[dev.cc] all: merge default (8d42099cdc23) into dev.ccdev.cc

TBR=austin
CC=golang-codereviews
https://codereview.appspot.com/178700044

2 files changed, 24 insertions, 7 deletions

diff --git a/src/runtime/proc1.go b/src/runtime/proc1.go
index 81b211d0d..aeded0e77 100644
--- a/src/runtime/proc1.go
+++ b/src/runtime/proc1.go
@@ -371,6 +371,11 @@ func casgstatus(gp *g, oldval, newval uint32) {
 	// loop if gp->atomicstatus is in a scan state giving
 	// GC time to finish and change the state to oldval.
 	for !cas(&gp.atomicstatus, oldval, newval) {
+		if oldval == _Gwaiting && gp.atomicstatus == _Grunnable {
+			systemstack(func() {
+				gothrow("casgstatus: waiting for Gwaiting but is Grunnable")
+			})
+		}
 		// Help GC if needed.
 		if gp.preemptscan && !gp.gcworkdone && (oldval == _Grunning || oldval == _Gsyscall) {
 			gp.preemptscan = false
@@ -381,6 +386,24 @@ func casgstatus(gp *g, oldval, newval uint32) {
 	}
 }
 
+// casgstatus(gp, oldstatus, Gcopystack), assuming oldstatus is Gwaiting or Grunnable.
+// Returns old status. Cannot call casgstatus directly, because we are racing with an
+// async wakeup that might come in from netpoll. If we see Gwaiting from the readgstatus,
+// it might have become Grunnable by the time we get to the cas. If we called casgstatus,
+// it would loop waiting for the status to go back to Gwaiting, which it never will.
+//go:nosplit
+func casgcopystack(gp *g) uint32 {
+	for {
+		oldstatus := readgstatus(gp) &^ _Gscan
+		if oldstatus != _Gwaiting && oldstatus != _Grunnable {
+			gothrow("copystack: bad status, not Gwaiting or Grunnable")
+		}
+		if cas(&gp.atomicstatus, oldstatus, _Gcopystack) {
+			return oldstatus
+		}
+	}
+}
+
 // stopg ensures that gp is stopped at a GC safe point where its stack can be scanned
 // or in the context of a moving collector the pointers can be flipped from pointing
 // to old object to pointing to new objects.
diff --git a/src/runtime/stack1.go b/src/runtime/stack1.go
index ad83e5895..1fd61ce1a 100644
--- a/src/runtime/stack1.go
+++ b/src/runtime/stack1.go
@@ -563,13 +563,7 @@ func copystack(gp *g, newsize uintptr) {
 	}
 	memmove(unsafe.Pointer(new.hi-used), unsafe.Pointer(old.hi-used), used)
 
-	oldstatus := readgstatus(gp)
-	oldstatus &^= _Gscan
-	if oldstatus == _Gwaiting || oldstatus == _Grunnable {
-		casgstatus(gp, oldstatus, _Gcopystack) // oldstatus is Gwaiting or Grunnable
-	} else {
-		gothrow("copystack: bad status, not Gwaiting or Grunnable")
-	}
+	oldstatus := casgcopystack(gp) // cas from Gwaiting or Grunnable to Gcopystack, return old status
 
 	// Swap out old stack for new one
 	gp.stack = new