1 files changed, 21 insertions, 14 deletions

diff --git a/src/runtime/stack.c b/src/runtime/stack.c
index e402691f4..e06e48a93 100644
--- a/src/runtime/stack.c
+++ b/src/runtime/stack.c
@@ -587,13 +587,13 @@ adjustsudogs(G *gp, AdjustInfo *adjinfo)
 }
 
 // Copies gp's stack to a new stack of a different size.
+// Caller must have changed gp status to Gcopystack.
 static void
 copystack(G *gp, uintptr newsize)
 {
 	Stack old, new;
 	uintptr used;
 	AdjustInfo adjinfo;
-	uint32 oldstatus;
 	bool (*cb)(Stkframe*, void*);
 	byte *p, *ep;
 
@@ -637,20 +637,11 @@ copystack(G *gp, uintptr newsize)
 	}
 	runtime·memmove((byte*)new.hi - used, (byte*)old.hi - used, used);
 
-	oldstatus = runtime·readgstatus(gp);
-	oldstatus &= ~Gscan;
-	if(oldstatus == Gwaiting || oldstatus == Grunnable)
-		runtime·casgstatus(gp, oldstatus, Gcopystack); // oldstatus is Gwaiting or Grunnable
-	else
-		runtime·throw("copystack: bad status, not Gwaiting or Grunnable");
-
 	// Swap out old stack for new one
 	gp->stack = new;
 	gp->stackguard0 = new.lo + StackGuard; // NOTE: might clobber a preempt request
 	gp->sched.sp = new.hi - used;
 
-	runtime·casgstatus(gp, Gcopystack, oldstatus); // oldstatus is Gwaiting or Grunnable
-
 	// free old stack
 	if(StackPoisonCopy) {
 		p = (byte*)old.lo;
@@ -700,6 +691,7 @@ void
 runtime·newstack(void)
 {
 	int32 oldsize, newsize;
+	uint32 oldstatus;
 	uintptr sp;
 	G *gp;
 	Gobuf morebuf;
@@ -789,12 +781,15 @@ runtime·newstack(void)
 		runtime·throw("stack overflow");
 	}
 
-	// Note that the concurrent GC might be scanning the stack as we try to replace it.
-	// copystack takes care of the appropriate coordination with the stack scanner.
+	oldstatus = runtime·readgstatus(gp);
+	oldstatus &= ~Gscan;
+	runtime·casgstatus(gp, oldstatus, Gcopystack); // oldstatus is Gwaiting or Grunnable
+	// The concurrent GC will not scan the stack while we are doing the copy since
+	// the gp is in a Gcopystack status.
 	copystack(gp, newsize);
 	if(StackDebug >= 1)
 		runtime·printf("stack grow done\n");
-	runtime·casgstatus(gp, Gwaiting, Grunning);
+	runtime·casgstatus(gp, Gcopystack, Grunning);
 	runtime·gogo(&gp->sched);
 }
 
@@ -825,6 +820,7 @@ void
 runtime·shrinkstack(G *gp)
 {
 	uintptr used, oldsize, newsize;
+	uint32 oldstatus;
 
 	if(runtime·readgstatus(gp) == Gdead) {
 		if(gp->stack.lo != 0) {
@@ -858,8 +854,19 @@ runtime·shrinkstack(G *gp)
 #endif
 	if(StackDebug > 0)
 		runtime·printf("shrinking stack %D->%D\n", (uint64)oldsize, (uint64)newsize);
+	// This is being done in a Gscan state and was initiated by the GC so no need to move to
+	// the Gcopystate.
+	// The world is stopped, so the goroutine must be Gwaiting or Grunnable,
+	// and what it is is not changing underfoot.
+
+	oldstatus = runtime·readgstatus(gp);
+	oldstatus &= ~Gscan;
+	if(oldstatus != Gwaiting && oldstatus != Grunnable)
+		runtime·throw("status is not Gwaiting or Grunnable");
+	runtime·casgstatus(gp, oldstatus, Gcopystack);
 	copystack(gp, newsize);
-}
+	runtime·casgstatus(gp, Gcopystack, oldstatus);
+ }
 
 // Do any delayed stack freeing that was queued up during GC.
 void