runtime: initialize extra M for cgo during mstart

Previously the extra m needed for cgo callbacks was created on the
first callback. This works for cgo, however the cgocallback mechanism
is also borrowed by badsignal which can run before any cgo calls are
made.

Now we initialize the extra M at runtime startup before any signal
handlers are registered, so badsignal cannot be called until the
extra M is ready.

Updates #10207.

Change-Id: Iddda2c80db6dc52d8b60e2b269670fbaa704c7b3
Reviewed-on: https://go-review.googlesource.com/7978Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>

src/runtime/cgocall.go

@@ -101,11 +101,6 @@ func cgocall_errno(fn, arg unsafe.Pointer) int32 {
 		racereleasemerge(unsafe.Pointer(&racecgosync))
 	}
 
-	// Create an extra M for callbacks on threads not created by Go on first cgo call.
-	if needextram == 1 && cas(&needextram, 1, 0) {
-		systemstack(newextram)
-	}
-
 	/*
 	 * Lock g to m to ensure we stay on the same stack if we do a
 	 * cgo callback. Add entry to defer stack in case of panic.

src/runtime/proc1.go

@@ -717,6 +717,11 @@ func mstart1() {
 	// Install signal handlers; after minit so that minit can
 	// prepare the thread to be able to handle the signals.
 	if _g_.m == &m0 {
+		// Create an extra M for callbacks on threads not created by Go.
+		if needextram == 1 {
+			needextram = 0
+			newextram()
+		}
 		initsig()
 	}
 

src/runtime/sigqueue.go

@@ -165,14 +165,5 @@ func signal_ignore(s uint32) {
 // This runs on a foreign stack, without an m or a g.  No stack split.
 //go:nosplit
 func badsignal(sig uintptr) {
-	// Some external libraries, for example, OpenBLAS, create worker threads in
-	// a global constructor. If we're doing cpu profiling, and the SIGPROF signal
-	// comes to one of the foreign threads before we make our first cgo call, the
-	// call to cgocallback below will bring down the whole process.
-	// It's better to miss a few SIGPROF signals than to abort in this case.
-	// See http://golang.org/issue/9456.
-	if _SIGPROF != 0 && sig == _SIGPROF && needextram != 0 {
-		return
-	}
 	cgocallback(unsafe.Pointer(funcPC(sigsend)), noescape(unsafe.Pointer(&sig)), unsafe.Sizeof(sig))
 }