From 3b2d8145a4c349058d76ce299ea7eea605572004 Mon Sep 17 00:00:00 2001 From: Ian Lance Taylor Date: Mon, 21 Dec 2020 21:20:59 -0800 Subject: [PATCH] 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 --- gcc/go/gofrontend/MERGE | 2 +- libgo/go/runtime/chan.go | 25 +++++++++++++++---------- libgo/go/runtime/runtime2.go | 6 ++++++ libgo/go/runtime/select.go | 19 +++++++------------ 4 files changed, 29 insertions(+), 23 deletions(-) diff --git a/gcc/go/gofrontend/MERGE b/gcc/go/gofrontend/MERGE index a7df8433403..bb537f152b9 100644 --- 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 8e104f14140..1b20aff6ea3 100644 --- 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 bf3fbac14c8..f30d1bcde6e 100644 --- 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 d5087fbc045..c31aa344b55 100644 --- 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) -- 2.11.4.GIT