runtime: best-effort detection of concurrent misuse of maps

If reports like #13062 are really concurrent misuse of maps,
we can detect that, at least some of the time, with a cheap check.

There is an extra pair of memory writes for writing to a map,
but to the same cache line as h.count, which is often being modified anyway,
and there is an extra memory read for reading from a map,
but to the same cache line as h.count, which is always being read anyway.
So the check should be basically invisible and may help reduce the
number of "mysterious runtime crash due to map misuse" reports.

Change-Id: I0e71b0d92eaa3b7bef48bf41b0f5ab790092487e
Reviewed-on: https://go-review.googlesource.com/17501Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/hashmap.go

@@ -95,6 +95,7 @@ const (
 	// flags
 	iterator    = 1 // there may be an iterator using buckets
 	oldIterator = 2 // there may be an iterator using oldbuckets
+	hashWriting = 4 // a goroutine is writing to the map
 
 	// sentinel bucket ID for iterator checks
 	noCheck = 1<<(8*sys.PtrSize) - 1
@@ -284,6 +285,9 @@ func mapaccess1(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
 	if h == nil || h.count == 0 {
 		return atomic.Loadp(unsafe.Pointer(&zeroptr))
 	}
+	if h.flags&hashWriting != 0 {
+		throw("concurrent map read and map write")
+	}
 	alg := t.key.alg
 	hash := alg.hash(key, uintptr(h.hash0))
 	m := uintptr(1)<<h.B - 1
@@ -335,6 +339,9 @@ func mapaccess2(t *maptype, h *hmap, key unsafe.Pointer) (unsafe.Pointer, bool)
 	if h == nil || h.count == 0 {
 		return atomic.Loadp(unsafe.Pointer(&zeroptr)), false
 	}
+	if h.flags&hashWriting != 0 {
+		throw("concurrent map read and map write")
+	}
 	alg := t.key.alg
 	hash := alg.hash(key, uintptr(h.hash0))
 	m := uintptr(1)<<h.B - 1
@@ -378,6 +385,9 @@ func mapaccessK(t *maptype, h *hmap, key unsafe.Pointer) (unsafe.Pointer, unsafe
 	if h == nil || h.count == 0 {
 		return nil, nil
 	}
+	if h.flags&hashWriting != 0 {
+		throw("concurrent map read and map write")
+	}
 	alg := t.key.alg
 	hash := alg.hash(key, uintptr(h.hash0))
 	m := uintptr(1)<<h.B - 1
@@ -431,6 +441,10 @@ func mapassign1(t *maptype, h *hmap, key unsafe.Pointer, val unsafe.Pointer) {
 		msanread(key, t.key.size)
 		msanread(val, t.elem.size)
 	}
+	if h.flags&hashWriting != 0 {
+		throw("concurrent map writes")
+	}
+	h.flags |= hashWriting
 
 	alg := t.key.alg
 	hash := alg.hash(key, uintptr(h.hash0))
@@ -481,7 +495,7 @@ again:
 				v2 = *((*unsafe.Pointer)(v2))
 			}
 			typedmemmove(t.elem, v2, val)
-			return
+			goto done
 		}
 		ovf := b.overflow(t)
 		if ovf == nil {
@@ -520,6 +534,12 @@ again:
 	typedmemmove(t.elem, insertv, val)
 	*inserti = top
 	h.count++
+
+done:
+	if h.flags&hashWriting == 0 {
+		throw("concurrent map writes")
+	}
+	h.flags &^= hashWriting
 }
 
 func mapdelete(t *maptype, h *hmap, key unsafe.Pointer) {
@@ -535,6 +555,11 @@ func mapdelete(t *maptype, h *hmap, key unsafe.Pointer) {
 	if h == nil || h.count == 0 {
 		return
 	}
+	if h.flags&hashWriting != 0 {
+		throw("concurrent map writes")
+	}
+	h.flags |= hashWriting
+
 	alg := t.key.alg
 	hash := alg.hash(key, uintptr(h.hash0))
 	bucket := hash & (uintptr(1)<<h.B - 1)
@@ -564,13 +589,19 @@ func mapdelete(t *maptype, h *hmap, key unsafe.Pointer) {
 			memclr(v, uintptr(t.valuesize))
 			b.tophash[i] = empty
 			h.count--
-			return
+			goto done
 		}
 		b = b.overflow(t)
 		if b == nil {
-			return
+			goto done
 		}
 	}
+
+done:
+	if h.flags&hashWriting == 0 {
+		throw("concurrent map writes")
+	}
+	h.flags &^= hashWriting
 }
 
 func mapiterinit(t *maptype, h *hmap, it *hiter) {

src/runtime/hashmap_fast.go

@@ -18,6 +18,9 @@ func mapaccess1_fast32(t *maptype, h *hmap, key uint32) unsafe.Pointer {
 	if h == nil || h.count == 0 {
 		return atomic.Loadp(unsafe.Pointer(&zeroptr))
 	}
+	if h.flags&hashWriting != 0 {
+		throw("concurrent map read and map write")
+	}
 	var b *bmap
 	if h.B == 0 {
 		// One-bucket table.  No need to hash.
@@ -60,6 +63,9 @@ func mapaccess2_fast32(t *maptype, h *hmap, key uint32) (unsafe.Pointer, bool) {
 	if h == nil || h.count == 0 {
 		return atomic.Loadp(unsafe.Pointer(&zeroptr)), false
 	}
+	if h.flags&hashWriting != 0 {
+		throw("concurrent map read and map write")
+	}
 	var b *bmap
 	if h.B == 0 {
 		// One-bucket table.  No need to hash.
@@ -102,6 +108,9 @@ func mapaccess1_fast64(t *maptype, h *hmap, key uint64) unsafe.Pointer {
 	if h == nil || h.count == 0 {
 		return atomic.Loadp(unsafe.Pointer(&zeroptr))
 	}
+	if h.flags&hashWriting != 0 {
+		throw("concurrent map read and map write")
+	}
 	var b *bmap
 	if h.B == 0 {
 		// One-bucket table.  No need to hash.
@@ -144,6 +153,9 @@ func mapaccess2_fast64(t *maptype, h *hmap, key uint64) (unsafe.Pointer, bool) {
 	if h == nil || h.count == 0 {
 		return atomic.Loadp(unsafe.Pointer(&zeroptr)), false
 	}
+	if h.flags&hashWriting != 0 {
+		throw("concurrent map read and map write")
+	}
 	var b *bmap
 	if h.B == 0 {
 		// One-bucket table.  No need to hash.
@@ -186,6 +198,9 @@ func mapaccess1_faststr(t *maptype, h *hmap, ky string) unsafe.Pointer {
 	if h == nil || h.count == 0 {
 		return atomic.Loadp(unsafe.Pointer(&zeroptr))
 	}
+	if h.flags&hashWriting != 0 {
+		throw("concurrent map read and map write")
+	}
 	key := stringStructOf(&ky)
 	if h.B == 0 {
 		// One-bucket table.
@@ -288,6 +303,9 @@ func mapaccess2_faststr(t *maptype, h *hmap, ky string) (unsafe.Pointer, bool) {
 	if h == nil || h.count == 0 {
 		return atomic.Loadp(unsafe.Pointer(&zeroptr)), false
 	}
+	if h.flags&hashWriting != 0 {
+		throw("concurrent map read and map write")
+	}
 	key := stringStructOf(&ky)
 	if h.B == 0 {
 		// One-bucket table.