runtime: handle selects with duplicate channels in shrinkstack

The shrinkstack code locks all the channels a goroutine is waiting for,
but didn't handle the case of the same channel appearing in the list
multiple times. This led to a deadlock. The channels are sorted so it's
easy to avoid locking the same channel twice.

Fixes #16286.

Change-Id: Ie514805d0532f61c942e85af5b7b8ac405e2ff65
Reviewed-on: https://go-review.googlesource.com/24815
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Austin Clements <[email protected]>

src/runtime/chan_test.go

@@ -593,8 +593,10 @@ func TestSelectStackAdjust(t *testing.T) {
 	// pointers are adjusted correctly by stack shrinking.
 	c := make(chan *int)
 	d := make(chan *int)
-	ready := make(chan bool)
-	go func() {
+	ready1 := make(chan bool)
+	ready2 := make(chan bool)
+
+	f := func(ready chan bool, dup bool) {
 		// Temporarily grow the stack to 10K.
 		stackGrowthRecursive((10 << 10) / (128 * 8))
 
@@ -604,10 +606,20 @@ func TestSelectStackAdjust(t *testing.T) {
 		val := 42
 		var cx *int
 		cx = &val
+
+		var c2 chan *int
+		var d2 chan *int
+		if dup {
+			c2 = c
+			d2 = d
+		}
+
 		// Receive from d. cx won't be affected.
 		select {
 		case cx = <-c:
+		case <-c2:
 		case <-d:
+		case <-d2:
 		}
 
 		// Check that pointer in cx was adjusted correctly.
@@ -622,10 +634,14 @@ func TestSelectStackAdjust(t *testing.T) {
 			}
 		}
 		ready <- true
-	}()
+	}
+
+	go f(ready1, false)
+	go f(ready2, true)
 
-	// Let the goroutine get into the select.
-	<-ready
+	// Let the goroutines get into the select.
+	<-ready1
+	<-ready2
 	time.Sleep(10 * time.Millisecond)
 
 	// Force concurrent GC a few times.
@@ -642,9 +658,10 @@ func TestSelectStackAdjust(t *testing.T) {
 done:
 	selectSink = nil
 
-	// Wake select.
-	d <- nil
-	<-ready
+	// Wake selects.
+	close(d)
+	<-ready1
+	<-ready2
 }
 
 func BenchmarkChanNonblocking(b *testing.B) {

src/runtime/stack.go

@@ -784,8 +784,12 @@ func syncadjustsudogs(gp *g, used uintptr, adjinfo *adjustinfo) uintptr {
 	// copystack; otherwise, gp may be in the middle of
 	// putting itself on wait queues and this would
 	// self-deadlock.
+	var lastc *hchan
 	for sg := gp.waiting; sg != nil; sg = sg.waitlink {
-		lock(&sg.c.lock)
+		if sg.c != lastc {
+			lock(&sg.c.lock)
+		}
+		lastc = sg.c
 	}
 
 	// Adjust sudogs.
@@ -803,8 +807,12 @@ func syncadjustsudogs(gp *g, used uintptr, adjinfo *adjustinfo) uintptr {
 	}
 
 	// Unlock channels.
+	lastc = nil
 	for sg := gp.waiting; sg != nil; sg = sg.waitlink {
-		unlock(&sg.c.lock)
+		if sg.c != lastc {
+			unlock(&sg.c.lock)
+		}
+		lastc = sg.c
 	}
 
 	return sgsize