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// Copyright 2009 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 runtime
// This file contains the implementation of Go select statements.
import (
"unsafe"
)
const debugSelect = false
// scase.kind values.
// Known to compiler.
// Changes here must also be made in src/cmd/compile/internal/gc/select.go's walkselect.
const (
caseNil = iota
caseRecv
caseSend
caseDefault
)
// Select case descriptor.
// Known to compiler.
// Changes here must also be made in src/cmd/internal/gc/select.go's scasetype.
type scase struct {
c *hchan // chan
elem unsafe.Pointer // data element
kind uint16
pc uintptr // race pc (for race detector / msan)
releasetime int64
}
var (
chansendpc = funcPC(chansend)
chanrecvpc = funcPC(chanrecv)
)
func selectsetpc(cas *scase) {
cas.pc = getcallerpc()
}
func sellock(scases []scase, lockorder []uint16) {
var c *hchan
for _, o := range lockorder {
c0 := scases[o].c
if c0 != nil && c0 != c {
c = c0
lock(&c.lock)
}
}
}
func selunlock(scases []scase, lockorder []uint16) {
// We must be very careful here to not touch sel after we have unlocked
// the last lock, because sel can be freed right after the last unlock.
// Consider the following situation.
// First M calls runtime·park() in runtime·selectgo() passing the sel.
// Once runtime·park() has unlocked the last lock, another M makes
// the G that calls select runnable again and schedules it for execution.
// When the G runs on another M, it locks all the locks and frees sel.
// Now if the first M touches sel, it will access freed memory.
for i := len(scases) - 1; i >= 0; i-- {
c := scases[lockorder[i]].c
if c == nil {
break
}
if i > 0 && c == scases[lockorder[i-1]].c {
continue // will unlock it on the next iteration
}
unlock(&c.lock)
}
}
func selparkcommit(gp *g, _ unsafe.Pointer) bool {
// This must not access gp's stack (see gopark). In
// particular, it must not access the *hselect. That's okay,
// because by the time this is called, gp.waiting has all
// channels in lock order.
var lastc *hchan
for sg := gp.waiting; sg != nil; sg = sg.waitlink {
if sg.c != lastc && lastc != nil {
// As soon as we unlock the channel, fields in
// any sudog with that channel may change,
// including c and waitlink. Since multiple
// sudogs may have the same channel, we unlock
// only after we've passed the last instance
// of a channel.
unlock(&lastc.lock)
}
lastc = sg.c
}
if lastc != nil {
unlock(&lastc.lock)
}
return true
}
func block() {
gopark(nil, nil, waitReasonSelectNoCases, traceEvGoStop, 1) // forever
}
// selectgo implements the select statement.
//
// cas0 points to an array of type [ncases]scase, and order0 points to
// an array of type [2*ncases]uint16. Both reside on the goroutine's
// stack (regardless of any escaping in selectgo).
//
// selectgo returns the index of the chosen scase, which matches the
// ordinal position of its respective select{recv,send,default} call.
// Also, if the chosen scase was a receive operation, it reports whether
// a value was received.
func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
if debugSelect {
print("select: cas0=", cas0, "\n")
}
cas1 := (*[1 << 16]scase)(unsafe.Pointer(cas0))
order1 := (*[1 << 17]uint16)(unsafe.Pointer(order0))
scases := cas1[:ncases:ncases]
pollorder := order1[:ncases:ncases]
lockorder := order1[ncases:][:ncases:ncases]
// Replace send/receive cases involving nil channels with
// caseNil so logic below can assume non-nil channel.
for i := range scases {
cas := &scases[i]
if cas.c == nil && cas.kind != caseDefault {
*cas = scase{}
}
}
var t0 int64
if blockprofilerate > 0 {
t0 = cputicks()
for i := 0; i < ncases; i++ {
scases[i].releasetime = -1
}
}
// The compiler rewrites selects that statically have
// only 0 or 1 cases plus default into simpler constructs.
// The only way we can end up with such small sel.ncase
// values here is for a larger select in which most channels
// have been nilled out. The general code handles those
// cases correctly, and they are rare enough not to bother
// optimizing (and needing to test).
// generate permuted order
for i := 1; i < ncases; i++ {
j := fastrandn(uint32(i + 1))
pollorder[i] = pollorder[j]
pollorder[j] = uint16(i)
}
// sort the cases by Hchan address to get the locking order.
// simple heap sort, to guarantee n log n time and constant stack footprint.
for i := 0; i < ncases; i++ {
j := i
// Start with the pollorder to permute cases on the same channel.
c := scases[pollorder[i]].c
for j > 0 && scases[lockorder[(j-1)/2]].c.sortkey() < c.sortkey() {
k := (j - 1) / 2
lockorder[j] = lockorder[k]
j = k
}
lockorder[j] = pollorder[i]
}
for i := ncases - 1; i >= 0; i-- {
o := lockorder[i]
c := scases[o].c
lockorder[i] = lockorder[0]
j := 0
for {
k := j*2 + 1
if k >= i {
break
}
if k+1 < i && scases[lockorder[k]].c.sortkey() < scases[lockorder[k+1]].c.sortkey() {
k++
}
if c.sortkey() < scases[lockorder[k]].c.sortkey() {
lockorder[j] = lockorder[k]
j = k
continue
}
break
}
lockorder[j] = o
}
if debugSelect {
for i := 0; i+1 < ncases; i++ {
if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() {
print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
throw("select: broken sort")
}
}
}
// lock all the channels involved in the select
sellock(scases, lockorder)
var (
gp *g
sg *sudog
c *hchan
k *scase
sglist *sudog
sgnext *sudog
qp unsafe.Pointer
nextp **sudog
)
loop:
// pass 1 - look for something already waiting
var dfli int
var dfl *scase
var casi int
var cas *scase
var recvOK bool
for i := 0; i < ncases; i++ {
casi = int(pollorder[i])
cas = &scases[casi]
c = cas.c
switch cas.kind {
case caseNil:
continue
case caseRecv:
sg = c.sendq.dequeue()
if sg != nil {
goto recv
}
if c.qcount > 0 {
goto bufrecv
}
if c.closed != 0 {
goto rclose
}
case caseSend:
if raceenabled {
racereadpc(c.raceaddr(), cas.pc, chansendpc)
}
if c.closed != 0 {
goto sclose
}
sg = c.recvq.dequeue()
if sg != nil {
goto send
}
if c.qcount < c.dataqsiz {
goto bufsend
}
case caseDefault:
dfli = casi
dfl = cas
}
}
if dfl != nil {
selunlock(scases, lockorder)
casi = dfli
cas = dfl
goto retc
}
// pass 2 - enqueue on all chans
gp = getg()
if gp.waiting != nil {
throw("gp.waiting != nil")
}
nextp = &gp.waiting
for _, casei := range lockorder {
casi = int(casei)
cas = &scases[casi]
if cas.kind == caseNil {
continue
}
c = cas.c
sg := acquireSudog()
sg.g = gp
sg.isSelect = true
// No stack splits between assigning elem and enqueuing
// sg on gp.waiting where copystack can find it.
sg.elem = cas.elem
sg.releasetime = 0
if t0 != 0 {
sg.releasetime = -1
}
sg.c = c
// Construct waiting list in lock order.
*nextp = sg
nextp = &sg.waitlink
switch cas.kind {
case caseRecv:
c.recvq.enqueue(sg)
case caseSend:
c.sendq.enqueue(sg)
}
}
// wait for someone to wake us up
gp.param = nil
gopark(selparkcommit, nil, waitReasonSelect, traceEvGoBlockSelect, 1)
sellock(scases, lockorder)
gp.selectDone = 0
sg = (*sudog)(gp.param)
gp.param = nil
// pass 3 - dequeue from unsuccessful chans
// otherwise they stack up on quiet channels
// record the successful case, if any.
// We singly-linked up the SudoGs in lock order.
casi = -1
cas = nil
sglist = gp.waiting
// Clear all elem before unlinking from gp.waiting.
for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
sg1.isSelect = false
sg1.elem = nil
sg1.c = nil
}
gp.waiting = nil
for _, casei := range lockorder {
k = &scases[casei]
if k.kind == caseNil {
continue
}
if sglist.releasetime > 0 {
k.releasetime = sglist.releasetime
}
if sg == sglist {
// sg has already been dequeued by the G that woke us up.
casi = int(casei)
cas = k
} else {
c = k.c
if k.kind == caseSend {
c.sendq.dequeueSudoG(sglist)
} else {
c.recvq.dequeueSudoG(sglist)
}
}
sgnext = sglist.waitlink
sglist.waitlink = nil
releaseSudog(sglist)
sglist = sgnext
}
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
}
c = cas.c
if debugSelect {
print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " kind=", cas.kind, "\n")
}
if cas.kind == caseRecv {
recvOK = true
}
if raceenabled {
if cas.kind == caseRecv && cas.elem != nil {
raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
} else if cas.kind == caseSend {
raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
}
}
if msanenabled {
if cas.kind == caseRecv && cas.elem != nil {
msanwrite(cas.elem, c.elemtype.size)
} else if cas.kind == caseSend {
msanread(cas.elem, c.elemtype.size)
}
}
selunlock(scases, lockorder)
goto retc
bufrecv:
// can receive from buffer
if raceenabled {
if cas.elem != nil {
raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
}
raceacquire(chanbuf(c, c.recvx))
racerelease(chanbuf(c, c.recvx))
}
if msanenabled && cas.elem != nil {
msanwrite(cas.elem, c.elemtype.size)
}
recvOK = true
qp = chanbuf(c, c.recvx)
if cas.elem != nil {
typedmemmove(c.elemtype, cas.elem, qp)
}
typedmemclr(c.elemtype, qp)
c.recvx++
if c.recvx == c.dataqsiz {
c.recvx = 0
}
c.qcount--
selunlock(scases, lockorder)
goto retc
bufsend:
// can send to buffer
if raceenabled {
raceacquire(chanbuf(c, c.sendx))
racerelease(chanbuf(c, c.sendx))
raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
}
if msanenabled {
msanread(cas.elem, c.elemtype.size)
}
typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem)
c.sendx++
if c.sendx == c.dataqsiz {
c.sendx = 0
}
c.qcount++
selunlock(scases, lockorder)
goto retc
recv:
// can receive from sleeping sender (sg)
recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
if debugSelect {
print("syncrecv: cas0=", cas0, " c=", c, "\n")
}
recvOK = true
goto retc
rclose:
// read at end of closed channel
selunlock(scases, lockorder)
recvOK = false
if cas.elem != nil {
typedmemclr(c.elemtype, cas.elem)
}
if raceenabled {
raceacquire(c.raceaddr())
}
goto retc
send:
// can send to a sleeping receiver (sg)
if raceenabled {
raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
}
if msanenabled {
msanread(cas.elem, c.elemtype.size)
}
send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
if debugSelect {
print("syncsend: cas0=", cas0, " c=", c, "\n")
}
goto retc
retc:
if cas.releasetime > 0 {
blockevent(cas.releasetime-t0, 1)
}
return casi, recvOK
sclose:
// send on closed channel
selunlock(scases, lockorder)
panic(plainError("send on closed channel"))
}
func (c *hchan) sortkey() uintptr {
// TODO(khr): if we have a moving garbage collector, we'll need to
// change this function.
return uintptr(unsafe.Pointer(c))
}
// A runtimeSelect is a single case passed to rselect.
// This must match ../reflect/value.go:/runtimeSelect
type runtimeSelect struct {
dir selectDir
typ unsafe.Pointer // channel type (not used here)
ch *hchan // channel
val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
}
// These values must match ../reflect/value.go:/SelectDir.
type selectDir int
const (
_ selectDir = iota
selectSend // case Chan <- Send
selectRecv // case <-Chan:
selectDefault // default
)
//go:linkname reflect_rselect reflect.rselect
func reflect_rselect(cases []runtimeSelect) (int, bool) {
if len(cases) == 0 {
block()
}
sel := make([]scase, len(cases))
order := make([]uint16, 2*len(cases))
for i := range cases {
rc := &cases[i]
switch rc.dir {
case selectDefault:
sel[i] = scase{kind: caseDefault}
case selectSend:
sel[i] = scase{kind: caseSend, c: rc.ch, elem: rc.val}
case selectRecv:
sel[i] = scase{kind: caseRecv, c: rc.ch, elem: rc.val}
}
if raceenabled || msanenabled {
selectsetpc(&sel[i])
}
}
return selectgo(&sel[0], &order[0], len(cases))
}
func (q *waitq) dequeueSudoG(sgp *sudog) {
x := sgp.prev
y := sgp.next
if x != nil {
if y != nil {
// middle of queue
x.next = y
y.prev = x
sgp.next = nil
sgp.prev = nil
return
}
// end of queue
x.next = nil
q.last = x
sgp.prev = nil
return
}
if y != nil {
// start of queue
y.prev = nil
q.first = y
sgp.next = nil
return
}
// x==y==nil. Either sgp is the only element in the queue,
// or it has already been removed. Use q.first to disambiguate.
if q.first == sgp {
q.first = nil
q.last = nil
}
}