runtime: add "success" field to sudog

This is the gofrontend version of https://golang.org/cl/245019.
Original CL description:

    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.

    For golang/go#40410

This is being brought over to gofrontend as a step toward upgrading to
Go1.16beta1, setting up for more compiler changes related to select handling.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/279734

diff --git a/gcc/go/gofrontend/MERGE b/gcc/go/gofrontend/MERGE
index a7df843..bb537f1 100644 (file)
--- a/gcc/go/gofrontend/MERGE
+++ b/gcc/go/gofrontend/MERGE
@@ -1,4 +1,4 @@
-d0e56e82bb298268ec0f306fef99a715c892d4a7
+eca96e39cb895805b617e0e1f184f893ed3e46bb
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

diff --git a/libgo/go/runtime/chan.go b/libgo/go/runtime/chan.go
index 8e104f1..1b20aff 100644 (file)
--- a/libgo/go/runtime/chan.go
+++ b/libgo/go/runtime/chan.go
@@ -285,18 +285,19 @@ func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
        }
        gp.waiting = nil
        gp.activeStackChans = false
-       if gp.param == nil {
-               if c.closed == 0 {
-                       throw("chansend: spurious wakeup")
-               }
-               panic(plainError("send on closed channel"))
-       }
+       closed := !mysg.success
        gp.param = nil
        if mysg.releasetime > 0 {
                blockevent(mysg.releasetime-t0, 2)
        }
        mysg.c = nil
        releaseSudog(mysg)
+       if closed {
+               if c.closed == 0 {
+                       throw("chansend: spurious wakeup")
+               }
+               panic(plainError("send on closed channel"))
+       }
        return true
 }
 
@@ -333,6 +334,7 @@ func send(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
        gp := sg.g
        unlockf()
        gp.param = unsafe.Pointer(sg)
+       sg.success = true
        if sg.releasetime != 0 {
                sg.releasetime = cputicks()
        }
@@ -406,7 +408,8 @@ func closechan(c *hchan) {
                        sg.releasetime = cputicks()
                }
                gp := sg.g
-               gp.param = nil
+               gp.param = unsafe.Pointer(sg)
+               sg.success = false
                if raceenabled {
                        raceacquireg(gp, c.raceaddr())
                }
@@ -424,7 +427,8 @@ func closechan(c *hchan) {
                        sg.releasetime = cputicks()
                }
                gp := sg.g
-               gp.param = nil
+               gp.param = unsafe.Pointer(sg)
+               sg.success = false
                if raceenabled {
                        raceacquireg(gp, c.raceaddr())
                }
@@ -607,11 +611,11 @@ func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool)
        if mysg.releasetime > 0 {
                blockevent(mysg.releasetime-t0, 2)
        }
-       closed := gp.param == nil
+       success := mysg.success
        gp.param = nil
        mysg.c = nil
        releaseSudog(mysg)
-       return true, !closed
+       return true, success
 }
 
 // recv processes a receive operation on a full channel c.
@@ -664,6 +668,7 @@ func recv(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
        gp := sg.g
        unlockf()
        gp.param = unsafe.Pointer(sg)
+       sg.success = true
        if sg.releasetime != 0 {
                sg.releasetime = cputicks()
        }

diff --git a/libgo/go/runtime/runtime2.go b/libgo/go/runtime/runtime2.go
index bf3fbac..f30d1bc 100644 (file)
--- a/libgo/go/runtime/runtime2.go
+++ b/libgo/go/runtime/runtime2.go
@@ -354,6 +354,12 @@ type sudog struct {
        // g.selectDone must be CAS'd to win the wake-up race.
        isSelect bool
 
+       // success indicates whether communication over channel c
+       // succeeded. It is true if the goroutine was awoken because a
+       // value was delivered over channel c, and false if awoken
+       // because c was closed.
+       success bool
+
        parent   *sudog // semaRoot binary tree
        waitlink *sudog // g.waiting list or semaRoot
        waittail *sudog // semaRoot

diff --git a/libgo/go/runtime/select.go b/libgo/go/runtime/select.go
index d5087fb..c31aa34 100644 (file)
--- a/libgo/go/runtime/select.go
+++ b/libgo/go/runtime/select.go
@@ -235,10 +235,10 @@ func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
                nextp  **sudog
        )
 
-loop:
        // pass 1 - look for something already waiting
        var casi int
        var cas *scase
+       var caseSuccess bool
        var caseReleaseTime int64 = -1
        var recvOK bool
        for _, casei := range pollorder {
@@ -335,6 +335,7 @@ 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 {
@@ -350,6 +351,7 @@ loop:
                        // sg has already been dequeued by the G that woke us up.
                        casi = int(casei)
                        cas = k
+                       caseSuccess = sglist.success
                        if sglist.releasetime > 0 {
                                caseReleaseTime = sglist.releasetime
                        }
@@ -368,16 +370,7 @@ 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
@@ -387,7 +380,9 @@ loop:
        }
 
        if cas.kind == caseRecv {
-               recvOK = true
+               recvOK = caseSuccess
+       } else if cas.kind == caseSend && !caseSuccess {
+               goto sclose
        }
 
        selunlock(scases, lockorder)