runtime: fix race condition

(Thanks to ken and rsc for pointing this out)

rsc:
	ken pointed out that there's a race in the new
	one-lock-per-channel code.  the issue is that
	if one goroutine has gone to sleep doing

	select {
	case <-c1:
	case <-c2:
	}

	and then two more goroutines try to send
	on c1 and c2 simultaneously, the way that
	the code makes sure only one wins is the
	selgen field manipulation in dequeue:

	       // if sgp is stale, ignore it
	       if(sgp->selgen != sgp->g->selgen) {
		       //prints("INVALID PSEUDOG POINTER\n");
		       freesg(c, sgp);
		       goto loop;
	       }

	       // invalidate any others
	       sgp->g->selgen++;

	but because the global lock is gone both
	goroutines will be fiddling with sgp->g->selgen
	at the same time.

This results in a 7% slowdown in the single threaded case for a
ping-pong microbenchmark.

Since the cas predominantly succeeds, adding a simple check first
didn't make any difference.

R=rsc
CC=golang-dev
https://golang.org/cl/180068

src/pkg/runtime/chan.c

@@ -24,7 +24,7 @@ typedef	struct	Scase	Scase;
 struct	SudoG
 {
 	G*	g;		// g and selgen constitute
-	int32	selgen;		// a weak pointer to g
+	uint32	selgen;		// a weak pointer to g
 	int16	offset;		// offset of case number
 	int8	isfree;		// offset of case number
 	SudoG*	link;
@@ -982,14 +982,12 @@ loop:
 	q->first = sgp->link;
 
 	// if sgp is stale, ignore it
-	if(sgp->selgen != sgp->g->selgen) {
+	if(!cas(&sgp->g->selgen, sgp->selgen, sgp->selgen + 1)) {
 		//prints("INVALID PSEUDOG POINTER\n");
 		freesg(c, sgp);
 		goto loop;
 	}
 
-	// invalidate any others
-	sgp->g->selgen++;
 	return sgp;
 }
 

src/pkg/runtime/runtime.h

@@ -167,7 +167,7 @@ struct	G
 	void*	param;		// passed parameter on wakeup
 	int16	status;
 	int32	goid;
-	int32	selgen;		// valid sudog pointer
+	uint32	selgen;		// valid sudog pointer
 	G*	schedlink;
 	bool	readyonstop;
 	M*	m;		// for debuggers, but offset not hard-coded

test/chan/doubleselect.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This test is designed to flush out the case where two cases of a select can
+// both end up running. See http://codereview.appspot.com/180068.
+package main
+
+import (
+	"flag"
+	"runtime"
+)
+
+var iterations *int = flag.Int("n", 100000, "number of iterations")
+
+// sender sends a counter to one of four different channels. If two
+// cases both end up running in the same iteration, the same value will be sent
+// to two different channels.
+func sender(n int, c1, c2, c3, c4 chan<- int) {
+	defer close(c1)
+	defer close(c2)
+
+	for i := 0; i < n; i++ {
+		select {
+		case c1 <- i:
+		case c2 <- i:
+		case c3 <- i:
+		case c4 <- i:
+		}
+	}
+}
+
+// mux receives the values from sender and forwards them onto another channel.
+// It would be simplier to just have sender's four cases all be the same
+// channel, but this doesn't actually trigger the bug.
+func mux(out chan<- int, in <-chan int) {
+	for {
+		v := <-in
+		if closed(in) {
+			close(out)
+			break
+		}
+		out <- v
+	}
+}
+
+// recver gets a steam of values from the four mux's and checks for duplicates.
+func recver(in <-chan int) {
+	seen := make(map[int]bool)
+
+	for {
+		v := <-in
+		if closed(in) {
+			break
+		}
+		if _, ok := seen[v]; ok {
+			panic("got duplicate value: ", v)
+		}
+		seen[v] = true
+	}
+}
+
+func main() {
+	runtime.GOMAXPROCS(2)
+
+	c1 := make(chan int)
+	c2 := make(chan int)
+	c3 := make(chan int)
+	c4 := make(chan int)
+	cmux := make(chan int)
+	go sender(*iterations, c1, c2, c3, c4)
+	go mux(cmux, c1)
+	go mux(cmux, c2)
+	go mux(cmux, c3)
+	go mux(cmux, c4)
+	// We keep the recver because it might catch more bugs in the future.
+	// However, the result of the bug linked to at the top is that we'll
+	// end up panicing with: "throw: bad g->status in ready".
+	recver(cmux)
+	print("PASS\n")
+}

test/golden.out

@@ -75,6 +75,9 @@ abcxyz-abcxyz-abcxyz-abcxyz-abcxyz-abcxyz-abcxyz
 
 == chan/
 
+=========== chan/doubleselect.go
+PASS
+
 =========== chan/nonblock.go
 PASS