runtime: update some comments

This updates some comments that became out of date when we moved the
mark bit out of the heap bitmap and started using the high bit for the
first word as a scan/dead bit.

Change-Id: I4a572d16db6114cadff006825466c1f18359f2db
Reviewed-on: https://go-review.googlesource.com/22662Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mbitmap.go

@@ -850,15 +850,12 @@ func (s *mspan) countFree() int {
 // because all free objects are marked as noscan during
 // heapBitsSweepSpan.
 // There can only be one allocation from a given span active at a time,
-// so this code is not racing with other instances of itself,
-// and we don't allocate from a span until it has been swept,
-// so this code is not racing with heapBitsSweepSpan.
-// It is, however, racing with the concurrent GC mark phase,
-// which can be setting the mark bit in the leading 2-bit entry
-// of an allocated block. The block we are modifying is not quite
-// allocated yet, so the GC marker is not racing with updates to x's bits,
-// but if the start or end of x shares a bitmap byte with an adjacent
-// object, the GC marker is racing with updates to those object's mark bits.
+// so this code is not racing with other instances of itself, and
+// the bitmap for a span always falls on byte boundaries.
+// Hence, it can access the bitmap with racing.
+//
+// TODO: This still has atomic accesses left over from when it could
+// race with GC accessing mark bits in the bitmap. Remove these.
 func heapBitsSetType(x, size, dataSize uintptr, typ *_type) {
 	const doubleCheck = false // slow but helpful; enable to test modifications to this code
 
@@ -1122,8 +1119,8 @@ func heapBitsSetType(x, size, dataSize uintptr, typ *_type) {
 	}
 
 	// Phase 1: Special case for leading byte (shift==0) or half-byte (shift==4).
-	// The leading byte is special because it contains the bits for words 0 and 1,
-	// which do not have the marked bits set.
+	// The leading byte is special because it contains the bits for word 1,
+	// which does not have the marked bits set.
 	// The leading half-byte is special because it's a half a byte and must be
 	// manipulated atomically.
 	switch {