runtime: tricky replacements of _g_ in signal_unix.go

sighandler has gp, the goroutine running when the signal arrived, and
gsignal, the goroutine executing the signal handler. The latter is
usually mp.gsignal, except in the case noted by the delayedSignal check.

Like previous CLs, cases where the getg() G is used only to access the M
are replaced with direct uses of mp.

Change-Id: I2dc7894da7004af17682712e07a0be5f9a235d81
Reviewed-on: https://go-review.googlesource.com/c/go/+/418580
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>

1 files changed, 19 insertions, 17 deletions

diff --git a/src/runtime/signal_unix.go b/src/runtime/signal_unix.go
index a220f8347e..ee5f3adead 100644
--- a/src/runtime/signal_unix.go
+++ b/src/runtime/signal_unix.go
@@ -596,7 +596,7 @@ var testSigusr1 func(gp *g) bool
 
 // sighandler is invoked when a signal occurs. The global g will be
 // set to a gsignal goroutine and we will be running on the alternate
-// signal stack. The parameter g will be the value of the global g
+// signal stack. The parameter gp will be the value of the global g
 // when the signal occurred. The sig, info, and ctxt parameters are
 // from the system signal handler: they are the parameters passed when
 // the SA is passed to the sigaction system call.
@@ -606,9 +606,11 @@ var testSigusr1 func(gp *g) bool
 //
 //go:nowritebarrierrec
 func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
-	_g_ := getg()
+	// The g executing the signal handler. This is almost always
+	// mp.gsignal. See delayedSignal for an exception.
+	gsignal := getg()
+	mp := gsignal.m
 	c := &sigctxt{info, ctxt}
-	mp := _g_.m
 
 	// Cgo TSAN (not the Go race detector) intercepts signals and calls the
 	// signal handler at a later time. When the signal handler is called, the
@@ -620,7 +622,7 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 	// signal delivery. We use that as an indicator of delayed signals.
 	// For delayed signals, the handler is called on the g0 stack (see
 	// adjustSignalStack).
-	delayedSignal := *cgo_yield != nil && mp != nil && _g_.stack == mp.g0.stack
+	delayedSignal := *cgo_yield != nil && mp != nil && gsignal.stack == mp.g0.stack
 
 	if sig == _SIGPROF {
 		// Some platforms (Linux) have per-thread timers, which we use in
@@ -710,8 +712,8 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 		return
 	}
 
-	_g_.m.throwing = throwTypeRuntime
-	_g_.m.caughtsig.set(gp)
+	mp.throwing = throwTypeRuntime
+	mp.caughtsig.set(gp)
 
 	if crashing == 0 {
 		startpanic_m()
@@ -723,12 +725,12 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 		print("Signal ", sig, "\n")
 	}
 
-	print("PC=", hex(c.sigpc()), " m=", _g_.m.id, " sigcode=", c.sigcode(), "\n")
-	if _g_.m.incgo && gp == _g_.m.g0 && _g_.m.curg != nil {
+	print("PC=", hex(c.sigpc()), " m=", mp.id, " sigcode=", c.sigcode(), "\n")
+	if mp.incgo && gp == mp.g0 && mp.curg != nil {
 		print("signal arrived during cgo execution\n")
 		// Switch to curg so that we get a traceback of the Go code
 		// leading up to the cgocall, which switched from curg to g0.
-		gp = _g_.m.curg
+		gp = mp.curg
 	}
 	if sig == _SIGILL || sig == _SIGFPE {
 		// It would be nice to know how long the instruction is.
@@ -760,10 +762,10 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 	if level > 0 {
 		goroutineheader(gp)
 		tracebacktrap(c.sigpc(), c.sigsp(), c.siglr(), gp)
-		if crashing > 0 && gp != _g_.m.curg && _g_.m.curg != nil && readgstatus(_g_.m.curg)&^_Gscan == _Grunning {
+		if crashing > 0 && gp != mp.curg && mp.curg != nil && readgstatus(mp.curg)&^_Gscan == _Grunning {
 			// tracebackothers on original m skipped this one; trace it now.
-			goroutineheader(_g_.m.curg)
-			traceback(^uintptr(0), ^uintptr(0), 0, _g_.m.curg)
+			goroutineheader(mp.curg)
+			traceback(^uintptr(0), ^uintptr(0), 0, mp.curg)
 		} else if crashing == 0 {
 			tracebackothers(gp)
 			print("\n")
@@ -1207,15 +1209,15 @@ func minitSignals() {
 // of whether it is already set). Record which choice was made in
 // newSigstack, so that it can be undone in unminit.
 func minitSignalStack() {
-	_g_ := getg()
+	mp := getg().m
 	var st stackt
 	sigaltstack(nil, &st)
 	if st.ss_flags&_SS_DISABLE != 0 || !iscgo {
-		signalstack(&_g_.m.gsignal.stack)
-		_g_.m.newSigstack = true
+		signalstack(&mp.gsignal.stack)
+		mp.newSigstack = true
 	} else {
-		setGsignalStack(&st, &_g_.m.goSigStack)
-		_g_.m.newSigstack = false
+		setGsignalStack(&st, &mp.goSigStack)
+		mp.newSigstack = false
 	}
 }