runtime: wait to update arena_used until after mapping bitmap

This avoids a race with gcmarkwb_m that was leading to faults.

Fixes #10212.

Change-Id: I6fcf8d09f2692227063ce29152cb57366ea22487
Reviewed-on: https://go-review.googlesource.com/10816
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/malloc.go

@@ -418,9 +418,9 @@ func mHeap_SysAlloc(h *mheap, n uintptr) unsafe.Pointer {
 		// Keep taking from our reservation.
 		p := h.arena_used
 		sysMap((unsafe.Pointer)(p), n, h.arena_reserved, &memstats.heap_sys)
-		h.arena_used += n
-		mHeap_MapBits(h)
-		mHeap_MapSpans(h)
+		mHeap_MapBits(h, p+n)
+		mHeap_MapSpans(h, p+n)
+		h.arena_used = p+n
 		if raceenabled {
 			racemapshadow((unsafe.Pointer)(p), n)
 		}
@@ -454,12 +454,12 @@ func mHeap_SysAlloc(h *mheap, n uintptr) unsafe.Pointer {
 	p_end := p + p_size
 	p += -p & (_PageSize - 1)
 	if uintptr(p)+n > uintptr(h.arena_used) {
-		h.arena_used = p + n
+		mHeap_MapBits(h, p+n)
+		mHeap_MapSpans(h, p+n)
+		h.arena_used = p+n
 		if p_end > h.arena_end {
 			h.arena_end = p_end
 		}
-		mHeap_MapBits(h)
-		mHeap_MapSpans(h)
 		if raceenabled {
 			racemapshadow((unsafe.Pointer)(p), n)
 		}

src/runtime/mbitmap.go

@@ -118,15 +118,20 @@ func subtract1(p *byte) *byte {
 
 // mHeap_MapBits is called each time arena_used is extended.
 // It maps any additional bitmap memory needed for the new arena memory.
+// It must be called with the expected new value of arena_used,
+// *before* h.arena_used has been updated.
+// Waiting to update arena_used until after the memory has been mapped
+// avoids faults when other threads try access the bitmap immediately
+// after observing the change to arena_used.
 //
 //go:nowritebarrier
-func mHeap_MapBits(h *mheap) {
+func mHeap_MapBits(h *mheap, arena_used uintptr) {
 	// Caller has added extra mappings to the arena.
 	// Add extra mappings of bitmap words as needed.
 	// We allocate extra bitmap pieces in chunks of bitmapChunk.
 	const bitmapChunk = 8192
 
-	n := (mheap_.arena_used - mheap_.arena_start) / heapBitmapScale
+	n := (arena_used - mheap_.arena_start) / heapBitmapScale
 	n = round(n, bitmapChunk)
 	n = round(n, _PhysPageSize)
 	if h.bitmap_mapped >= n {

src/runtime/mheap.go

@@ -279,10 +279,18 @@ func mHeap_Init(h *mheap, spans_size uintptr) {
 	sp.cap = int(spans_size / ptrSize)
 }
 
-func mHeap_MapSpans(h *mheap) {
+// mHeap_MapSpans makes sure that the spans are mapped
+// up to the new value of arena_used.
+//
+// It must be called with the expected new value of arena_used,
+// *before* h.arena_used has been updated.
+// Waiting to update arena_used until after the memory has been mapped
+// avoids faults when other threads try access the bitmap immediately
+// after observing the change to arena_used.
+func mHeap_MapSpans(h *mheap, arena_used uintptr) {
 	// Map spans array, PageSize at a time.
-	n := uintptr(unsafe.Pointer(h.arena_used))
-	n -= uintptr(unsafe.Pointer(h.arena_start))
+	n := arena_used
+	n -= h.arena_start
 	n = n / _PageSize * ptrSize
 	n = round(n, _PhysPageSize)
 	if h.spans_mapped >= n {