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>

1 files changed, 7 insertions, 2 deletions

diff --git a/src/runtime/mbitmap.go b/src/runtime/mbitmap.go
index a1d5d8fc81..c97bf0a450 100644
--- a/src/runtime/mbitmap.go
+++ b/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 {