runtime: remove write barrier on G in sighandler

sighandler may run during a stop-the-world without a P, so it's not
allowed to have write barriers. Fix the G write to disable the write
barrier (this is safe because the G is reachable from allgs) and mark
the function nowritebarrier.

Change-Id: I907f05d3829e24eeb15fa4d020598af36710e87e
Reviewed-on: https://go-review.googlesource.com/8020Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/os3_plan9.go

@@ -6,6 +6,8 @@ package runtime
 
 import "unsafe"
 
+// May run during STW, so write barriers are not allowed.
+//go:nowritebarrier
 func sighandler(_ureg *ureg, note *byte, gp *g) int {
 	_g_ := getg()
 	var t sigTabT
@@ -79,7 +81,7 @@ func sighandler(_ureg *ureg, note *byte, gp *g) int {
 	}
 Throw:
 	_g_.m.throwing = 1
-	_g_.m.caughtsig = gp
+	setGNoWriteBarrier(&_g_.m.caughtsig, gp)
 	startpanic()
 	print(notestr, "\n")
 	print("PC=", hex(c.pc()), "\n")

src/runtime/signal_386.go

@@ -24,6 +24,8 @@ func dumpregs(c *sigctxt) {
 	print("gs     ", hex(c.gs()), "\n")
 }
 
+// May run during STW, so write barriers are not allowed.
+//go:nowritebarrier
 func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 	_g_ := getg()
 	c := &sigctxt{info, ctxt}
@@ -98,7 +100,7 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 	}
 
 	_g_.m.throwing = 1
-	_g_.m.caughtsig = gp
+	setGNoWriteBarrier(&_g_.m.caughtsig, gp)
 	startpanic()
 
 	if sig < uint32(len(sigtable)) {

src/runtime/signal_amd64x.go

@@ -37,6 +37,8 @@ func dumpregs(c *sigctxt) {
 
 var crashing int32
 
+// May run during STW, so write barriers are not allowed.
+//go:nowritebarrier
 func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 	_g_ := getg()
 	c := &sigctxt{info, ctxt}
@@ -134,7 +136,7 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 	}
 
 	_g_.m.throwing = 1
-	_g_.m.caughtsig = gp
+	setGNoWriteBarrier(&_g_.m.caughtsig, gp)
 
 	if crashing == 0 {
 		startpanic()

src/runtime/signal_arm.go

@@ -32,6 +32,8 @@ func dumpregs(c *sigctxt) {
 	print("fault   ", hex(c.fault()), "\n")
 }
 
+// May run during STW, so write barriers are not allowed.
+//go:nowritebarrier
 func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 	_g_ := getg()
 	c := &sigctxt{info, ctxt}
@@ -93,7 +95,7 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 	}
 
 	_g_.m.throwing = 1
-	_g_.m.caughtsig = gp
+	setGNoWriteBarrier(&_g_.m.caughtsig, gp)
 	startpanic()
 
 	if sig < uint32(len(sigtable)) {

src/runtime/signal_arm64.go

@@ -45,6 +45,8 @@ func dumpregs(c *sigctxt) {
 	print("fault   ", hex(c.fault()), "\n")
 }
 
+// May run during STW, so write barriers are not allowed.
+//go:nowritebarrier
 func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 	_g_ := getg()
 	c := &sigctxt{info, ctxt}
@@ -106,7 +108,7 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 	}
 
 	_g_.m.throwing = 1
-	_g_.m.caughtsig = gp
+	setGNoWriteBarrier(&_g_.m.caughtsig, gp)
 	startpanic()
 
 	if sig < uint32(len(sigtable)) {

src/runtime/signal_ppc64x.go

@@ -50,6 +50,8 @@ func dumpregs(c *sigctxt) {
 	print("trap ", hex(c.trap()), "\n")
 }
 
+// May run during STW, so write barriers are not allowed.
+//go:nowritebarrier
 func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 	_g_ := getg()
 	c := &sigctxt{info, ctxt}
@@ -111,7 +113,7 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 	}
 
 	_g_.m.throwing = 1
-	_g_.m.caughtsig = gp
+	setGNoWriteBarrier(&_g_.m.caughtsig, gp)
 	startpanic()
 
 	if sig < uint32(len(sigtable)) {