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runtime: simplify chan ops, take 2

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This change is the same as CL #9345 which was reverted,
except for a small bug fix.

The only change is to the body of sendDirect and its callsite.
Also added a test.

The problem was during a channel send operation.  The target
of the send was a sleeping goroutine waiting to receive.  We
basically do:
1) Read the destination pointer out of the sudog structure
2) Copy the value we're sending to that destination pointer
Unfortunately, the previous change had a goroutine suspend
point between 1 & 2 (the call to sendDirect).  At that point
the destination goroutine's stack could be copied (shrunk).
The pointer we read in step 1 is no longer valid for step 2.

Fixed by not allowing any suspension points between 1 & 2.
I suspect the old code worked correctly basically by accident.

Fixes #13169

The original 9345:

This change removes the retry mechanism we use for buffered channels.
Instead, any sender waking up a receiver or vice versa completes the
full protocol with its counterpart.  This means the counterpart does
not need to relock the channel when it wakes up.  (Currently
buffered channels need to relock on wakeup.)

For sends on a channel with waiting receivers, this change replaces
two copies (sender->queue, queue->receiver) with one (sender->receiver).
For receives on channels with a waiting sender, two copies are still required.

This change unifies to a large degree the algorithm for buffered
and unbuffered channels, simplifying the overall implementation.

Fixes #11506

Change-Id: I57dfa3fc219cffa4d48301ee15fe5479299efa09
Reviewed-on: https://go-review.googlesource.com/16740
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Keith Randall 2015-11-07 21:28:21 -08:00
parent 1b4d28f8cf
commit e410a527b2
3 changed files with 328 additions and 359 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

520
src/runtime/chan.go
View file

@ -6,6 +6,11 @@ package runtime
// This file contains the implementation of Go channels.
// Invariants:
// At least one of c.sendq and c.recvq is empty.
// For buffered channels, also:
// c.qcount > 0 implies that c.recvq is empty.
// c.qcount < c.dataqsiz implies that c.sendq is empty.
import "unsafe"
const (
@ -153,135 +158,117 @@ func chansend(t *chantype, c *hchan, ep unsafe.Pointer, block bool, callerpc uin
}
lock(&c.lock)
if c.closed != 0 {
unlock(&c.lock)
panic("send on closed channel")
}
if c.dataqsiz == 0 { // synchronous channel
sg := c.recvq.dequeue()
if sg != nil { // found a waiting receiver
if raceenabled {
racesync(c, sg)
}
unlock(&c.lock)
recvg := sg.g
if sg.elem != nil {
syncsend(c, sg, ep)
}
recvg.param = unsafe.Pointer(sg)
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
goready(recvg, 3)
return true
}
if !block {
unlock(&c.lock)
return false
}
// no receiver available: block on this channel.
gp := getg()
mysg := acquireSudog()
mysg.releasetime = 0
if t0 != 0 {
mysg.releasetime = -1
}
mysg.elem = ep
mysg.waitlink = nil
gp.waiting = mysg
mysg.g = gp
mysg.selectdone = nil
gp.param = nil
c.sendq.enqueue(mysg)
goparkunlock(&c.lock, "chan send", traceEvGoBlockSend, 3)
// someone woke us up.
if mysg != gp.waiting {
throw("G waiting list is corrupted!")
}
gp.waiting = nil
if gp.param == nil {
if c.closed == 0 {
throw("chansend: spurious wakeup")
}
panic("send on closed channel")
}
gp.param = nil
if mysg.releasetime > 0 {
blockevent(int64(mysg.releasetime)-t0, 2)
}
releaseSudog(mysg)
if sg := c.recvq.dequeue(); sg != nil {
// Found a waiting receiver. We pass the value we want to send
// directly to the receiver, bypassing the channel buffer (if any).
send(c, sg, ep, func() { unlock(&c.lock) })
return true
}
// asynchronous channel
// wait for some space to write our data
var t1 int64
for futile := byte(0); c.qcount >= c.dataqsiz; futile = traceFutileWakeup {
if !block {
unlock(&c.lock)
return false
if c.qcount < c.dataqsiz {
// Space is available in the channel buffer. Enqueue the element to send.
qp := chanbuf(c, c.sendx)
if raceenabled {
raceacquire(qp)
racerelease(qp)
}
gp := getg()
mysg := acquireSudog()
mysg.releasetime = 0
if t0 != 0 {
mysg.releasetime = -1
typedmemmove(c.elemtype, qp, ep)
c.sendx++
if c.sendx == c.dataqsiz {
c.sendx = 0
}
mysg.g = gp
mysg.elem = nil
mysg.selectdone = nil
c.sendq.enqueue(mysg)
goparkunlock(&c.lock, "chan send", traceEvGoBlockSend|futile, 3)
// someone woke us up - try again
if mysg.releasetime > 0 {
t1 = mysg.releasetime
}
releaseSudog(mysg)
lock(&c.lock)
if c.closed != 0 {
unlock(&c.lock)
panic("send on closed channel")
}
}
// write our data into the channel buffer
if raceenabled {
raceacquire(chanbuf(c, c.sendx))
racerelease(chanbuf(c, c.sendx))
}
typedmemmove(c.elemtype, chanbuf(c, c.sendx), ep)
c.sendx++
if c.sendx == c.dataqsiz {
c.sendx = 0
}
c.qcount++
// wake up a waiting receiver
sg := c.recvq.dequeue()
if sg != nil {
recvg := sg.g
c.qcount++
unlock(&c.lock)
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
goready(recvg, 3)
} else {
return true
}
if !block {
unlock(&c.lock)
return false
}
if t1 > 0 {
blockevent(t1-t0, 2)
// Block on the channel. Some receiver will complete our operation for us.
gp := getg()
mysg := acquireSudog()
mysg.releasetime = 0
if t0 != 0 {
mysg.releasetime = -1
}
mysg.elem = ep
mysg.waitlink = nil
mysg.g = gp
mysg.selectdone = nil
gp.waiting = mysg
gp.param = nil
c.sendq.enqueue(mysg)
goparkunlock(&c.lock, "chan send", traceEvGoBlockSend, 3)
// someone woke us up.
if mysg != gp.waiting {
throw("G waiting list is corrupted")
}
gp.waiting = nil
if gp.param == nil {
if c.closed == 0 {
throw("chansend: spurious wakeup")
}
panic("send on closed channel")
}
gp.param = nil
if mysg.releasetime > 0 {
blockevent(int64(mysg.releasetime)-t0, 2)
}
releaseSudog(mysg)
return true
}
func syncsend(c *hchan, sg *sudog, elem unsafe.Pointer) {
// Send on unbuffered channel is the only operation
// send processes a send operation on an empty channel c.
// The value ep sent by the sender is copied to the receiver sg.
// The receiver is then woken up to go on its merry way.
// Channel c must be empty and locked. send unlocks c with unlockf.
// sg must already be dequeued from c.
// ep must be non-nil and point to the heap or the caller's stack.
func send(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func()) {
if raceenabled {
if c.dataqsiz == 0 {
racesync(c, sg)
} else {
// Pretend we go through the buffer, even though
// we copy directly. Note that we need to increment
// the head/tail locations only when raceenabled.
qp := chanbuf(c, c.recvx)
raceacquire(qp)
racerelease(qp)
raceacquireg(sg.g, qp)
racereleaseg(sg.g, qp)
c.recvx++
if c.recvx == c.dataqsiz {
c.recvx = 0
}
c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz
}
}
unlockf()
if sg.elem != nil {
sendDirect(c.elemtype, sg, ep)
sg.elem = nil
}
gp := sg.g
gp.param = unsafe.Pointer(sg)
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
goready(gp, 4)
}
func sendDirect(t *_type, sg *sudog, src unsafe.Pointer) {
// Send on an unbuffered or empty-buffered channel is the only operation
// in the entire runtime where one goroutine
// writes to the stack of another goroutine. The GC assumes that
// stack writes only happen when the goroutine is running and are
@ -290,9 +277,13 @@ func syncsend(c *hchan, sg *sudog, elem unsafe.Pointer) {
// typedmemmove will call heapBitsBulkBarrier, but the target bytes
// are not in the heap, so that will not help. We arrange to call
// memmove and typeBitsBulkBarrier instead.
memmove(sg.elem, elem, c.elemtype.size)
typeBitsBulkBarrier(c.elemtype, uintptr(sg.elem), c.elemtype.size)
sg.elem = nil
// Once we read sg.elem out of sg, it will no longer
// be updated if the destination's stack gets copied (shrunk).
// So make sure that no preemption points can happen between read & use.
dst := sg.elem
memmove(dst, src, t.size)
typeBitsBulkBarrier(t, uintptr(dst), t.size)
}
func closechan(c *hchan) {
@ -320,27 +311,36 @@ func closechan(c *hchan) {
if sg == nil {
break
}
gp := sg.g
sg.elem = nil
gp.param = nil
if sg.elem != nil {
memclr(sg.elem, uintptr(c.elemsize))
sg.elem = nil
}
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
gp := sg.g
gp.param = nil
if raceenabled {
raceacquireg(gp, unsafe.Pointer(c))
}
goready(gp, 3)
}
// release all writers
// release all writers (they will panic)
for {
sg := c.sendq.dequeue()
if sg == nil {
break
}
gp := sg.g
sg.elem = nil
gp.param = nil
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
gp := sg.g
gp.param = nil
if raceenabled {
raceacquireg(gp, unsafe.Pointer(c))
}
goready(gp, 3)
}
unlock(&c.lock)
@ -363,8 +363,10 @@ func chanrecv2(t *chantype, c *hchan, elem unsafe.Pointer) (received bool) {
// If block == false and no elements are available, returns (false, false).
// Otherwise, if c is closed, zeros *ep and returns (true, false).
// Otherwise, fills in *ep with an element and returns (true, true).
// A non-nil ep must point to the heap or the caller's stack.
func chanrecv(t *chantype, c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) {
// raceenabled: don't need to check ep, as it is always on the stack.
// raceenabled: don't need to check ep, as it is always on the stack
// or is new memory allocated by reflect.
if debugChan {
print("chanrecv: chan=", c, "\n")
@ -402,167 +404,139 @@ func chanrecv(t *chantype, c *hchan, ep unsafe.Pointer, block bool) (selected, r
}
lock(&c.lock)
if c.dataqsiz == 0 { // synchronous channel
if c.closed != 0 {
return recvclosed(c, ep)
if c.closed != 0 && c.qcount == 0 {
if raceenabled {
raceacquire(unsafe.Pointer(c))
}
sg := c.sendq.dequeue()
if sg != nil {
if raceenabled {
racesync(c, sg)
}
unlock(&c.lock)
if ep != nil {
typedmemmove(c.elemtype, ep, sg.elem)
}
sg.elem = nil
gp := sg.g
gp.param = unsafe.Pointer(sg)
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
goready(gp, 3)
selected = true
received = true
return
}
if !block {
unlock(&c.lock)
return
}
// no sender available: block on this channel.
gp := getg()
mysg := acquireSudog()
mysg.releasetime = 0
if t0 != 0 {
mysg.releasetime = -1
}
mysg.elem = ep
mysg.waitlink = nil
gp.waiting = mysg
mysg.g = gp
mysg.selectdone = nil
gp.param = nil
c.recvq.enqueue(mysg)
goparkunlock(&c.lock, "chan receive", traceEvGoBlockRecv, 3)
// someone woke us up
if mysg != gp.waiting {
throw("G waiting list is corrupted!")
}
gp.waiting = nil
if mysg.releasetime > 0 {
blockevent(mysg.releasetime-t0, 2)
}
haveData := gp.param != nil
gp.param = nil
releaseSudog(mysg)
if haveData {
// a sender sent us some data. It already wrote to ep.
selected = true
received = true
return
}
lock(&c.lock)
if c.closed == 0 {
throw("chanrecv: spurious wakeup")
}
return recvclosed(c, ep)
}
// asynchronous channel
// wait for some data to appear
var t1 int64
for futile := byte(0); c.qcount <= 0; futile = traceFutileWakeup {
if c.closed != 0 {
selected, received = recvclosed(c, ep)
if t1 > 0 {
blockevent(t1-t0, 2)
}
return
}
if !block {
unlock(&c.lock)
return
}
// wait for someone to send an element
gp := getg()
mysg := acquireSudog()
mysg.releasetime = 0
if t0 != 0 {
mysg.releasetime = -1
}
mysg.elem = nil
mysg.g = gp
mysg.selectdone = nil
c.recvq.enqueue(mysg)
goparkunlock(&c.lock, "chan receive", traceEvGoBlockRecv|futile, 3)
// someone woke us up - try again
if mysg.releasetime > 0 {
t1 = mysg.releasetime
}
releaseSudog(mysg)
lock(&c.lock)
}
if raceenabled {
raceacquire(chanbuf(c, c.recvx))
racerelease(chanbuf(c, c.recvx))
}
if ep != nil {
typedmemmove(c.elemtype, ep, chanbuf(c, c.recvx))
}
memclr(chanbuf(c, c.recvx), uintptr(c.elemsize))
c.recvx++
if c.recvx == c.dataqsiz {
c.recvx = 0
}
c.qcount--
// ping a sender now that there is space
sg := c.sendq.dequeue()
if sg != nil {
gp := sg.g
unlock(&c.lock)
if sg.releasetime != 0 {
sg.releasetime = cputicks()
if ep != nil {
memclr(ep, uintptr(c.elemsize))
}
goready(gp, 3)
} else {
unlock(&c.lock)
return true, false
}
if t1 > 0 {
blockevent(t1-t0, 2)
if sg := c.sendq.dequeue(); sg != nil {
// Found a waiting sender. If buffer is size 0, receive value
// directly from sender. Otherwise, recieve from head of queue
// and add sender's value to the tail of the queue (both map to
// the same buffer slot because the queue is full).
recv(c, sg, ep, func() { unlock(&c.lock) })
return true, true
}
selected = true
received = true
return
if c.qcount > 0 {
// Receive directly from queue
qp := chanbuf(c, c.recvx)
if raceenabled {
raceacquire(qp)
racerelease(qp)
}
if ep != nil {
typedmemmove(c.elemtype, ep, qp)
}
memclr(qp, uintptr(c.elemsize))
c.recvx++
if c.recvx == c.dataqsiz {
c.recvx = 0
}
c.qcount--
unlock(&c.lock)
return true, true
}
if !block {
unlock(&c.lock)
return false, false
}
// no sender available: block on this channel.
gp := getg()
mysg := acquireSudog()
mysg.releasetime = 0
if t0 != 0 {
mysg.releasetime = -1
}
mysg.elem = ep
mysg.waitlink = nil
gp.waiting = mysg
mysg.g = gp
mysg.selectdone = nil
gp.param = nil
c.recvq.enqueue(mysg)
goparkunlock(&c.lock, "chan receive", traceEvGoBlockRecv, 3)
// someone woke us up
if mysg != gp.waiting {
throw("G waiting list is corrupted")
}
gp.waiting = nil
if mysg.releasetime > 0 {
blockevent(mysg.releasetime-t0, 2)
}
closed := gp.param == nil
gp.param = nil
releaseSudog(mysg)
return true, !closed
}
// recvclosed is a helper function for chanrecv. Handles cleanup
// when the receiver encounters a closed channel.
// Caller must hold c.lock, recvclosed will release the lock.
func recvclosed(c *hchan, ep unsafe.Pointer) (selected, recevied bool) {
if raceenabled {
raceacquire(unsafe.Pointer(c))
// recv processes a receive operation on a full channel c.
// There are 2 parts:
// 1) The value sent by the sender sg is put into the channel
// and the sender is woken up to go on its merry way.
// 2) The value received by the receiver (the current G) is
// written to ep.
// For synchronous channels, both values are the same.
// For asynchronous channels, the receiver gets its data from
// the channel buffer and the sender's data is put in the
// channel buffer.
// Channel c must be full and locked. recv unlocks c with unlockf.
// sg must already be dequeued from c.
// A non-nil ep must point to the heap or the caller's stack.
func recv(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func()) {
if c.dataqsiz == 0 {
if raceenabled {
racesync(c, sg)
}
unlockf()
if ep != nil {
// copy data from sender
// ep points to our own stack or heap, so nothing
// special (ala sendDirect) needed here.
typedmemmove(c.elemtype, ep, sg.elem)
}
} else {
// Queue is full. Take the item at the
// head of the queue. Make the sender enqueue
// its item at the tail of the queue. Since the
// queue is full, those are both the same slot.
qp := chanbuf(c, c.recvx)
if raceenabled {
raceacquire(qp)
racerelease(qp)
raceacquireg(sg.g, qp)
racereleaseg(sg.g, qp)
}
// copy data from queue to receiver
if ep != nil {
typedmemmove(c.elemtype, ep, qp)
}
// copy data from sender to queue
typedmemmove(c.elemtype, qp, sg.elem)
c.recvx++
if c.recvx == c.dataqsiz {
c.recvx = 0
}
c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz
unlockf()
}
unlock(&c.lock)
if ep != nil {
memclr(ep, uintptr(c.elemsize))
sg.elem = nil
gp := sg.g
gp.param = unsafe.Pointer(sg)
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
return true, false
goready(gp, 4)
}
// compiler implements

118
src/runtime/select.go
View file

@ -304,7 +304,7 @@ func selectgoImpl(sel *hselect) (uintptr, uint16) {
k *scase
sglist *sudog
sgnext *sudog
futile byte
qp unsafe.Pointer
)
loop:
@ -317,15 +317,12 @@ loop:
switch cas.kind {
case caseRecv:
if c.dataqsiz > 0 {
if c.qcount > 0 {
goto asyncrecv
}
} else {
sg = c.sendq.dequeue()
if sg != nil {
goto syncrecv
}
sg = c.sendq.dequeue()
if sg != nil {
goto recv
}
if c.qcount > 0 {
goto bufrecv
}
if c.closed != 0 {
goto rclose
@ -338,15 +335,12 @@ loop:
if c.closed != 0 {
goto sclose
}
if c.dataqsiz > 0 {
if c.qcount < c.dataqsiz {
goto asyncsend
}
} else {
sg = c.recvq.dequeue()
if sg != nil {
goto syncsend
}
sg = c.recvq.dequeue()
if sg != nil {
goto send
}
if c.qcount < c.dataqsiz {
goto bufsend
}
case caseDefault:
@ -363,6 +357,9 @@ loop:
// pass 2 - enqueue on all chans
gp = getg()
done = 0
if gp.waiting != nil {
throw("gp.waiting != nil")
}
for i := 0; i < int(sel.ncase); i++ {
cas = &scases[pollorder[i]]
c = cas.c
@ -389,7 +386,7 @@ loop:
// wait for someone to wake us up
gp.param = nil
gopark(selparkcommit, unsafe.Pointer(sel), "select", traceEvGoBlockSelect|futile, 2)
gopark(selparkcommit, unsafe.Pointer(sel), "select", traceEvGoBlockSelect, 2)
// someone woke us up
sellock(sel)
@ -432,16 +429,13 @@ loop:
}
if cas == nil {
futile = traceFutileWakeup
// This can happen if we were woken up by a close().
// TODO: figure that out explicitly so we don't need this loop.
goto loop
}
c = cas.c
if c.dataqsiz > 0 {
throw("selectgo: shouldn't happen")
}
if debugSelect {
print("wait-return: sel=", sel, " c=", c, " cas=", cas, " kind=", cas.kind, "\n")
}
@ -470,7 +464,7 @@ loop:
selunlock(sel)
goto retc
asyncrecv:
bufrecv:
// can receive from buffer
if raceenabled {
if cas.elem != nil {
@ -485,29 +479,20 @@ asyncrecv:
if cas.receivedp != nil {
*cas.receivedp = true
}
qp = chanbuf(c, c.recvx)
if cas.elem != nil {
typedmemmove(c.elemtype, cas.elem, chanbuf(c, c.recvx))
typedmemmove(c.elemtype, cas.elem, qp)
}
memclr(chanbuf(c, c.recvx), uintptr(c.elemsize))
memclr(qp, uintptr(c.elemsize))
c.recvx++
if c.recvx == c.dataqsiz {
c.recvx = 0
}
c.qcount--
sg = c.sendq.dequeue()
if sg != nil {
gp = sg.g
selunlock(sel)
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
goready(gp, 3)
} else {
selunlock(sel)
}
selunlock(sel)
goto retc
asyncsend:
bufsend:
// can send to buffer
if raceenabled {
raceacquire(chanbuf(c, c.sendx))
@ -523,47 +508,18 @@ asyncsend:
c.sendx = 0
}
c.qcount++
sg = c.recvq.dequeue()
if sg != nil {
gp = sg.g
selunlock(sel)
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
goready(gp, 3)
} else {
selunlock(sel)
}
selunlock(sel)
goto retc
syncrecv:
recv:
// can receive from sleeping sender (sg)
if raceenabled {
if cas.elem != nil {
raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
}
racesync(c, sg)
}
if msanenabled && cas.elem != nil {
msanwrite(cas.elem, c.elemtype.size)
}
selunlock(sel)
recv(c, sg, cas.elem, func() { selunlock(sel) })
if debugSelect {
print("syncrecv: sel=", sel, " c=", c, "\n")
}
if cas.receivedp != nil {
*cas.receivedp = true
}
if cas.elem != nil {
typedmemmove(c.elemtype, cas.elem, sg.elem)
}
sg.elem = nil
gp = sg.g
gp.param = unsafe.Pointer(sg)
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
goready(gp, 3)
goto retc
rclose:
@ -580,29 +536,19 @@ rclose:
}
goto retc
syncsend:
// can send to sleeping receiver (sg)
send:
// can send to a sleeping receiver (sg)
if raceenabled {
raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
racesync(c, sg)
}
if msanenabled {
msanread(cas.elem, c.elemtype.size)
}
selunlock(sel)
send(c, sg, cas.elem, func() { selunlock(sel) })
if debugSelect {
print("syncsend: sel=", sel, " c=", c, "\n")
}
if sg.elem != nil {
syncsend(c, sg, cas.elem)
}
sg.elem = nil
gp = sg.g
gp.param = unsafe.Pointer(sg)
if sg.releasetime != 0 {
sg.releasetime = cputicks()
}
goready(gp, 3)
goto retc
retc:
if cas.releasetime > 0 {

49
test/fixedbugs/issue13169.go Normal file
View file

@ -0,0 +1,49 @@
// run
// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main
type T struct {
a, b, c int
}
func usestack() {
usestack1(32)
}
func usestack1(d int) byte {
if d == 0 {
return 0
}
var b [1024]byte
usestack1(d - 1)
return b[3]
}
const n = 100000
func main() {
c := make(chan interface{})
done := make(chan bool)
for i := 0; i < 10; i++ {
go func() {
for j := 0; j < n; j++ {
c <- new(T)
}
done <- true
}()
go func() {
for j := 0; j < n; j++ {
_ = (<-c).(*T)
usestack()
}
done <- true
}()
}
for i := 0; i < 20; i++ {
<-done
}
}