haveStackLock != nil {
-		gcUnlockStackBarriers(haveStackLock)
+		n = gentraceback(pc, sp, lr, gp, 0, &stk[0], len(stk), nil, nil, _TraceTrap|_TraceJumpStack)
 	}
 
 	if n <= 0 {
@@ -3287,10 +3261,7 @@ func sigprof(pc, sp, lr uintptr, gp *g, mp *m) {
 		if GOOS == "windows" && mp.libcallg != 0 && mp.libcallpc != 0 && mp.libcallsp != 0 {
 			// Libcall, i.e. runtime syscall on windows.
 			// Collect Go stack that leads to the call.
-			if gcTryLockStackBarriers(mp.libcallg.ptr()) {
-				n = gentraceback(mp.libcallpc, mp.libcallsp, 0, mp.libcallg.ptr(), 0, &stk[0], len(stk), nil, nil, 0)
-				gcUnlockStackBarriers(mp.libcallg.ptr())
-			}
+			n = gentraceback(mp.libcallpc, mp.libcallsp, 0, mp.libcallg.ptr(), 0, &stk[0], len(stk), nil, nil, 0)
 		}
 		if n == 0 {
 			// If all of the above has failed, account it against abstract "System" or "GC".

src/runtime/runtime1.go

@@ -313,22 +313,20 @@ type dbgVar struct {
 // existing int var for that value, which may
 // already have an initial value.
 var debug struct {
-	allocfreetrace    int32
-	cgocheck          int32
-	efence            int32
-	gccheckmark       int32
-	gcpacertrace      int32
-	gcshrinkstackoff  int32
-	gcstackbarrieroff int32
-	gcstackbarrierall int32
-	gcrescanstacks    int32
-	gcstoptheworld    int32
-	gctrace           int32
-	invalidptr        int32
-	sbrk              int32
-	scavenge          int32
-	scheddetail       int32
-	schedtrace        int32
+	allocfreetrace   int32
+	cgocheck         int32
+	efence           int32
+	gccheckmark      int32
+	gcpacertrace     int32
+	gcshrinkstackoff int32
+	gcrescanstacks   int32
+	gcstoptheworld   int32
+	gctrace          int32
+	invalidptr       int32
+	sbrk             int32
+	scavenge         int32
+	scheddetail      int32
+	schedtrace       int32
 }
 
 var dbgvars = []dbgVar{
@@ -338,8 +336,6 @@ var dbgvars = []dbgVar{
 	{"gccheckmark", &debug.gccheckmark},
 	{"gcpacertrace", &debug.gcpacertrace},
 	{"gcshrinkstackoff", &debug.gcshrinkstackoff},
-	{"gcstackbarrieroff", &debug.gcstackbarrieroff},
-	{"gcstackbarrierall", &debug.gcstackbarrierall},
 	{"gcrescanstacks", &debug.gcrescanstacks},
 	{"gcstoptheworld", &debug.gcstoptheworld},
 	{"gctrace", &debug.gctrace},
@@ -390,17 +386,6 @@ func parsedebugvars() {
 	setTraceback(gogetenv("GOTRACEBACK"))
 	traceback_env = traceback_cache
 
-	if debug.gcrescanstacks == 0 {
-		// Without rescanning, there's no need for stack
-		// barriers.
-		debug.gcstackbarrieroff = 1
-		debug.gcstackbarrierall = 0
-	}
-
-	if debug.gcstackbarrierall > 0 {
-		firstStackBarrierOffset = 0
-	}
-
 	// For cgocheck > 1, we turn on the write barrier at all times
 	// and check all pointer writes.
 	if debug.cgocheck > 1 {

src/runtime/runtime2.go

@@ -326,12 +326,6 @@ type stack struct {
 	hi uintptr
 }
 
-// stkbar records the state of a G's stack barrier.
-type stkbar struct {
-	savedLRPtr uintptr // location overwritten by stack barrier PC
-	savedLRVal uintptr // value overwritten at savedLRPtr
-}
-
 type g struct {
 	// Stack parameters.
 	// stack describes the actual stack memory: [stack.lo, stack.hi).
@@ -351,8 +345,6 @@ type g struct {
 	sched          gobuf
 	syscallsp      uintptr        // if status==Gsyscall, syscallsp = sched.sp to use during gc
 	syscallpc      uintptr        // if status==Gsyscall, syscallpc = sched.pc to use during gc
-	stkbar         []stkbar       // stack barriers, from low to high (see top of mstkbar.go)
-	stkbarPos      uintptr        // index of lowest stack barrier not hit
 	stktopsp       uintptr        // expected sp at top of stack, to check in traceback
 	param          unsafe.Pointer // passed parameter on wakeup
 	atomicstatus   uint32

src/runtime/stack.go

@@ -320,7 +320,7 @@ func stackcache_clear(c *mcache) {
 // resources and must not split the stack.
 //
 //go:systemstack
-func stackalloc(n uint32) (stack, []stkbar) {
+func stackalloc(n uint32) stack {
 	// Stackalloc must be called on scheduler stack, so that we
 	// never try to grow the stack during the code that stackalloc runs.
 	// Doing so would cause a deadlock (issue 1547).
@@ -335,21 +335,12 @@ func stackalloc(n uint32) (stack, []stkbar) {
 		print("stackalloc ", n, "\n")
 	}
 
-	// Compute the size of stack barrier array.
-	maxstkbar := gcMaxStackBarriers(int(n))
-	nstkbar := unsafe.Sizeof(stkbar{}) * uintptr(maxstkbar)
-	var stkbarSlice slice
-
 	if debug.efence != 0 || stackFromSystem != 0 {
 		v := sysAlloc(round(uintptr(n), _PageSize), &memstats.stacks_sys)
 		if v == nil {
 			throw("out of memory (stackalloc)")
 		}
-		top := uintptr(n) - nstkbar
-		if maxstkbar != 0 {
-			stkbarSlice = slice{add(v, top), 0, maxstkbar}
-		}
-		return stack{uintptr(v), uintptr(v) + top}, *(*[]stkbar)(unsafe.Pointer(&stkbarSlice))
+		return stack{uintptr(v), uintptr(v) + uintptr(n)}
 	}
 
 	// Small stacks are allocated with a fixed-size free-list allocator.
@@ -415,11 +406,7 @@ func stackalloc(n uint32) (stack, []stkbar) {
 	if stackDebug >= 1 {
 		print("  allocated ", v, "\n")
 	}
-	top := uintptr(n) - nstkbar
-	if maxstkbar != 0 {
-		stkbarSlice = slice{add(v, top), 0, maxstkbar}
-	}
-	return stack{uintptr(v), uintptr(v) + top}, *(*[]stkbar)(unsafe.Pointer(&stkbarSlice))
+	return stack{uintptr(v), uintptr(v) + uintptr(n)}
 }
 
 // stackfree frees an n byte stack allocation at stk.
@@ -774,12 +761,6 @@ func adjustsudogs(gp *g, adjinfo *adjustinfo) {
 	}
 }
 
-func adjuststkbar(gp *g, adjinfo *adjustinfo) {
-	for i := int(gp.stkbarPos); i < len(gp.stkbar); i++ {
-		adjustpointer(adjinfo, unsafe.Pointer(&gp.stkbar[i].savedLRPtr))
-	}
-}
-
 func fillstack(stk stack, b byte) {
 	for p := stk.lo; p < stk.hi; p++ {
 		*(*byte)(unsafe.Pointer(p)) = b
@@ -866,7 +847,7 @@ func copystack(gp *g, newsize uintptr, sync bool) {
 	used := old.hi - gp.sched.sp
 
 	// allocate new stack
-	new, newstkbar := stackalloc(uint32(newsize))
+	new := stackalloc(uint32(newsize))
 	if stackPoisonCopy != 0 {
 		fillstack(new, 0xfd)
 	}
@@ -900,39 +881,27 @@ func copystack(gp *g, newsize uintptr, sync bool) {
 	// Copy the stack (or the rest of it) to the new location
 	memmove(unsafe.Pointer(new.hi-ncopy), unsafe.Pointer(old.hi-ncopy), ncopy)
 
-	// Disallow sigprof scans of this stack and block if there's
-	// one in progress.
-	gcLockStackBarriers(gp)
-
 	// Adjust remaining structures that have pointers into stacks.
 	// We have to do most of these before we traceback the new
 	// stack because gentraceback uses them.
 	adjustctxt(gp, &adjinfo)
 	adjustdefers(gp, &adjinfo)
 	adjustpanics(gp, &adjinfo)
-	adjuststkbar(gp, &adjinfo)
 	if adjinfo.sghi != 0 {
 		adjinfo.sghi += adjinfo.delta
 	}
 
-	// copy old stack barriers to new stack barrier array
-	newstkbar = newstkbar[:len(gp.stkbar)]
-	copy(newstkbar, gp.stkbar)
-
 	// Swap out old stack for new one
 	gp.stack = new
 	gp.stackguard0 = new.lo + _StackGuard // NOTE: might clobber a preempt request
 	gp.sched.sp = new.hi - used
 	oldsize := gp.stackAlloc
 	gp.stackAlloc = newsize
-	gp.stkbar = newstkbar
 	gp.stktopsp += adjinfo.delta
 
 	// Adjust pointers in the new stack.
 	gentraceback(^uintptr(0), ^uintptr(0), 0, gp, 0, nil, 0x7fffffff, adjustframe, noescape(unsafe.Pointer(&adjinfo)), 0)
 
-	gcUnlockStackBarriers(gp)
-
 	// free old stack
 	if stackPoisonCopy != 0 {
 		fillstack(old, 0xfc)
@@ -1132,8 +1101,6 @@ func shrinkstack(gp *g) {
 			stackfree(gp.stack, gp.stackAlloc)
 			gp.stack.lo = 0
 			gp.stack.hi = 0
-			gp.stkbar = nil
-			gp.stkbarPos = 0
 		}
 		return
 	}

src/runtime/stack_test.go

@@ -315,17 +315,17 @@ func TestPanicFar(t *testing.T) {
 	defer func() {
 		// At this point we created a large stack and unwound
 		// it via recovery. Force a stack walk, which will
-		// check the consistency of stack barriers.
+		// check the stack's consistency.
 		Callers(0, pc)
 	}()
 	defer func() {
 		recover()
 	}()
 	useStackAndCall(100, func() {
-		// Kick off the GC and make it do something nontrivial
-		// to keep stack barriers installed for a while.
+		// Kick off the GC and make it do something nontrivial.
+		// (This used to force stack barriers to stick around.)
 		xtree = makeTree(18)
-		// Give the GC time to install stack barriers.
+		// Give the GC time to start scanning stacks.
 		time.Sleep(time.Millisecond)
 		panic(1)
 	})

src/runtime/stubs.go

@@ -164,7 +164,7 @@ type neverCallThisFunction struct{}
 // This function must never be called directly. Call goexit1 instead.
 // gentraceback assumes that goexit terminates the stack. A direct
 // call on the stack will cause gentraceback to stop walking the stack
-// prematurely and if there are leftover stack barriers it may panic.
+// prematurely and if there is leftover state it may panic.
 func goexit(neverCallThisFunction)
 
 // Not all cgocallback_gofunc frames are actually cgocallback_gofunc,
@@ -241,13 +241,6 @@ func morestack()
 func morestack_noctxt()
 func rt0_go()
 
-// stackBarrier records that the stack has been unwound past a certain
-// point. It is installed over a return PC on the stack. It must
-// retrieve the original return PC from g.stkbuf, increment
-// g.stkbufPos to record that the barrier was hit, and jump to the
-// original return PC.
-func stackBarrier()
-
 // return0 is a stub used to return 0 from deferproc.
 // It is called at the very end of deferproc to signal
 // the calling Go function that it should not jump

src/runtime/trace.go

@@ -572,12 +572,7 @@ func traceStackID(mp *m, buf []uintptr, skip int) uint64 {
 		nstk = callers(skip+1, buf[:])
 	} else if gp != nil {
 		gp = mp.curg
-		// This may happen when tracing a system call,
-		// so we must lock the stack.
-		if gcTryLockStackBarriers(gp) {
-			nstk = gcallers(gp, skip, buf[:])
-			gcUnlockStackBarriers(gp)
-		}
+		nstk = gcallers(gp, skip, buf[:])
 	}
 	if nstk > 0 {
 		nstk-- // skip runtime.goexit

src/runtime/traceback.go

@@ -51,7 +51,6 @@ var (
 	gcBgMarkWorkerPC     uintptr
 	systemstack_switchPC uintptr
 	systemstackPC        uintptr
-	stackBarrierPC       uintptr
 	cgocallback_gofuncPC uintptr
 
 	gogoPC uintptr
@@ -78,7 +77,6 @@ func tracebackinit() {
 	gcBgMarkWorkerPC = funcPC(gcBgMarkWorker)
 	systemstack_switchPC = funcPC(systemstack_switch)
 	systemstackPC = funcPC(systemstack)
-	stackBarrierPC = funcPC(stackBarrier)
 	cgocallback_gofuncPC = funcPC(cgocallback_gofunc)
 
 	// used by sigprof handler
@@ -143,11 +141,6 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 	}
 	level, _, _ := gotraceback()
 
-	// Fix up returns to the stack barrier by fetching the
-	// original return PC from gp.stkbar.
-	stkbarG := gp
-	stkbar := stkbarG.stkbar[stkbarG.stkbarPos:]
-
 	if pc0 == ^uintptr(0) && sp0 == ^uintptr(0) { // Signal to fetch saved values from gp.
 		if gp.syscallsp != 0 {
 			pc0 = gp.syscallpc
@@ -193,34 +186,6 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 	}
 
 	f := findfunc(frame.pc)
-	if f != nil && f.entry == stackBarrierPC {
-		// We got caught in the middle of a stack barrier
-		// (presumably by a signal), so stkbar may be
-		// inconsistent with the barriers on the stack.
-		// Simulate the completion of the barrier.
-		//
-		// On x86, SP will be exactly one word above
-		// savedLRPtr. On LR machines, SP will be above
-		// savedLRPtr by some frame size.
-		var stkbarPos uintptr
-		if len(stkbar) > 0 && stkbar[0].savedLRPtr < sp0 {
-			// stackBarrier has not incremented stkbarPos.
-			stkbarPos = gp.stkbarPos
-		} else if gp.stkbarPos > 0 && gp.stkbar[gp.stkbarPos-1].savedLRPtr < sp0 {
-			// stackBarrier has incremented stkbarPos.
-			stkbarPos = gp.stkbarPos - 1
-		} else {
-			printlock()
-			print("runtime: failed to unwind through stackBarrier at SP ", hex(sp0), "; ")
-			gcPrintStkbars(gp, int(gp.stkbarPos))
-			print("\n")
-			throw("inconsistent state in stackBarrier")
-		}
-
-		frame.pc = gp.stkbar[stkbarPos].savedLRVal
-		stkbar = gp.stkbar[stkbarPos+1:]
-		f = findfunc(frame.pc)
-	}
 	if f == nil {
 		if callback != nil {
 			print("runtime: unknown pc ", hex(frame.pc), "\n")
@@ -257,8 +222,6 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 			if flags&_TraceJumpStack != 0 && f.entry == systemstackPC && gp == g.m.g0 && gp.m.curg != nil {
 				sp = gp.m.curg.sched.sp
 				frame.sp = sp
-				stkbarG = gp.m.curg
-				stkbar = stkbarG.stkbar[stkbarG.stkbarPos:]
 				cgoCtxt = gp.m.curg.cgoCtxt
 			}
 			frame.fp = sp + uintptr(funcspdelta(f, frame.pc, &cache))
@@ -295,17 +258,6 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 					frame.lr = uintptr(*(*sys.Uintreg)(unsafe.Pointer(lrPtr)))
 				}
 			}
-			if frame.lr == stackBarrierPC {
-				// Recover original PC.
-				if len(stkbar) == 0 || stkbar[0].savedLRPtr != lrPtr {
-					print("found next stack barrier at ", hex(lrPtr), "; expected ")
-					gcPrintStkbars(stkbarG, len(stkbarG.stkbar)-len(stkbar))
-					print("\n")
-					throw("missed stack barrier")
-				}
-				frame.lr = stkbar[0].savedLRVal
-				stkbar = stkbar[1:]
-			}
 			flr = findfunc(frame.lr)
 			if flr == nil {
 				// This happens if you get a profiling interrupt at just the wrong time.
@@ -530,13 +482,6 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 		throw("traceback has leftover defers")
 	}
 
-	if callback != nil && n < max && len(stkbar) > 0 {
-		print("runtime: g", gp.goid, ": leftover stack barriers ")
-		gcPrintStkbars(stkbarG, len(stkbarG.stkbar)-len(stkbar))
-		print("\n")
-		throw("traceback has leftover stack barriers")
-	}
-
 	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")