runtime: remove old stack scanning code

This removes scang and preemptscan, since the stack scanning code now
uses suspendG.

For #10958, #24543.

Change-Id: Ic868bf5d6dcce40662a82cb27bb996cb74d0720e
Reviewed-on: https://go-review.googlesource.com/c/go/+/201138
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 1 insertions, 28 deletions

diff --git a/src/runtime/stack.go b/src/runtime/stack.go
index 3b92c89ff0..2c2a88e6e1 100644
--- a/src/runtime/stack.go
+++ b/src/runtime/stack.go
@@ -916,7 +916,7 @@ func round2(x int32) int32 {
 // Stack growth is multiplicative, for constant amortized cost.
 //
 // g->atomicstatus will be Grunning or Gscanrunning upon entry.
-// If the GC is trying to stop this g then it will set preemptscan to true.
+// If the scheduler is trying to stop this g, then it will set preemptStop.
 //
 // This must be nowritebarrierrec because it can be called as part of
 // stack growth from other nowritebarrierrec functions, but the
@@ -1022,34 +1022,7 @@ func newstack() {
 			preemptPark(gp) // never returns
 		}
 
-		// Synchronize with scang.
-		casgstatus(gp, _Grunning, _Gwaiting)
-		if gp.preemptscan {
-			for !castogscanstatus(gp, _Gwaiting, _Gscanwaiting) {
-				// Likely to be racing with the GC as
-				// it sees a _Gwaiting and does the
-				// stack scan. If so, gcworkdone will
-				// be set and gcphasework will simply
-				// return.
-			}
-			if !gp.gcscandone {
-				// gcw is safe because we're on the
-				// system stack.
-				gcw := &gp.m.p.ptr().gcw
-				scanstack(gp, gcw)
-				gp.gcscandone = true
-			}
-			gp.preemptscan = false
-			gp.preempt = false
-			casfrom_Gscanstatus(gp, _Gscanwaiting, _Gwaiting)
-			// This clears gcscanvalid.
-			casgstatus(gp, _Gwaiting, _Grunning)
-			gp.stackguard0 = gp.stack.lo + _StackGuard
-			gogo(&gp.sched) // never return
-		}
-
 		// Act like goroutine called runtime.Gosched.
-		casgstatus(gp, _Gwaiting, _Grunning)
 		gopreempt_m(gp) // never return
 	}