runtime: make NumGoroutine wait for system goroutines to register

    
    In libgo system goroutines register themselves after they start.
    That means that there is a small race between the goroutine being
    seen by the scheduler and the scheduler knowing that the goroutine
    is a system goroutine. That in turn means that runtime.NumGoroutines
    can overestimate the number of goroutines at times.
    
    This patch fixes the overestimate by counting the number of system
    goroutines waiting to start, and pausing NumGoroutines until those
    goroutines have all registered.
    
    This is kind of a lot of mechanism for this not very important
    problem, but I couldn't think of a better approach.
    
    The test for this is TestNumGoroutine in runtime/proc_test.go.
    The test is not currently run, but it will be soon.
    
    Reviewed-on: https://go-review.googlesource.com/46457

From-SVN: r249565

gcc/go/gofrontend/MERGE

-54d83c2d67c35ad4f622936d2fbf81c17354fff9
+681c8a7b0a9d52c0b81e7a4b1c55fe65ed889573
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/debug.go

@@ -54,6 +54,7 @@ func NumCgoCall() int64 {
 
 // NumGoroutine returns the number of goroutines that currently exist.
 func NumGoroutine() int {
+	waitForSystemGoroutines()
 	return int(gcount())
 }
 

libgo/go/runtime/mfinal.go

@@ -106,6 +106,7 @@ var (
 func createfing() {
 	// start the finalizer goroutine exactly once
 	if fingCreate == 0 && atomic.Cas(&fingCreate, 0, 1) {
+		expectSystemGoroutine()
 		go runfinq()
 	}
 }

libgo/go/runtime/mgc.go

@@ -209,6 +209,7 @@ func readgogc() int32 {
 // It kicks off the background sweeper goroutine and enables GC.
 func gcenable() {
 	c := make(chan int, 1)
+	expectSystemGoroutine()
 	go bgsweep(c)
 	<-c
 	memstats.enablegc = true // now that runtime is initialized, GC is okay
@@ -1399,6 +1400,7 @@ func gcBgMarkStartWorkers() {
 			break
 		}
 		if p.gcBgMarkWorker == 0 {
+			expectSystemGoroutine()
 			go gcBgMarkWorker(p)
 			notetsleepg(&work.bgMarkReady, -1)
 			noteclear(&work.bgMarkReady)

libgo/go/runtime/proc.go

@@ -233,6 +233,7 @@ func os_beforeExit() {
 
 // start forcegc helper goroutine
 func init() {
+	expectSystemGoroutine()
 	go forcegchelper()
 }
 
@@ -2728,6 +2729,28 @@ func newproc(fn uintptr, arg unsafe.Pointer) *g {
 	return newg
 }
 
+// expectedSystemGoroutines counts the number of goroutines expected
+// to mark themselves as system goroutines. After they mark themselves
+// by calling setSystemGoroutine, this is decremented. NumGoroutines
+// uses this to wait for all system goroutines to mark themselves
+// before it counts them.
+var expectedSystemGoroutines uint32
+
+// expectSystemGoroutine is called when starting a goroutine that will
+// call setSystemGoroutine. It increments expectedSystemGoroutines.
+func expectSystemGoroutine() {
+	atomic.Xadd(&expectedSystemGoroutines, +1)
+}
+
+// waitForSystemGoroutines waits for all currently expected system
+// goroutines to register themselves.
+func waitForSystemGoroutines() {
+	for atomic.Load(&expectedSystemGoroutines) > 0 {
+		Gosched()
+		osyield()
+	}
+}
+
 // setSystemGoroutine marks this goroutine as a "system goroutine".
 // In the gc toolchain this is done by comparing startpc to a list of
 // saved special PCs. In gccgo that approach does not work as startpc
@@ -2738,6 +2761,7 @@ func newproc(fn uintptr, arg unsafe.Pointer) *g {
 func setSystemGoroutine() {
 	getg().isSystemGoroutine = true
 	atomic.Xadd(&sched.ngsys, +1)
+	atomic.Xadd(&expectedSystemGoroutines, -1)
 }
 
 // Put on gfree list.

libgo/go/runtime/time.go

@@ -113,6 +113,7 @@ func addtimerLocked(t *timer) {
 	}
 	if !timers.created {
 		timers.created = true
+		expectSystemGoroutine()
 		go timerproc()
 	}
 }