runtime: clean up canpanic

* The gp argument to canpanic is always equivalent to getg(), so no need
to pass it at all.
* gp must not be nil or _g_.m would have crashed, so no need to check
for nil.
* Use acquirem to better reason about preemption.

Change-Id: Ic7dc8dc1e56ab4c1644965f6aeba16807cdb2df4
Reviewed-on: https://go-review.googlesource.com/c/go/+/418575
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>

src/runtime/os_js.go

@@ -50,7 +50,7 @@ const _SIGSEGV = 0xb
 
 func sigpanic() {
 	g := getg()
-	if !canpanic(g) {
+	if !canpanic() {
 		throw("unexpected signal during runtime execution")
 	}
 

src/runtime/os_plan9.go

@@ -76,7 +76,7 @@ func os_sigpipe() {
 
 func sigpanic() {
 	g := getg()
-	if !canpanic(g) {
+	if !canpanic() {
 		throw("unexpected signal during runtime execution")
 	}
 

src/runtime/panic.go

@@ -1290,29 +1290,32 @@ func dopanic_m(gp *g, pc, sp uintptr) bool {
 // panicking.
 //
 //go:nosplit
-func canpanic(gp *g) bool {
-	// Note that g is m->gsignal, different from gp.
-	// Note also that g->m can change at preemption, so m can go stale
-	// if this function ever makes a function call.
-	_g_ := getg()
-	mp := _g_.m
+func canpanic() bool {
+	gp := getg()
+	mp := acquirem()
 
 	// Is it okay for gp to panic instead of crashing the program?
 	// Yes, as long as it is running Go code, not runtime code,
 	// and not stuck in a system call.
-	if gp == nil || gp != mp.curg {
+	if gp != mp.curg {
+		releasem(mp)
 		return false
 	}
-	if mp.locks != 0 || mp.mallocing != 0 || mp.throwing != throwTypeNone || mp.preemptoff != "" || mp.dying != 0 {
+	// N.B. mp.locks != 1 instead of 0 to account for acquirem.
+	if mp.locks != 1 || mp.mallocing != 0 || mp.throwing != throwTypeNone || mp.preemptoff != "" || mp.dying != 0 {
+		releasem(mp)
 		return false
 	}
 	status := readgstatus(gp)
 	if status&^_Gscan != _Grunning || gp.syscallsp != 0 {
+		releasem(mp)
 		return false
 	}
 	if GOOS == "windows" && mp.libcallsp != 0 {
+		releasem(mp)
 		return false
 	}
+	releasem(mp)
 	return true
 }
 

src/runtime/signal_unix.go

@@ -815,7 +815,7 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 //go:linkname sigpanic
 func sigpanic() {
 	g := getg()
-	if !canpanic(g) {
+	if !canpanic() {
 		throw("unexpected signal during runtime execution")
 	}
 

src/runtime/signal_windows.go

@@ -245,7 +245,7 @@ func winthrow(info *exceptionrecord, r *context, gp *g) {
 
 func sigpanic() {
 	g := getg()
-	if !canpanic(g) {
+	if !canpanic() {
 		throw("unexpected signal during runtime execution")
 	}