runtime: add "success" field to sudog

The current wakeup protocol for channel communications is that the
second goroutine sets gp.param to the sudog when a value is
successfully communicated over the channel, and to nil when the wakeup
is due to closing the channel.

Setting nil to indicate channel closure works okay for chansend and
chanrecv, because they're only communicating with one channel, so they
know it must be the channel that was closed. However, it means
selectgo has to re-poll all of the channels to figure out which one
was closed.

This commit adds a "success" field to sudog, and changes the wakeup
protocol to always set gp.param to sg, and to use sg.success to
indicate successful communication vs channel closure.

While here, this also reorganizes the chansend code slightly so that
the sudog is still released to the pool if the send blocks and then is
awoken because the channel closed.

Updates #40410.

Change-Id: I6cd9a20ebf9febe370a15af1b8afe24c5539efc6
Reviewed-on: https://go-review.googlesource.com/c/go/+/245019
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 7 insertions, 12 deletions

diff --git a/src/runtime/select.go b/src/runtime/select.go
index a069e3e050..081db7bad4 100644
--- a/src/runtime/select.go
+++ b/src/runtime/select.go
@@ -221,12 +221,12 @@ func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
 		nextp  **sudog
 	)
 
-loop:
 	// pass 1 - look for something already waiting
 	var dfli int
 	var dfl *scase
 	var casi int
 	var cas *scase
+	var caseSuccess bool
 	var recvOK bool
 	for i := 0; i < ncases; i++ {
 		casi = int(pollorder[i])
@@ -331,6 +331,7 @@ loop:
 	// We singly-linked up the SudoGs in lock order.
 	casi = -1
 	cas = nil
+	caseSuccess = false
 	sglist = gp.waiting
 	// Clear all elem before unlinking from gp.waiting.
 	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
@@ -352,6 +353,7 @@ loop:
 			// sg has already been dequeued by the G that woke us up.
 			casi = int(casei)
 			cas = k
+			caseSuccess = sglist.success
 		} else {
 			c = k.c
 			if k.kind == caseSend {
@@ -367,16 +369,7 @@ loop:
 	}
 
 	if cas == nil {
-		// We can wake up with gp.param == nil (so cas == nil)
-		// when a channel involved in the select has been closed.
-		// It is easiest to loop and re-run the operation;
-		// we'll see that it's now closed.
-		// Maybe some day we can signal the close explicitly,
-		// but we'd have to distinguish close-on-reader from close-on-writer.
-		// It's easiest not to duplicate the code and just recheck above.
-		// We know that something closed, and things never un-close,
-		// so we won't block again.
-		goto loop
+		throw("selectgo: bad wakeup")
 	}
 
 	c = cas.c
@@ -386,7 +379,9 @@ loop:
 	}
 
 	if cas.kind == caseRecv {
-		recvOK = true
+		recvOK = caseSuccess
+	} else if cas.kind == caseSend && !caseSuccess {
+		goto sclose
 	}
 
 	if raceenabled {