runtime: make mSpanList more go:notinheap-friendly

Currently mspan links to its previous mspan using a **mspan field that
points to the previous span's next field. This simplifies some of the
list manipulation code, but is going to make it very hard to convince
the compiler that mspan list manipulations don't need write barriers.

Fix this by using a more traditional ("boring") linked list that uses
a simple *mspan pointer to the previous mspan. This complicates some
of the list manipulation slightly, but it will let us eliminate all
write barriers from the mspan list manipulation code by marking mspan
go:notinheap.

Change-Id: I0d0b212db5f20002435d2a0ed2efc8aa0364b905
Reviewed-on: https://go-review.googlesource.com/30940Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mheap.go

@@ -122,15 +122,14 @@ var mSpanStateNames = []string{
 
 // mSpanList heads a linked list of spans.
 //
-// Linked list structure is based on BSD's "tail queue" data structure.
 type mSpanList struct {
 	first *mspan // first span in list, or nil if none
-	last  **mspan // last span's next field, or first if none
+	last  *mspan // last span in list, or nil if none
 }
 
 type mspan struct {
 	next *mspan     // next span in list, or nil if none
-	prev **mspan    // previous span's next field, or list head's first field if none
+	prev *mspan     // previous span in list, or nil if none
 	list *mSpanList // For debugging. TODO: Remove.
 
 	startAddr     uintptr   // address of first byte of span aka s.base()
@@ -997,28 +996,30 @@ func (span *mspan) init(base uintptr, npages uintptr) {
 }
 
 func (span *mspan) inList() bool {
-	return span.prev != nil
+	return span.list != nil
 }
 
 // Initialize an empty doubly-linked list.
 func (list *mSpanList) init() {
 	list.first = nil
-	list.last = &list.first
+	list.last = nil
 }
 
 func (list *mSpanList) remove(span *mspan) {
-	if span.prev == nil || span.list != list {
+	if span.list != list {
 		println("runtime: failed MSpanList_Remove", span, span.prev, span.list, list)
 		throw("MSpanList_Remove")
 	}
-	if span.next != nil {
-		span.next.prev = span.prev
+	if list.first == span {
+		list.first = span.next
 	} else {
-		// TODO: After we remove the span.list != list check above,
-		// we could at least still check list.last == &span.next here.
+		span.prev.next = span.next
+	}
+	if list.last == span {
 		list.last = span.prev
+	} else {
+		span.next.prev = span.prev
 	}
-	*span.prev = span.next
 	span.next = nil
 	span.prev = nil
 	span.list = nil
@@ -1035,12 +1036,14 @@ func (list *mSpanList) insert(span *mspan) {
 	}
 	span.next = list.first
 	if list.first != nil {
-		list.first.prev = &span.next
+		// The list contains at least one span; link it in.
+		// The last span in the list doesn't change.
+		list.first.prev = span
 	} else {
-		list.last = &span.next
+		// The list contains no spans, so this is also the last span.
+		list.last = span
 	}
 	list.first = span
-	span.prev = &list.first
 	span.list = list
 }
 
@@ -1049,10 +1052,15 @@ func (list *mSpanList) insertBack(span *mspan) {
 		println("failed MSpanList_InsertBack", span, span.next, span.prev, span.list)
 		throw("MSpanList_InsertBack")
 	}
-	span.next = nil
 	span.prev = list.last
-	*list.last = span
-	list.last = &span.next
+	if list.last != nil {
+		// The list contains at least one span.
+		list.last.next = span
+	} else {
+		// The list contains no spans, so this is also the first span.
+		list.first = span
+	}
+	list.last = span
 	span.list = list
 }