runtime: move selectdone into g

Writing to selectdone on the stack of another goroutine meant a
pretty subtle dance between the select code and the stack copying
code. Instead move the selectdone variable into the g struct.

Change-Id: Id246aaf18077c625adef7ca2d62794afef1bdd1b
Reviewed-on: https://go-review.googlesource.com/53390Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/chan.go

@@ -222,7 +222,7 @@ func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
 	mysg.elem = ep
 	mysg.waitlink = nil
 	mysg.g = gp
-	mysg.selectdone = nil
+	mysg.isSelect = false
 	mysg.c = c
 	gp.waiting = mysg
 	gp.param = nil
@@ -507,7 +507,7 @@ func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool)
 	mysg.waitlink = nil
 	gp.waiting = mysg
 	mysg.g = gp
-	mysg.selectdone = nil
+	mysg.isSelect = false
 	mysg.c = c
 	gp.param = nil
 	c.recvq.enqueue(mysg)
@@ -711,10 +711,16 @@ func (q *waitq) dequeue() *sudog {
 			sgp.next = nil // mark as removed (see dequeueSudog)
 		}
 
-		// if sgp participates in a select and is already signaled, ignore it
-		if sgp.selectdone != nil {
-			// claim the right to signal
-			if *sgp.selectdone != 0 || !atomic.Cas(sgp.selectdone, 0, 1) {
+		// if a goroutine was put on this queue because of a
+		// select, there is a small window between the goroutine
+		// being woken up by a different case and it grabbing the
+		// channel locks. Once it has the lock
+		// it removes itself from the queue, so we won't see it after that.
+		// We use a flag in the G struct to tell us when someone
+		// else has won the race to signal this goroutine but the goroutine
+		// hasn't removed itself from the queue yet.
+		if sgp.isSelect {
+			if !atomic.Cas(&sgp.g.selectDone, 0, 1) {
 				continue
 			}
 		}

src/runtime/proc.go

@@ -332,8 +332,8 @@ func releaseSudog(s *sudog) {
 	if s.elem != nil {
 		throw("runtime: sudog with non-nil elem")
 	}
-	if s.selectdone != nil {
-		throw("runtime: sudog with non-nil selectdone")
+	if s.isSelect {
+		throw("runtime: sudog with non-false isSelect")
 	}
 	if s.next != nil {
 		throw("runtime: sudog with non-nil next")

src/runtime/runtime2.go

@@ -272,11 +272,14 @@ type sudog struct {
 	// channel this sudog is blocking on. shrinkstack depends on
 	// this for sudogs involved in channel ops.
 
-	g          *g
-	selectdone *uint32 // CAS to 1 to win select race (may point to stack)
-	next       *sudog
-	prev       *sudog
-	elem       unsafe.Pointer // data element (may point to stack)
+	g *g
+
+	// isSelect indicates g is participating in a select, so
+	// g.selectDone must be CAS'd to win the wake-up race.
+	isSelect bool
+	next     *sudog
+	prev     *sudog
+	elem     unsafe.Pointer // data element (may point to stack)
 
 	// The following fields are never accessed concurrently.
 	// For channels, waitlink is only accessed by g.
@@ -367,6 +370,7 @@ type g struct {
 	cgoCtxt        []uintptr      // cgo traceback context
 	labels         unsafe.Pointer // profiler labels
 	timer          *timer         // cached timer for time.Sleep
+	selectDone     uint32         // are we participating in a select and did someone win the race?
 
 	// Per-G GC state
 

src/runtime/select.go

@@ -286,7 +286,6 @@ func selectgo(sel *hselect) int {
 
 	var (
 		gp     *g
-		done   uint32
 		sg     *sudog
 		c      *hchan
 		k      *scase
@@ -353,7 +352,6 @@ loop:
 
 	// pass 2 - enqueue on all chans
 	gp = getg()
-	done = 0
 	if gp.waiting != nil {
 		throw("gp.waiting != nil")
 	}
@@ -367,8 +365,7 @@ loop:
 		c = cas.c
 		sg := acquireSudog()
 		sg.g = gp
-		// Note: selectdone is adjusted for stack copies in stack1.go:adjustsudogs
-		sg.selectdone = (*uint32)(noescape(unsafe.Pointer(&done)))
+		sg.isSelect = true
 		// No stack splits between assigning elem and enqueuing
 		// sg on gp.waiting where copystack can find it.
 		sg.elem = cas.elem
@@ -394,62 +391,9 @@ loop:
 	gp.param = nil
 	gopark(selparkcommit, nil, "select", traceEvGoBlockSelect, 1)
 
-	// While we were asleep, some goroutine came along and completed
-	// one of the cases in the select and woke us up (called ready).
-	// As part of that process, the goroutine did a cas on done above
-	// (aka *sg.selectdone for all queued sg) to win the right to
-	// complete the select. Now done = 1.
-	//
-	// If we copy (grow) our own stack, we will update the
-	// selectdone pointers inside the gp.waiting sudog list to point
-	// at the new stack. Another goroutine attempting to
-	// complete one of our (still linked in) select cases might
-	// see the new selectdone pointer (pointing at the new stack)
-	// before the new stack has real data; if the new stack has done = 0
-	// (before the old values are copied over), the goroutine might
-	// do a cas via sg.selectdone and incorrectly believe that it has
-	// won the right to complete the select, executing a second
-	// communication and attempting to wake us (call ready) again.
-	//
-	// Then things break.
-	//
-	// The best break is that the goroutine doing ready sees the
-	// _Gcopystack status and throws, as in #17007.
-	// A worse break would be for us to continue on, start running real code,
-	// block in a semaphore acquisition (sema.go), and have the other
-	// goroutine wake us up without having really acquired the semaphore.
-	// That would result in the goroutine spuriously running and then
-	// queue up another spurious wakeup when the semaphore really is ready.
-	// In general the situation can cascade until something notices the
-	// problem and causes a crash.
-	//
-	// A stack shrink does not have this problem, because it locks
-	// all the channels that are involved first, blocking out the
-	// possibility of a cas on selectdone.
-	//
-	// A stack growth before gopark above does not have this
-	// problem, because we hold those channel locks (released by
-	// selparkcommit).
-	//
-	// A stack growth after sellock below does not have this
-	// problem, because again we hold those channel locks.
-	//
-	// The only problem is a stack growth during sellock.
-	// To keep that from happening, run sellock on the system stack.
-	//
-	// It might be that we could avoid this if copystack copied the
-	// stack before calling adjustsudogs. In that case,
-	// syncadjustsudogs would need to recopy the tiny part that
-	// it copies today, resulting in a little bit of extra copying.
-	//
-	// An even better fix, not for the week before a release candidate,
-	// would be to put space in every sudog and make selectdone
-	// point at (say) the space in the first sudog.
-
-	systemstack(func() {
-		sellock(scases, lockorder)
-	})
+	sellock(scases, lockorder)
 
+	gp.selectDone = 0
 	sg = (*sudog)(gp.param)
 	gp.param = nil
 
@@ -462,7 +406,7 @@ loop:
 	sglist = gp.waiting
 	// Clear all elem before unlinking from gp.waiting.
 	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
-		sg1.selectdone = nil
+		sg1.isSelect = false
 		sg1.elem = nil
 		sg1.c = nil
 	}

src/runtime/stack.go

@@ -751,7 +751,6 @@ func adjustsudogs(gp *g, adjinfo *adjustinfo) {
 	// might be in the stack.
 	for s := gp.waiting; s != nil; s = s.waitlink {
 		adjustpointer(adjinfo, unsafe.Pointer(&s.elem))
-		adjustpointer(adjinfo, unsafe.Pointer(&s.selectdone))
 	}
 }
 
@@ -768,10 +767,6 @@ func findsghi(gp *g, stk stack) uintptr {
 		if stk.lo <= p && p < stk.hi && p > sghi {
 			sghi = p
 		}
-		p = uintptr(unsafe.Pointer(sg.selectdone)) + unsafe.Sizeof(sg.selectdone)
-		if stk.lo <= p && p < stk.hi && p > sghi {
-			sghi = p
-		}
 	}
 	return sghi
 }