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>

src/runtime/proc.go

@@ -802,14 +802,6 @@ func casgstatus(gp *g, oldval, newval uint32) {
 		if oldval == _Gwaiting && gp.atomicstatus == _Grunnable {
 			throw("casgstatus: waiting for Gwaiting but is Grunnable")
 		}
-		// Help GC if needed.
-		// if gp.preemptscan && !gp.gcworkdone && (oldval == _Grunning || oldval == _Gsyscall) {
-		// 	gp.preemptscan = false
-		// 	systemstack(func() {
-		// 		gcphasework(gp)
-		// 	})
-		// }
-		// But meanwhile just yield.
 		if i == 0 {
 			nextYield = nanotime() + yieldDelay
 		}
@@ -867,111 +859,6 @@ func casGFromPreempted(gp *g, old, new uint32) bool {
 	return atomic.Cas(&gp.atomicstatus, _Gpreempted, _Gwaiting)
 }
 
-// scang blocks until gp's stack has been scanned.
-// It might be scanned by scang or it might be scanned by the goroutine itself.
-// Either way, the stack scan has completed when scang returns.
-func scang(gp *g, gcw *gcWork) {
-	// Invariant; we (the caller, markroot for a specific goroutine) own gp.gcscandone.
-	// Nothing is racing with us now, but gcscandone might be set to true left over
-	// from an earlier round of stack scanning (we scan twice per GC).
-	// We use gcscandone to record whether the scan has been done during this round.
-
-	gp.gcscandone = false
-
-	// See https://golang.org/cl/21503 for justification of the yield delay.
-	const yieldDelay = 10 * 1000
-	var nextYield int64
-
-	// Endeavor to get gcscandone set to true,
-	// either by doing the stack scan ourselves or by coercing gp to scan itself.
-	// gp.gcscandone can transition from false to true when we're not looking
-	// (if we asked for preemption), so any time we lock the status using
-	// castogscanstatus we have to double-check that the scan is still not done.
-loop:
-	for i := 0; !gp.gcscandone; i++ {
-		switch s := readgstatus(gp); s {
-		default:
-			dumpgstatus(gp)
-			throw("stopg: invalid status")
-
-		case _Gdead:
-			// No stack.
-			gp.gcscandone = true
-			break loop
-
-		case _Gcopystack:
-		// Stack being switched. Go around again.
-
-		case _Grunnable, _Gsyscall, _Gwaiting:
-			// Claim goroutine by setting scan bit.
-			// Racing with execution or readying of gp.
-			// The scan bit keeps them from running
-			// the goroutine until we're done.
-			if castogscanstatus(gp, s, s|_Gscan) {
-				if !gp.gcscandone {
-					scanstack(gp, gcw)
-					gp.gcscandone = true
-				}
-				restartg(gp)
-				break loop
-			}
-
-		case _Gscanwaiting:
-		// newstack is doing a scan for us right now. Wait.
-
-		case _Grunning:
-			// Goroutine running. Try to preempt execution so it can scan itself.
-			// The preemption handler (in newstack) does the actual scan.
-
-			// Optimization: if there is already a pending preemption request
-			// (from the previous loop iteration), don't bother with the atomics.
-			if gp.preemptscan && gp.preempt && gp.stackguard0 == stackPreempt {
-				break
-			}
-
-			// Ask for preemption and self scan.
-			if castogscanstatus(gp, _Grunning, _Gscanrunning) {
-				if !gp.gcscandone {
-					gp.preemptscan = true
-					gp.preempt = true
-					gp.stackguard0 = stackPreempt
-				}
-				casfrom_Gscanstatus(gp, _Gscanrunning, _Grunning)
-			}
-		}
-
-		if i == 0 {
-			nextYield = nanotime() + yieldDelay
-		}
-		if nanotime() < nextYield {
-			procyield(10)
-		} else {
-			osyield()
-			nextYield = nanotime() + yieldDelay/2
-		}
-	}
-
-	gp.preemptscan = false // cancel scan request if no longer needed
-}
-
-// The GC requests that this routine be moved from a scanmumble state to a mumble state.
-func restartg(gp *g) {
-	s := readgstatus(gp)
-	switch s {
-	default:
-		dumpgstatus(gp)
-		throw("restartg: unexpected status")
-
-	case _Gdead:
-	// ok
-
-	case _Gscanrunnable,
-		_Gscanwaiting,
-		_Gscansyscall:
-		casfrom_Gscanstatus(gp, s, s&^_Gscan)
-	}
-}
-
 // stopTheWorld stops all P's from executing goroutines, interrupting
 // all goroutines at GC safe points and records reason as the reason
 // for the stop. On return, only the current goroutine's P is running.

src/runtime/runtime2.go

@@ -421,7 +421,6 @@ type g struct {
 	preempt        bool       // preemption signal, duplicates stackguard0 = stackpreempt
 	preemptStop    bool       // transition to _Gpreempted on preemption; otherwise, just deschedule
 	paniconfault   bool       // panic (instead of crash) on unexpected fault address
-	preemptscan    bool       // preempted g does scan for gc
 	gcscandone     bool       // g has scanned stack; protected by _Gscan bit in status
 	gcscanvalid    bool       // false at start of gc cycle, true if G has not run since last scan; TODO: remove?
 	throwsplit     bool       // must not split stack

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
 	}