runtime: update comments and make crashing atomic

Author: zzkcode

src/runtime/signal_unix.go

@@ -597,7 +597,7 @@ func adjustSignalStack(sig uint32, mp *m, gsigStack *gsignalStack) bool {
 
 // crashing is the number of m's we have waited for when implementing
 // GOTRACEBACK=crash when a signal is received.
-var crashing int32
+var crashing atomic.Int32
 
 // testSigtrap and testSigusr1 are used by the runtime tests. If
 // non-nil, it is called on SIGTRAP/SIGUSR1. If it returns true, the
@@ -730,7 +730,7 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 	mp.throwing = throwTypeRuntime
 	mp.caughtsig.set(gp)
 
-	if crashing == 0 {
+	if crashing.Load() == 0 {
 		startpanic_m()
 	}
 
@@ -740,11 +740,11 @@ 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 != mp.curg && mp.curg != nil && readgstatus(mp.curg)&^_Gscan == _Grunning {
+		if crashing.Load() > 0 && gp != mp.curg && mp.curg != nil && readgstatus(mp.curg)&^_Gscan == _Grunning {
 			// tracebackothers on original m skipped this one; trace it now.
 			goroutineheader(mp.curg)
 			traceback(^uintptr(0), ^uintptr(0), 0, mp.curg)
-		} else if crashing == 0 {
+		} else if crashing.Load() == 0 {
 			tracebackothers(gp)
 			print("\n")
 		}
@@ -753,29 +753,29 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 
 	if docrash {
 		isCrashThread := false
-		if crashing == 0 {
+		if crashing.Load() == 0 {
 			isCrashThread = true
 		}
-		crashing++
-		if crashing < mcount()-int32(extraMLength.Load()) {
+		crashing.Add(1)
+		if crashing.Load() < mcount()-int32(extraMLength.Load()) {
 			// There are other m's that need to dump their stacks.
 			// Relay SIGQUIT to the next m by sending it to the current process.
 			// All m's that have already received SIGQUIT have signal masks blocking
 			// receipt of any signals, so the SIGQUIT will go to an m that hasn't seen it yet.
 			// The first m will wait until all ms received the SIGQUIT, then crash/exit.
 			// Just in case the relaying gets botched, each m involved in
 			// the relay sleeps for 5 seconds and then does the crash/exit itself.
-			// In expected operation, the first m will wait until the last m has received the SIGQUIT,
+			// The faulting m is crashing first so it is the faulting thread in the core dump (see issue #63277):
+			// in expected operation, the first m will wait until the last m has received the SIGQUIT,
 			// and then run crash/exit and the process is gone.
 			// However, if it spends more than 5 seconds to send SIGQUIT to all ms,
 			// any of ms may crash/exit the process after waiting for 5 seconds.
 			print("\n-----\n\n")
 			raiseproc(_SIGQUIT)
 		}
-		// fix #63277
 		if isCrashThread {
 			i := 0
-			for (crashing < mcount()-int32(extraMLength.Load()) && i < 10) {
+			for (crashing.Load() < mcount()-int32(extraMLength.Load())) && i < 10 {
 				i++
 				usleep(500 * 1000)
 			}