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>
diff --git a/src/runtime/chan.go b/src/runtime/chan.go
index f6f4ffd..0afe5d9 100644
--- a/src/runtime/chan.go
+++ b/src/runtime/chan.go
@@ -263,18 +263,19 @@
}
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
}
@@ -311,6 +312,7 @@
gp := sg.g
unlockf()
gp.param = unsafe.Pointer(sg)
+ sg.success = true
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
@@ -384,7 +386,8 @@
sg.releasetime = cputicks()
}
gp := sg.g
- gp.param = nil
+ gp.param = unsafe.Pointer(sg)
+ sg.success = false
if raceenabled {
raceacquireg(gp, c.raceaddr())
}
@@ -402,7 +405,8 @@
sg.releasetime = cputicks()
}
gp := sg.g
- gp.param = nil
+ gp.param = unsafe.Pointer(sg)
+ sg.success = false
if raceenabled {
raceacquireg(gp, c.raceaddr())
}
@@ -575,11 +579,11 @@
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.
@@ -632,6 +636,7 @@
gp := sg.g
unlockf()
gp.param = unsafe.Pointer(sg)
+ sg.success = true
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index 9598784..b7d0739 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -366,6 +366,12 @@
// 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/src/runtime/select.go b/src/runtime/select.go
index a069e3e..081db7b 100644
--- a/src/runtime/select.go
+++ b/src/runtime/select.go
@@ -221,12 +221,12 @@
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 @@
// 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 @@
// 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 @@
}
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 @@
}
if cas.kind == caseRecv {
- recvOK = true
+ recvOK = caseSuccess
+ } else if cas.kind == caseSend && !caseSuccess {
+ goto sclose
}
if raceenabled {
