runtime: only increment extraMInUse when actually in use

Currently lockextra always increments extraMInUse, even if the M won't
be used (or doesn't even exist), such as in addExtraM. addExtraM fails
to decrement extraMInUse, so it stays elevated forever.

Fix this bug and simplify the model by moving extraMInUse out of
lockextra to getExtraM, where we know the M will actually be used.

While we're here, remove the nilokay argument from getExtraM, which is
always false.

Fixes #60540.

Change-Id: I7a5d97456b3bc6ea1baeb06b5b2975e3b8dd96a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/499677
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/proc.go

@@ -1985,10 +1985,10 @@ func needm(signal bool) {
 	sigsave(&sigmask)
 	sigblock(false)
 
-	// nilokay=false is safe here because of the invariant above,
+	// getExtraM is safe here because of the invariant above,
 	// that the extra list always contains or will soon contain
 	// at least one m.
-	mp, last := getExtraM(false)
+	mp, last := getExtraM()
 
 	// Set needextram when we've just emptied the list,
 	// so that the eventual call into cgocallbackg will
@@ -2260,7 +2260,6 @@ func lockextra(nilokay bool) *m {
 			continue
 		}
 		if extraM.CompareAndSwap(old, locked) {
-			extraMInUse.Add(1)
 			return (*m)(unsafe.Pointer(old))
 		}
 		osyield_no_g()
@@ -2277,13 +2276,13 @@ func unlockextra(mp *m, delta int32) {
 // Return an M from the extra M list. Returns last == true if the list becomes
 // empty because of this call.
 //
+// Spins waiting for an extra M, so caller must ensure that the list always
+// contains or will soon contain at least one M.
+//
 //go:nosplit
-func getExtraM(nilokay bool) (mp *m, last bool) {
-	mp = lockextra(nilokay)
-	if mp == nil {
-		unlockextra(nil, 0)
-		return nil, true
-	}
+func getExtraM() (mp *m, last bool) {
+	mp = lockextra(false)
+	extraMInUse.Add(1)
 	unlockextra(mp.schedlink.ptr(), -1)
 	return mp, mp.schedlink.ptr() == nil
 }

src/runtime/testdata/testprogcgo/callback.go

@@ -32,6 +32,8 @@ import (
 	"fmt"
 	"os"
 	"runtime"
+	"sync/atomic"
+	_ "unsafe" // for go:linkname
 )
 
 func init() {
@@ -40,6 +42,11 @@ func init() {
 
 //export go_callback
 func go_callback() {
+	if e := extraMInUse.Load(); e == 0 {
+		fmt.Printf("in callback extraMInUse got %d want >0\n", e)
+		os.Exit(1)
+	}
+
 	runtime.GC()
 	grow()
 	runtime.GC()
@@ -69,6 +76,12 @@ func CgoCallbackGC() {
 	if os.Getenv("RUNTIME_TEST_SHORT") != "" {
 		P = 10
 	}
+
+	if e := extraMInUse.Load(); e != 0 {
+		fmt.Printf("before testing extraMInUse got %d want 0\n", e)
+		os.Exit(1)
+	}
+
 	done := make(chan bool)
 	// allocate a bunch of stack frames and spray them with pointers
 	for i := 0; i < P; i++ {
@@ -90,5 +103,14 @@ func CgoCallbackGC() {
 	for i := 0; i < P; i++ {
 		<-done
 	}
+
+	if e := extraMInUse.Load(); e != 0 {
+		fmt.Printf("after testing extraMInUse got %d want 0\n", e)
+		os.Exit(1)
+	}
+
 	fmt.Printf("OK\n")
 }
+
+//go:linkname extraMInUse runtime.extraMInUse
+var extraMInUse atomic.Uint32