runtime: put g.waiting list in lock order

Currently the g.waiting list created by a select is in poll order.
However, nothing depends on this, and we're going to need access to
the channel lock order in other places shortly, so modify select to
put the waiting list in channel lock order.

For #12967.

Change-Id: If0d38816216ecbb37a36624d9b25dd96e0a775ec
Reviewed-on: https://go-review.googlesource.com/20037Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Austin Clements <austin@google.com>

src/runtime/runtime2.go

@@ -314,7 +314,7 @@ type g struct {
 	gopc           uintptr // pc of go statement that created this goroutine
 	startpc        uintptr // pc of goroutine function
 	racectx        uintptr
-	waiting        *sudog // sudog structures this g is waiting on (that have a valid elem ptr)
+	waiting        *sudog // sudog structures this g is waiting on (that have a valid elem ptr); in lock order
 
 	// Per-G gcController state
 

src/runtime/select.go

@@ -319,6 +319,7 @@ func selectgoImpl(sel *hselect) (uintptr, uint16) {
 		sglist *sudog
 		sgnext *sudog
 		qp     unsafe.Pointer
+		nextp  **sudog
 	)
 
 loop:
@@ -374,8 +375,9 @@ loop:
 	if gp.waiting != nil {
 		throw("gp.waiting != nil")
 	}
-	for i := 0; i < int(sel.ncase); i++ {
-		cas = &scases[pollorder[i]]
+	nextp = &gp.waiting
+	for _, casei := range lockorder {
+		cas = &scases[casei]
 		c = cas.c
 		sg := acquireSudog()
 		sg.g = gp
@@ -388,9 +390,10 @@ loop:
 		if t0 != 0 {
 			sg.releasetime = -1
 		}
-		sg.waitlink = gp.waiting
 		sg.c = c
-		gp.waiting = sg
+		// Construct waiting list in lock order.
+		*nextp = sg
+		nextp = &sg.waitlink
 
 		switch cas.kind {
 		case caseRecv:
@@ -413,8 +416,7 @@ loop:
 	// pass 3 - dequeue from unsuccessful chans
 	// otherwise they stack up on quiet channels
 	// record the successful case, if any.
-	// We singly-linked up the SudoGs in case order, so when
-	// iterating through the linked list they are in reverse order.
+	// We singly-linked up the SudoGs in lock order.
 	cas = nil
 	sglist = gp.waiting
 	// Clear all elem before unlinking from gp.waiting.
@@ -424,8 +426,9 @@ loop:
 		sg1.c = nil
 	}
 	gp.waiting = nil
-	for i := int(sel.ncase) - 1; i >= 0; i-- {
-		k = &scases[pollorder[i]]
+
+	for _, casei := range lockorder {
+		k = &scases[casei]
 		if sglist.releasetime > 0 {
 			k.releasetime = sglist.releasetime
 		}