runtime: remove g.gcscanvalid

Currently, gcscanvalid is used to resolve a race between attempts to
scan a stack. Now that there's a clear owner of the stack scan
operation, there's no longer any danger of racing or attempting to
scan a stack more than once, so this CL eliminates gcscanvalid.

I double-checked my reasoning by first adding a throw if gcscanvalid
was set in scanstack and verifying that all.bash still passed.

For #10958, #24543.
Fixes #24363.

Change-Id: I76794a5fcda325ed7cfc2b545e2a839b8b3bc713
Reviewed-on: https://go-review.googlesource.com/c/go/+/201139
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/mgc.go

@@ -2168,8 +2168,7 @@ func gcResetMarkState() {
 	// allgs doesn't change.
 	lock(&allglock)
 	for _, gp := range allgs {
-		gp.gcscandone = false  // set to true in gcphasework
-		gp.gcscanvalid = false // stack has not been scanned
+		gp.gcscandone = false // set to true in gcphasework
 		gp.gcAssistBytes = 0
 	}
 	unlock(&allglock)

src/runtime/mgcmark.go

@@ -125,8 +125,7 @@ func gcMarkRootCheck() {
 fail:
 	println("gp", gp, "goid", gp.goid,
 		"status", readgstatus(gp),
-		"gcscandone", gp.gcscandone,
-		"gcscanvalid", gp.gcscanvalid)
+		"gcscandone", gp.gcscandone)
 	unlock(&allglock) // Avoid self-deadlock with traceback.
 	throw("scan missed a g")
 }
@@ -674,10 +673,6 @@ func gcFlushBgCredit(scanWork int64) {
 //go:nowritebarrier
 //go:systemstack
 func scanstack(gp *g, gcw *gcWork) {
-	if gp.gcscanvalid {
-		return
-	}
-
 	if readgstatus(gp)&_Gscan == 0 {
 		print("runtime:scanstack: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", hex(readgstatus(gp)), "\n")
 		throw("scanstack - bad status")
@@ -817,8 +812,6 @@ func scanstack(gp *g, gcw *gcWork) {
 	if state.buf != nil || state.freeBuf != nil {
 		throw("remaining pointer buffers")
 	}
-
-	gp.gcscanvalid = true
 }
 
 // Scan a stack frame: local variables and function arguments/results.

src/runtime/proc.go

@@ -710,18 +710,6 @@ func readgstatus(gp *g) uint32 {
 	return atomic.Load(&gp.atomicstatus)
 }
 
-// Ownership of gcscanvalid:
-//
-// If gp is running (meaning status == _Grunning or _Grunning|_Gscan),
-// then gp owns gp.gcscanvalid, and other goroutines must not modify it.
-//
-// Otherwise, a second goroutine can lock the scan state by setting _Gscan
-// in the status bit and then modify gcscanvalid, and then unlock the scan state.
-//
-// Note that the first condition implies an exception to the second:
-// if a second goroutine changes gp's status to _Grunning|_Gscan,
-// that second goroutine still does not have the right to modify gcscanvalid.
-
 // The Gscanstatuses are acting like locks and this releases them.
 // If it proves to be a performance hit we should be able to make these
 // simple atomic stores but for now we are going to throw if
@@ -781,17 +769,6 @@ func casgstatus(gp *g, oldval, newval uint32) {
 		})
 	}
 
-	if oldval == _Grunning && gp.gcscanvalid {
-		// If oldvall == _Grunning, then the actual status must be
-		// _Grunning or _Grunning|_Gscan; either way,
-		// we own gp.gcscanvalid, so it's safe to read.
-		// gp.gcscanvalid must not be true when we are running.
-		systemstack(func() {
-			print("runtime: casgstatus ", hex(oldval), "->", hex(newval), " gp.status=", hex(gp.atomicstatus), " gp.gcscanvalid=true\n")
-			throw("casgstatus")
-		})
-	}
-
 	// See https://golang.org/cl/21503 for justification of the yield delay.
 	const yieldDelay = 5 * 1000
 	var nextYield int64
@@ -814,9 +791,6 @@ func casgstatus(gp *g, oldval, newval uint32) {
 			nextYield = nanotime() + yieldDelay/2
 		}
 	}
-	if newval == _Grunning {
-		gp.gcscanvalid = false
-	}
 }
 
 // casgstatus(gp, oldstatus, Gcopystack), assuming oldstatus is Gwaiting or Grunnable.
@@ -1585,7 +1559,6 @@ func oneNewExtraM() {
 	gp.syscallpc = gp.sched.pc
 	gp.syscallsp = gp.sched.sp
 	gp.stktopsp = gp.sched.sp
-	gp.gcscanvalid = true
 	// malg returns status as _Gidle. Change to _Gdead before
 	// adding to allg where GC can see it. We use _Gdead to hide
 	// this from tracebacks and stack scans since it isn't a
@@ -2815,9 +2788,6 @@ func goexit0(gp *g) {
 		gp.gcAssistBytes = 0
 	}
 
-	// Note that gp's stack scan is now "valid" because it has no
-	// stack.
-	gp.gcscanvalid = true
 	dropg()
 
 	if GOARCH == "wasm" { // no threads yet on wasm
@@ -3462,7 +3432,6 @@ func newproc1(fn *funcval, argp unsafe.Pointer, narg int32, callergp *g, callerp
 	if isSystemGoroutine(newg, false) {
 		atomic.Xadd(&sched.ngsys, +1)
 	}
-	newg.gcscanvalid = false
 	casgstatus(newg, _Gdead, _Grunnable)
 
 	if _p_.goidcache == _p_.goidcacheend {

src/runtime/runtime2.go

@@ -422,7 +422,6 @@ type g struct {
 	preemptStop    bool       // transition to _Gpreempted on preemption; otherwise, just deschedule
 	paniconfault   bool       // panic (instead of crash) on unexpected fault address
 	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
 	raceignore     int8       // ignore race detection events
 	sysblocktraced bool       // StartTrace has emitted EvGoInSyscall about this goroutine

src/runtime/sizeof_test.go

@@ -21,7 +21,7 @@ func TestSizeof(t *testing.T) {
 		_32bit uintptr     // size on 32bit platforms
 		_64bit uintptr     // size on 64bit platforms
 	}{
-		{runtime.G{}, 216, 376}, // g, but exported for testing
+		{runtime.G{}, 212, 368}, // g, but exported for testing
 	}
 
 	for _, tt := range tests {