runtime: atomically set span state and use as publication barrier

When everything is working correctly, any pointer the garbage
collector encounters can only point into a fully initialized heap
span, since the span must have been initialized before that pointer
could escape the heap allocator and become visible to the GC.

However, in various cases, we try to be defensive against bad
pointers. In findObject, this is just a sanity check: we never expect
to find a bad pointer, but programming errors can lead to them. In
spanOfHeap, we don't necessarily trust the pointer and we're trying to
check if it really does point to the heap, though it should always
point to something. Conservative scanning takes this to a new level,
since it can only guess that a word may be a pointer and verify this.

In all of these cases, we have a problem that the span lookup and
check can race with span initialization, since the span becomes
visible to lookups before it's fully initialized.

Furthermore, we're about to start initializing the span without the
heap lock held, which is going to introduce races where accesses were
previously protected by the heap lock.

To address this, this CL makes accesses to mspan.state atomic, and
ensures that the span is fully initialized before setting the state to
mSpanInUse. All loads are now atomic, and in any case where we don't
trust the pointer, it first atomically loads the span state and checks
that it's mSpanInUse, after which it will have synchronized with span
initialization and can safely check the other span fields.

For #10958, #24543, but a good fix in general.

Change-Id: I518b7c63555b02064b98aa5f802c92b758fef853
Reviewed-on: https://go-review.googlesource.com/c/go/+/203286
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/cgocheck.go

@@ -133,7 +133,7 @@ func cgoCheckTypedBlock(typ *_type, src unsafe.Pointer, off, size uintptr) {
 	}
 
 	s := spanOfUnchecked(uintptr(src))
-	if s.state == mSpanManual {
+	if s.state.get() == mSpanManual {
 		// There are no heap bits for value stored on the stack.
 		// For a channel receive src might be on the stack of some
 		// other goroutine, so we can't unwind the stack even if

src/runtime/export_test.go

@@ -256,7 +256,7 @@ func CountPagesInUse() (pagesInUse, counted uintptr) {
 	pagesInUse = uintptr(mheap_.pagesInUse)
 
 	for _, s := range mheap_.allspans {
-		if s.state == mSpanInUse {
+		if s.state.get() == mSpanInUse {
 			counted += s.npages
 		}
 	}
@@ -318,7 +318,7 @@ func ReadMemStatsSlow() (base, slow MemStats) {
 
 		// Add up current allocations in spans.
 		for _, s := range mheap_.allspans {
-			if s.state != mSpanInUse {
+			if s.state.get() != mSpanInUse {
 				continue
 			}
 			if sizeclass := s.spanclass.sizeclass(); sizeclass == 0 {
@@ -542,7 +542,7 @@ func UnscavHugePagesSlow() (uintptr, uintptr) {
 		lock(&mheap_.lock)
 		base = mheap_.free.unscavHugePages
 		for _, s := range mheap_.allspans {
-			if s.state == mSpanFree && !s.scavenged {
+			if s.state.get() == mSpanFree && !s.scavenged {
 				slow += s.hugePages()
 			}
 		}

src/runtime/heapdump.go

@@ -435,7 +435,7 @@ func dumproots() {
 
 	// mspan.types
 	for _, s := range mheap_.allspans {
-		if s.state == mSpanInUse {
+		if s.state.get() == mSpanInUse {
 			// Finalizers
 			for sp := s.specials; sp != nil; sp = sp.next {
 				if sp.kind != _KindSpecialFinalizer {
@@ -458,7 +458,7 @@ var freemark [_PageSize / 8]bool
 
 func dumpobjs() {
 	for _, s := range mheap_.allspans {
-		if s.state != mSpanInUse {
+		if s.state.get() != mSpanInUse {
 			continue
 		}
 		p := s.base()
@@ -621,7 +621,7 @@ func dumpmemprof_callback(b *bucket, nstk uintptr, pstk *uintptr, size, allocs,
 func dumpmemprof() {
 	iterate_memprof(dumpmemprof_callback)
 	for _, s := range mheap_.allspans {
-		if s.state != mSpanInUse {
+		if s.state.get() != mSpanInUse {
 			continue
 		}
 		for sp := s.specials; sp != nil; sp = sp.next {
@@ -642,7 +642,7 @@ var dumphdr = []byte("go1.7 heap dump\n")
 func mdump() {
 	// make sure we're done sweeping
 	for _, s := range mheap_.allspans {
-		if s.state == mSpanInUse {
+		if s.state.get() == mSpanInUse {
 			s.ensureSwept()
 		}
 	}

src/runtime/mbitmap.go

@@ -243,6 +243,10 @@ func (s *mspan) nextFreeIndex() uintptr {
 }
 
 // isFree reports whether the index'th object in s is unallocated.
+//
+// The caller must ensure s.state is mSpanInUse, and there must have
+// been no preemption points since ensuring this (which could allow a
+// GC transition, which would allow the state to change).
 func (s *mspan) isFree(index uintptr) bool {
 	if index < s.freeindex {
 		return false
@@ -361,12 +365,13 @@ func badPointer(s *mspan, p, refBase, refOff uintptr) {
 	// in allocated spans.
 	printlock()
 	print("runtime: pointer ", hex(p))
-	if s.state != mSpanInUse {
+	state := s.state.get()
+	if state != mSpanInUse {
 		print(" to unallocated span")
 	} else {
 		print(" to unused region of span")
 	}
-	print(" span.base()=", hex(s.base()), " span.limit=", hex(s.limit), " span.state=", s.state, "\n")
+	print(" span.base()=", hex(s.base()), " span.limit=", hex(s.limit), " span.state=", state, "\n")
 	if refBase != 0 {
 		print("runtime: found in object at *(", hex(refBase), "+", hex(refOff), ")\n")
 		gcDumpObject("object", refBase, refOff)
@@ -397,9 +402,12 @@ func findObject(p, refBase, refOff uintptr) (base uintptr, s *mspan, objIndex ui
 		return
 	}
 	// If p is a bad pointer, it may not be in s's bounds.
-	if p < s.base() || p >= s.limit || s.state != mSpanInUse {
+	//
+	// Check s.state to synchronize with span initialization
+	// before checking other fields. See also spanOfHeap.
+	if state := s.state.get(); state != mSpanInUse || p < s.base() || p >= s.limit {
 		// Pointers into stacks are also ok, the runtime manages these explicitly.
-		if s.state == mSpanManual {
+		if state == mSpanManual {
 			return
 		}
 		// The following ensures that we are rigorous about what data
@@ -620,7 +628,7 @@ func bulkBarrierPreWrite(dst, src, size uintptr) {
 			}
 		}
 		return
-	} else if s.state != mSpanInUse || dst < s.base() || s.limit <= dst {
+	} else if s.state.get() != mSpanInUse || dst < s.base() || s.limit <= dst {
 		// dst was heap memory at some point, but isn't now.
 		// It can't be a global. It must be either our stack,
 		// or in the case of direct channel sends, it could be

src/runtime/mgcmark.go

@@ -321,7 +321,9 @@ func markrootSpans(gcw *gcWork, shard int) {
 	// entered the scan phase, so addfinalizer will have ensured
 	// the above invariants for them.
 	for _, s := range spans {
-		if s.state != mSpanInUse {
+		// This is racing with spans being initialized, so
+		// check the state carefully.
+		if s.state.get() != mSpanInUse {
 			continue
 		}
 		// Check that this span was swept (it may be cached or uncached).
@@ -1310,15 +1312,15 @@ func gcDumpObject(label string, obj, off uintptr) {
 		return
 	}
 	print(" s.base()=", hex(s.base()), " s.limit=", hex(s.limit), " s.spanclass=", s.spanclass, " s.elemsize=", s.elemsize, " s.state=")
-	if 0 <= s.state && int(s.state) < len(mSpanStateNames) {
-		print(mSpanStateNames[s.state], "\n")
+	if state := s.state.get(); 0 <= state && int(state) < len(mSpanStateNames) {
+		print(mSpanStateNames[state], "\n")
 	} else {
-		print("unknown(", s.state, ")\n")
+		print("unknown(", state, ")\n")
 	}
 
 	skipped := false
 	size := s.elemsize
-	if s.state == mSpanManual && size == 0 {
+	if s.state.get() == mSpanManual && size == 0 {
 		// We're printing something from a stack frame. We
 		// don't know how big it is, so just show up to an
 		// including off.
@@ -1406,7 +1408,7 @@ var useCheckmark = false
 func initCheckmarks() {
 	useCheckmark = true
 	for _, s := range mheap_.allspans {
-		if s.state == mSpanInUse {
+		if s.state.get() == mSpanInUse {
 			heapBitsForAddr(s.base()).initCheckmarkSpan(s.layout())
 		}
 	}
@@ -1415,7 +1417,7 @@ func initCheckmarks() {
 func clearCheckmarks() {
 	useCheckmark = false
 	for _, s := range mheap_.allspans {
-		if s.state == mSpanInUse {
+		if s.state.get() == mSpanInUse {
 			heapBitsForAddr(s.base()).clearCheckmarkSpan(s.layout())
 		}
 	}

src/runtime/mgcsweep.go

@@ -114,12 +114,12 @@ func sweepone() uintptr {
 			atomic.Store(&mheap_.sweepdone, 1)
 			break
 		}
-		if s.state != mSpanInUse {
+		if state := s.state.get(); state != mSpanInUse {
 			// This can happen if direct sweeping already
 			// swept this span, but in that case the sweep
 			// generation should always be up-to-date.
 			if !(s.sweepgen == sg || s.sweepgen == sg+3) {
-				print("runtime: bad span s.state=", s.state, " s.sweepgen=", s.sweepgen, " sweepgen=", sg, "\n")
+				print("runtime: bad span s.state=", state, " s.sweepgen=", s.sweepgen, " sweepgen=", sg, "\n")
 				throw("non in-use span in unswept list")
 			}
 			continue
@@ -211,8 +211,8 @@ func (s *mspan) sweep(preserve bool) bool {
 		throw("mspan.sweep: m is not locked")
 	}
 	sweepgen := mheap_.sweepgen
-	if s.state != mSpanInUse || s.sweepgen != sweepgen-1 {
-		print("mspan.sweep: state=", s.state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
+	if state := s.state.get(); state != mSpanInUse || s.sweepgen != sweepgen-1 {
+		print("mspan.sweep: state=", state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
 		throw("mspan.sweep: bad span state")
 	}
 
@@ -351,8 +351,8 @@ func (s *mspan) sweep(preserve bool) bool {
 	if freeToHeap || nfreed == 0 {
 		// The span must be in our exclusive ownership until we update sweepgen,
 		// check for potential races.
-		if s.state != mSpanInUse || s.sweepgen != sweepgen-1 {
-			print("mspan.sweep: state=", s.state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
+		if state := s.state.get(); state != mSpanInUse || s.sweepgen != sweepgen-1 {
+			print("mspan.sweep: state=", state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
 			throw("mspan.sweep: bad span state after sweep")
 		}
 		// Serialization point.

src/runtime/mheap.go

@@ -305,6 +305,14 @@ type arenaHint struct {
 // * During GC (gcphase != _GCoff), a span *must not* transition from
 //   manual or in-use to free. Because concurrent GC may read a pointer
 //   and then look up its span, the span state must be monotonic.
+//
+// Setting mspan.state to mSpanInUse or mSpanManual must be done
+// atomically and only after all other span fields are valid.
+// Likewise, if inspecting a span is contingent on it being
+// mSpanInUse, the state should be loaded atomically and checked
+// before depending on other fields. This allows the garbage collector
+// to safely deal with potentially invalid pointers, since resolving
+// such pointers may race with a span being allocated.
 type mSpanState uint8
 
 const (
@@ -323,6 +331,21 @@ var mSpanStateNames = []string{
 	"mSpanFree",
 }
 
+// mSpanStateBox holds an mSpanState and provides atomic operations on
+// it. This is a separate type to disallow accidental comparison or
+// assignment with mSpanState.
+type mSpanStateBox struct {
+	s mSpanState
+}
+
+func (b *mSpanStateBox) set(s mSpanState) {
+	atomic.Store8((*uint8)(&b.s), uint8(s))
+}
+
+func (b *mSpanStateBox) get() mSpanState {
+	return mSpanState(atomic.Load8((*uint8)(&b.s)))
+}
+
 // mSpanList heads a linked list of spans.
 //
 //go:notinheap
@@ -408,7 +431,7 @@ type mspan struct {
 	baseMask    uint16        // if non-0, elemsize is a power of 2, & this will get object allocation base
 	allocCount  uint16        // number of allocated objects
 	spanclass   spanClass     // size class and noscan (uint8)
-	state       mSpanState // mspaninuse etc
+	state       mSpanStateBox // mSpanInUse etc; accessed atomically (get/set methods)
 	needzero    uint8         // needs to be zeroed before allocation
 	divShift    uint8         // for divide by elemsize - divMagic.shift
 	divShift2   uint8         // for divide by elemsize - divMagic.shift2
@@ -483,7 +506,7 @@ func (h *mheap) coalesce(s *mspan) {
 		// The size is potentially changing so the treap needs to delete adjacent nodes and
 		// insert back as a combined node.
 		h.free.removeSpan(other)
-		other.state = mSpanDead
+		other.state.set(mSpanDead)
 		h.spanalloc.free(unsafe.Pointer(other))
 	}
 
@@ -525,7 +548,7 @@ func (h *mheap) coalesce(s *mspan) {
 
 	// Coalesce with earlier, later spans.
 	var hpBefore uintptr
-	if before := spanOf(s.base() - 1); before != nil && before.state == mSpanFree {
+	if before := spanOf(s.base() - 1); before != nil && before.state.get() == mSpanFree {
 		if s.scavenged == before.scavenged {
 			hpBefore = before.hugePages()
 			merge(before, s, before)
@@ -536,7 +559,7 @@ func (h *mheap) coalesce(s *mspan) {
 
 	// Now check to see if next (greater addresses) span is free and can be coalesced.
 	var hpAfter uintptr
-	if after := spanOf(s.base() + s.npages*pageSize); after != nil && after.state == mSpanFree {
+	if after := spanOf(s.base() + s.npages*pageSize); after != nil && after.state.get() == mSpanFree {
 		if s.scavenged == after.scavenged {
 			hpAfter = after.hugePages()
 			merge(s, after, after)
@@ -733,7 +756,7 @@ func inHeapOrStack(b uintptr) bool {
 	if s == nil || b < s.base() {
 		return false
 	}
-	switch s.state {
+	switch s.state.get() {
 	case mSpanInUse, mSpanManual:
 		return b < s.limit
 	default:
@@ -800,9 +823,12 @@ func spanOfUnchecked(p uintptr) *mspan {
 //go:nosplit
 func spanOfHeap(p uintptr) *mspan {
 	s := spanOf(p)
-	// If p is not allocated, it may point to a stale span, so we
-	// have to check the span's bounds and state.
-	if s == nil || p < s.base() || p >= s.limit || s.state != mSpanInUse {
+	// s is nil if it's never been allocated. Otherwise, we check
+	// its state first because we don't trust this pointer, so we
+	// have to synchronize with span initialization. Then, it's
+	// still possible we picked up a stale span pointer, so we
+	// have to check the span's bounds.
+	if s == nil || s.state.get() != mSpanInUse || p < s.base() || p >= s.limit {
 		return nil
 	}
 	return s
@@ -1042,7 +1068,6 @@ func (h *mheap) alloc_m(npage uintptr, spanclass spanClass, large bool) *mspan {
 		// able to map interior pointer to containing span.
 		atomic.Store(&s.sweepgen, h.sweepgen)
 		h.sweepSpans[h.sweepgen/2%2].push(s) // Add to swept in-use list.
-		s.state = mSpanInUse
 		s.allocCount = 0
 		s.spanclass = spanclass
 		s.elemsize = elemSize
@@ -1066,6 +1091,18 @@ func (h *mheap) alloc_m(npage uintptr, spanclass spanClass, large bool) *mspan {
 		s.gcmarkBits = gcmarkBits
 		s.allocBits = allocBits
 
+		// Now that the span is filled in, set its state. This
+		// is a publication barrier for the other fields in
+		// the span. While valid pointers into this span
+		// should never be visible until the span is returned,
+		// if the garbage collector finds an invalid pointer,
+		// access to the span may race with initialization of
+		// the span. We resolve this race by atomically
+		// setting the state after the span is fully
+		// initialized, and atomically checking the state in
+		// any situation where a pointer is suspect.
+		s.state.set(mSpanInUse)
+
 		// Mark in-use span in arena page bitmap.
 		arena, pageIdx, pageMask := pageIndexOf(s.base())
 		arena.pageInUse[pageIdx] |= pageMask
@@ -1143,13 +1180,13 @@ func (h *mheap) allocManual(npage uintptr, stat *uint64) *mspan {
 	lock(&h.lock)
 	s := h.allocSpanLocked(npage, stat)
 	if s != nil {
-		s.state = mSpanManual
 		s.manualFreeList = 0
 		s.allocCount = 0
 		s.spanclass = 0
 		s.nelems = 0
 		s.elemsize = 0
 		s.limit = s.base() + s.npages<<_PageShift
+		s.state.set(mSpanManual) // Publish the span
 		// Manually managed memory doesn't count toward heap_sys.
 		memstats.heap_sys -= uint64(s.npages << _PageShift)
 	}
@@ -1201,7 +1238,7 @@ func (h *mheap) allocSpanLocked(npage uintptr, stat *uint64) *mspan {
 
 HaveSpan:
 	s := t.span()
-	if s.state != mSpanFree {
+	if s.state.get() != mSpanFree {
 		throw("candidate mspan for allocation is not free")
 	}
 
@@ -1332,7 +1369,7 @@ func (h *mheap) growAddSpan(v unsafe.Pointer, size uintptr) {
 	s := (*mspan)(h.spanalloc.alloc())
 	s.init(uintptr(v), size/pageSize)
 	h.setSpans(s.base(), s.npages, s)
-	s.state = mSpanFree
+	s.state.set(mSpanFree)
 	// [v, v+size) is always in the Prepared state. The new span
 	// must be marked scavenged so the allocator transitions it to
 	// Ready when allocating from it.
@@ -1395,7 +1432,7 @@ func (h *mheap) freeManual(s *mspan, stat *uint64) {
 }
 
 func (h *mheap) freeSpanLocked(s *mspan, acctinuse, acctidle bool) {
-	switch s.state {
+	switch s.state.get() {
 	case mSpanManual:
 		if s.allocCount != 0 {
 			throw("mheap.freeSpanLocked - invalid stack free")
@@ -1420,7 +1457,7 @@ func (h *mheap) freeSpanLocked(s *mspan, acctinuse, acctidle bool) {
 	if acctidle {
 		memstats.heap_idle += uint64(s.npages << _PageShift)
 	}
-	s.state = mSpanFree
+	s.state.set(mSpanFree)
 
 	// Coalesce span with neighbors.
 	h.coalesce(s)
@@ -1481,7 +1518,7 @@ func (h *mheap) scavengeSplit(t treapIter, size uintptr) *mspan {
 		h.setSpan(n.base(), n)
 		h.setSpan(n.base()+nbytes-1, n)
 		n.needzero = s.needzero
-		n.state = s.state
+		n.state.set(s.state.get())
 	})
 	return n
 }
@@ -1580,7 +1617,6 @@ func (span *mspan) init(base uintptr, npages uintptr) {
 	span.allocCount = 0
 	span.spanclass = 0
 	span.elemsize = 0
-	span.state = mSpanDead
 	span.scavenged = false
 	span.speciallock.key = 0
 	span.specials = nil
@@ -1588,6 +1624,7 @@ func (span *mspan) init(base uintptr, npages uintptr) {
 	span.freeindex = 0
 	span.allocBits = nil
 	span.gcmarkBits = nil
+	span.state.set(mSpanDead)
 }
 
 func (span *mspan) inList() bool {

src/runtime/signal_unix.go

@@ -305,7 +305,7 @@ func sigFetchG(c *sigctxt) *g {
 			// work.
 			sp := getcallersp()
 			s := spanOf(sp)
-			if s != nil && s.state == mSpanManual && s.base() < sp && sp < s.limit {
+			if s != nil && s.state.get() == mSpanManual && s.base() < sp && sp < s.limit {
 				gp := *(**g)(unsafe.Pointer(s.base()))
 				return gp
 			}

src/runtime/stack.go

@@ -219,7 +219,7 @@ func stackpoolalloc(order uint8) gclinkptr {
 // Adds stack x to the free pool. Must be called with stackpool[order].item.mu held.
 func stackpoolfree(x gclinkptr, order uint8) {
 	s := spanOfUnchecked(uintptr(x))
-	if s.state != mSpanManual {
+	if s.state.get() != mSpanManual {
 		throw("freeing stack not in a stack span")
 	}
 	if s.manualFreeList.ptr() == nil {
@@ -467,7 +467,7 @@ func stackfree(stk stack) {
 		}
 	} else {
 		s := spanOfUnchecked(uintptr(v))
-		if s.state != mSpanManual {
+		if s.state.get() != mSpanManual {
 			println(hex(s.base()), v)
 			throw("bad span state")
 		}