runtime: add world-stopped assertions

Stopping the world is an implicit lock for many operations, so we should
assert the world is stopped in functions that require it.

This is enabled along with the rest of lock ranking, though it is a bit
orthogonal and likely cheap enough to enable all the time should we
choose.

Requiring a lock _or_ world stop is common, so that can be expressed as
well.

Updates #40677

Change-Id: If0a58544f4251d367f73c4120c9d39974c6cd091
Reviewed-on: https://go-review.googlesource.com/c/go/+/248577
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Trust: Michael Pratt <mpratt@google.com>

src/runtime/heapdump.go

@@ -431,6 +431,9 @@ func finq_callback(fn *funcval, obj unsafe.Pointer, nret uintptr, fint *_type, o
 }
 
 func dumproots() {
+	// To protect mheap_.allspans.
+	assertWorldStopped()
+
 	// TODO(mwhudson): dump datamask etc from all objects
 	// data segment
 	dumpint(tagData)
@@ -468,6 +471,9 @@ func dumproots() {
 var freemark [_PageSize / 8]bool
 
 func dumpobjs() {
+	// To protect mheap_.allspans.
+	assertWorldStopped()
+
 	for _, s := range mheap_.allspans {
 		if s.state.get() != mSpanInUse {
 			continue
@@ -552,6 +558,8 @@ func dumpms() {
 
 //go:systemstack
 func dumpmemstats(m *MemStats) {
+	assertWorldStopped()
+
 	// These ints should be identical to the exported
 	// MemStats structure and should be ordered the same
 	// way too.
@@ -634,6 +642,9 @@ func dumpmemprof_callback(b *bucket, nstk uintptr, pstk *uintptr, size, allocs,
 }
 
 func dumpmemprof() {
+	// To protect mheap_.allspans.
+	assertWorldStopped()
+
 	iterate_memprof(dumpmemprof_callback)
 	for _, s := range mheap_.allspans {
 		if s.state.get() != mSpanInUse {
@@ -655,6 +666,8 @@ func dumpmemprof() {
 var dumphdr = []byte("go1.7 heap dump\n")
 
 func mdump(m *MemStats) {
+	assertWorldStopped()
+
 	// make sure we're done sweeping
 	for _, s := range mheap_.allspans {
 		if s.state.get() == mSpanInUse {
@@ -676,6 +689,8 @@ func mdump(m *MemStats) {
 }
 
 func writeheapdump_m(fd uintptr, m *MemStats) {
+	assertWorldStopped()
+
 	_g_ := getg()
 	casgstatus(_g_.m.curg, _Grunning, _Gwaiting)
 	_g_.waitreason = waitReasonDumpingHeap

src/runtime/lockrank_off.go

@@ -46,3 +46,19 @@ func assertLockHeld(l *mutex) {
 //go:nosplit
 func assertRankHeld(r lockRank) {
 }
+
+//go:nosplit
+func worldStopped() {
+}
+
+//go:nosplit
+func worldStarted() {
+}
+
+//go:nosplit
+func assertWorldStopped() {
+}
+
+//go:nosplit
+func assertWorldStoppedOrLockHeld(l *mutex) {
+}

src/runtime/lockrank_on.go

@@ -7,9 +7,14 @@
 package runtime
 
 import (
+	"runtime/internal/atomic"
 	"unsafe"
 )
 
+// worldIsStopped is accessed atomically to track world-stops. 1 == world
+// stopped.
+var worldIsStopped uint32
+
 // lockRankStruct is embedded in mutex
 type lockRankStruct struct {
 	// static lock ranking of the lock
@@ -284,3 +289,77 @@ func assertRankHeld(r lockRank) {
 		throw("not holding required lock!")
 	})
 }
+
+// worldStopped notes that the world is stopped.
+//
+// Caller must hold worldsema.
+//
+// nosplit to ensure it can be called in as many contexts as possible.
+//go:nosplit
+func worldStopped() {
+	if stopped := atomic.Xadd(&worldIsStopped, 1); stopped != 1 {
+		print("world stop count=", stopped, "\n")
+		throw("recursive world stop")
+	}
+}
+
+// worldStarted that the world is starting.
+//
+// Caller must hold worldsema.
+//
+// nosplit to ensure it can be called in as many contexts as possible.
+//go:nosplit
+func worldStarted() {
+	if stopped := atomic.Xadd(&worldIsStopped, -1); stopped != 0 {
+		print("world stop count=", stopped, "\n")
+		throw("released non-stopped world stop")
+	}
+}
+
+// nosplit to ensure it can be called in as many contexts as possible.
+//go:nosplit
+func checkWorldStopped() bool {
+	stopped := atomic.Load(&worldIsStopped)
+	if stopped > 1 {
+		print("inconsistent world stop count=", stopped, "\n")
+		throw("inconsistent world stop count")
+	}
+
+	return stopped == 1
+}
+
+// assertWorldStopped throws if the world is not stopped. It does not check
+// which M stopped the world.
+//
+// nosplit to ensure it can be called in as many contexts as possible.
+//go:nosplit
+func assertWorldStopped() {
+	if checkWorldStopped() {
+		return
+	}
+
+	throw("world not stopped")
+}
+
+// assertWorldStoppedOrLockHeld throws if the world is not stopped and the
+// passed lock is not held.
+//
+// nosplit to ensure it can be called in as many contexts as possible.
+//go:nosplit
+func assertWorldStoppedOrLockHeld(l *mutex) {
+	if checkWorldStopped() {
+		return
+	}
+
+	gp := getg()
+	systemstack(func() {
+		held := checkLockHeld(gp, l)
+		if !held {
+			printlock()
+			print("caller requires world stop or lock ", l, " (rank ", l.rank.String(), "), holding:\n")
+			println("<no world stop>")
+			printHeldLocks(gp)
+			throw("no world stop or required lock!")
+		}
+	})
+}

src/runtime/mcheckmark.go

@@ -34,6 +34,8 @@ var useCheckmark = false
 //
 // The world must be stopped.
 func startCheckmarks() {
+	assertWorldStopped()
+
 	// Clear all checkmarks.
 	for _, ai := range mheap_.allArenas {
 		arena := mheap_.arenas[ai.l1()][ai.l2()]

src/runtime/mgc.go

@@ -2164,6 +2164,8 @@ func gcMark(start_time int64) {
 //
 //go:systemstack
 func gcSweep(mode gcMode) {
+	assertWorldStopped()
+
 	if gcphase != _GCoff {
 		throw("gcSweep being done but phase is not GCoff")
 	}

src/runtime/mgcmark.go

@@ -54,6 +54,8 @@ const (
 //
 // The world must be stopped.
 func gcMarkRootPrepare() {
+	assertWorldStopped()
+
 	work.nFlushCacheRoots = 0
 
 	// Compute how many data and BSS root blocks there are.
@@ -1535,6 +1537,8 @@ func gcmarknewobject(span *mspan, obj, size, scanSize uintptr) {
 //
 // The world must be stopped.
 func gcMarkTinyAllocs() {
+	assertWorldStopped()
+
 	for _, p := range allp {
 		c := p.mcache
 		if c == nil || c.tiny == 0 {

src/runtime/mgcsweep.go

@@ -123,6 +123,8 @@ func (h *mheap) nextSpanForSweep() *mspan {
 //
 //go:nowritebarrier
 func finishsweep_m() {
+	assertWorldStopped()
+
 	// Sweeping must be complete before marking commences, so
 	// sweep any unswept spans. If this is a concurrent GC, there
 	// shouldn't be any spans left to sweep, so this should finish

src/runtime/mstats.go

@@ -601,6 +601,8 @@ func readGCStats_m(pauses *[]uint64) {
 //
 //go:nowritebarrier
 func updatememstats() {
+	assertWorldStopped()
+
 	// Flush mcaches to mcentral before doing anything else.
 	//
 	// Flushing to the mcentral may in general cause stats to
@@ -706,6 +708,8 @@ func updatememstats() {
 //
 //go:nowritebarrier
 func flushmcache(i int) {
+	assertWorldStopped()
+
 	p := allp[i]
 	c := p.mcache
 	if c == nil {
@@ -721,6 +725,8 @@ func flushmcache(i int) {
 //
 //go:nowritebarrier
 func flushallmcaches() {
+	assertWorldStopped()
+
 	for i := 0; i < int(gomaxprocs); i++ {
 		flushmcache(i)
 	}
@@ -876,10 +882,10 @@ func (m *consistentHeapStats) release(c *mcache) {
 // unsafeRead aggregates the delta for this shard into out.
 //
 // Unsafe because it does so without any synchronization. The
-// only safe time to call this is if the world is stopped or
-// we're freezing the world or going down anyway (and we just
-// want _some_ estimate).
+// world must be stopped.
 func (m *consistentHeapStats) unsafeRead(out *heapStatsDelta) {
+	assertWorldStopped()
+
 	for i := range m.stats {
 		out.merge(&m.stats[i])
 	}
@@ -890,6 +896,8 @@ func (m *consistentHeapStats) unsafeRead(out *heapStatsDelta) {
 // Unsafe because the world must be stopped and values should
 // be donated elsewhere before clearing.
 func (m *consistentHeapStats) unsafeClear() {
+	assertWorldStopped()
+
 	for i := range m.stats {
 		m.stats[i] = heapStatsDelta{}
 	}

src/runtime/proc.go

@@ -587,6 +587,9 @@ func schedinit() {
 
 	sched.maxmcount = 10000
 
+	// The world starts stopped.
+	worldStopped()
+
 	moduledataverify()
 	stackinit()
 	mallocinit()
@@ -617,6 +620,9 @@ func schedinit() {
 	}
 	unlock(&sched.lock)
 
+	// World is effectively started now, as P's can run.
+	worldStarted()
+
 	// For cgocheck > 1, we turn on the write barrier at all times
 	// and check all pointer writes. We can't do this until after
 	// procresize because the write barrier needs a P.
@@ -1082,9 +1088,13 @@ func stopTheWorldWithSema() {
 	if bad != "" {
 		throw(bad)
 	}
+
+	worldStopped()
 }
 
 func startTheWorldWithSema(emitTraceEvent bool) int64 {
+	assertWorldStopped()
+
 	mp := acquirem() // disable preemption because it can be holding p in a local var
 	if netpollinited() {
 		list := netpoll(0) // non-blocking
@@ -1105,6 +1115,8 @@ func startTheWorldWithSema(emitTraceEvent bool) int64 {
 	}
 	unlock(&sched.lock)
 
+	worldStarted()
+
 	for p1 != nil {
 		p := p1
 		p1 = p1.link.ptr()
@@ -4539,6 +4551,7 @@ func (pp *p) init(id int32) {
 // sched.lock must be held and the world must be stopped.
 func (pp *p) destroy() {
 	assertLockHeld(&sched.lock)
+	assertWorldStopped()
 
 	// Move all runnable goroutines to the global queue
 	for pp.runqhead != pp.runqtail {
@@ -4629,6 +4642,7 @@ func (pp *p) destroy() {
 // Returns list of Ps with local work, they need to be scheduled by the caller.
 func procresize(nprocs int32) *p {
 	assertLockHeld(&sched.lock)
+	assertWorldStopped()
 
 	old := gomaxprocs
 	if old < 0 || nprocs <= 0 {