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
http://codereview.appspot.com/180068

1 files changed, 3 insertions, 0 deletions

diff --git a/test/golden.out b/test/golden.out
index 9813c8313..063feccd0 100644
--- a/test/golden.out
+++ b/test/golden.out
@@ -75,6 +75,9 @@ abcxyz-abcxyz-abcxyz-abcxyz-abcxyz-abcxyz-abcxyz
 
 == chan/
 
+=========== chan/doubleselect.go
+PASS
+
 =========== chan/nonblock.go
 PASS