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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 (
"runtime/internal/sys"
"unsafe"
)
const (
debugSelect = false
// scase.kind
caseRecv = iota
caseSend
caseDefault
)
// Select statement header.
// Known to compiler.
// Changes here must also be made in src/cmd/internal/gc/select.go's selecttype.
type hselect struct {
tcase uint16 // total count of scase[]
ncase uint16 // currently filled scase[]
pollorder *uint16 // case poll order
lockorder *uint16 // channel lock order
scase [1]scase // one per case (in order of appearance)
}
// Select case descriptor.
// Known to compiler.
// Changes here must also be made in src/cmd/internal/gc/select.go's selecttype.
type scase struct {
elem unsafe.Pointer // data element
c *hchan // chan
pc uintptr // return pc
kind uint16
so uint16 // vararg of selected bool
receivedp *bool // pointer to received bool (recv2)
releasetime int64
}
var (
chansendpc = funcPC(chansend)
chanrecvpc = funcPC(chanrecv)
)
func selectsize(size uintptr) uintptr {
selsize := unsafe.Sizeof(hselect{}) +
(size-1)*unsafe.Sizeof(hselect{}.scase[0]) +
size*unsafe.Sizeof(*hselect{}.lockorder) +
size*unsafe.Sizeof(*hselect{}.pollorder)
return round(selsize, sys.Int64Align)
}
func newselect(sel *hselect, selsize int64, size int32) {
if selsize != int64(selectsize(uintptr(size))) {
print("runtime: bad select size ", selsize, ", want ", selectsize(uintptr(size)), "\n")
throw("bad select size")
}
sel.tcase = uint16(size)
sel.ncase = 0
sel.lockorder = (*uint16)(add(unsafe.Pointer(&sel.scase), uintptr(size)*unsafe.Sizeof(hselect{}.scase[0])))
sel.pollorder = (*uint16)(add(unsafe.Pointer(sel.lockorder), uintptr(size)*unsafe.Sizeof(*hselect{}.lockorder)))
if debugSelect {
print("newselect s=", sel, " size=", size, "\n")
}
}
//go:nosplit
func selectsend(sel *hselect, c *hchan, elem unsafe.Pointer) (selected bool) {
// nil cases do not compete
if c != nil {
selectsendImpl(sel, c, getcallerpc(unsafe.Pointer(&sel)), elem, uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
}
return
}
// cut in half to give stack a chance to split
func selectsendImpl(sel *hselect, c *hchan, pc uintptr, elem unsafe.Pointer, so uintptr) {
i := sel.ncase
if i >= sel.tcase {
throw("selectsend: too many cases")
}
sel.ncase = i + 1
cas := (*scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)*unsafe.Sizeof(sel.scase[0])))
cas.pc = pc
cas.c = c
cas.so = uint16(so)
cas.kind = caseSend
cas.elem = elem
if debugSelect {
print("selectsend s=", sel, " pc=", hex(cas.pc), " chan=", cas.c, " so=", cas.so, "\n")
}
}
//go:nosplit
func selectrecv(sel *hselect, c *hchan, elem unsafe.Pointer) (selected bool) {
// nil cases do not compete
if c != nil {
selectrecvImpl(sel, c, getcallerpc(unsafe.Pointer(&sel)), elem, nil, uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
}
return
}
//go:nosplit
func selectrecv2(sel *hselect, c *hchan, elem unsafe.Pointer, received *bool) (selected bool) {
// nil cases do not compete
if c != nil {
selectrecvImpl(sel, c, getcallerpc(unsafe.Pointer(&sel)), elem, received, uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
}
return
}
func selectrecvImpl(sel *hselect, c *hchan, pc uintptr, elem unsafe.Pointer, received *bool, so uintptr) {
i := sel.ncase
if i >= sel.tcase {
throw("selectrecv: too many cases")
}
sel.ncase = i + 1
cas := (*scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)*unsafe.Sizeof(sel.scase[0])))
cas.pc = pc
cas.c = c
cas.so = uint16(so)
cas.kind = caseRecv
cas.elem = elem
cas.receivedp = received
if debugSelect {
print("selectrecv s=", sel, " pc=", hex(cas.pc), " chan=", cas.c, " so=", cas.so, "\n")
}
}
//go:nosplit
func selectdefault(sel *hselect) (selected bool) {
selectdefaultImpl(sel, getcallerpc(unsafe.Pointer(&sel)), uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
return
}
func selectdefaultImpl(sel *hselect, callerpc uintptr, so uintptr) {
i := sel.ncase
if i >= sel.tcase {
throw("selectdefault: too many cases")
}
sel.ncase = i + 1
cas := (*scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)*unsafe.Sizeof(sel.scase[0])))
cas.pc = callerpc
cas.c = nil
cas.so = uint16(so)
cas.kind = caseDefault
if debugSelect {
print("selectdefault s=", sel, " pc=", hex(cas.pc), " so=", cas.so, "\n")
}
}
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.
n := len(scases)
r := 0
// skip the default case
if n > 0 && scases[lockorder[0]].c == nil {
r = 1
}
for i := n - 1; i >= r; i-- {
c := scases[lockorder[i]].c
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, "select (no cases)", traceEvGoStop, 1) // forever
}
// selectgo implements the select statement.
//
// *sel is on the current goroutine's stack (regardless of any
// escaping in selectgo).
//
// selectgo does not return. Instead, it overwrites its return PC and
// returns directly to the triggered select case. Because of this, it
// cannot appear at the top of a split stack.
//
//go:nosplit
func selectgo(sel *hselect) {
pc, offset := selectgoImpl(sel)
*(*bool)(add(unsafe.Pointer(&sel), uintptr(offset))) = true
setcallerpc(unsafe.Pointer(&sel), pc)
}
// selectgoImpl returns scase.pc and scase.so for the select
// case which fired.
func selectgoImpl(sel *hselect) (uintptr, uint16) {
if debugSelect {
print("select: sel=", sel, "\n")
}
scaseslice := slice{unsafe.Pointer(&sel.scase), int(sel.ncase), int(sel.ncase)}
scases := *(*[]scase)(unsafe.Pointer(&scaseslice))
var t0 int64
if blockprofilerate > 0 {
t0 = cputicks()
for i := 0; i < int(sel.ncase); 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
pollslice := slice{unsafe.Pointer(sel.pollorder), int(sel.ncase), int(sel.ncase)}
pollorder := *(*[]uint16)(unsafe.Pointer(&pollslice))
for i := 1; i < int(sel.ncase); i++ {
j := int(fastrand1()) % (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.
lockslice := slice{unsafe.Pointer(sel.lockorder), int(sel.ncase), int(sel.ncase)}
lockorder := *(*[]uint16)(unsafe.Pointer(&lockslice))
for i := 0; i < int(sel.ncase); 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 := int(sel.ncase) - 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
}
/*
for i := 0; i+1 < int(sel.ncase); 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
done uint32
sg *sudog
c *hchan
k *scase
sglist *sudog
sgnext *sudog
qp unsafe.Pointer
nextp **sudog
)
loop:
// pass 1 - look for something already waiting
var dfl *scase
var cas *scase
for i := 0; i < int(sel.ncase); i++ {
cas = &scases[pollorder[i]]
c = cas.c
switch cas.kind {
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(unsafe.Pointer(c), 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:
dfl = cas
}
}
if dfl != nil {
selunlock(scases, lockorder)
cas = dfl
goto retc
}
// pass 2 - enqueue on all chans
gp = getg()
done = 0
if gp.waiting != nil {
throw("gp.waiting != nil")
}
nextp = &gp.waiting
for _, casei := range lockorder {
cas = &scases[casei]
c = cas.c
sg := acquireSudog()
sg.g = gp
// Note: selectdone is adjusted for stack copies in stack1.go:adjustsudogs
sg.selectdone = (*uint32)(noescape(unsafe.Pointer(&done)))
// 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, "select", traceEvGoBlockSelect, 2)
// While we were asleep, some goroutine came along and completed
// one of the cases in the select and woke us up (called ready).
// As part of that process, the goroutine did a cas on done above
// (aka *sg.selectdone for all queued sg) to win the right to
// complete the select. Now done = 1.
//
// If we copy (grow) our own stack, we will update the
// selectdone pointers inside the gp.waiting sudog list to point
// at the new stack. Another goroutine attempting to
// complete one of our (still linked in) select cases might
// see the new selectdone pointer (pointing at the new stack)
// before the new stack has real data; if the new stack has done = 0
// (before the old values are copied over), the goroutine might
// do a cas via sg.selectdone and incorrectly believe that it has
// won the right to complete the select, executing a second
// communication and attempting to wake us (call ready) again.
//
// Then things break.
//
// The best break is that the goroutine doing ready sees the
// _Gcopystack status and throws, as in #17007.
// A worse break would be for us to continue on, start running real code,
// block in a semaphore acquisition (sema.go), and have the other
// goroutine wake us up without having really acquired the semaphore.
// That would result in the goroutine spuriously running and then
// queue up another spurious wakeup when the semaphore really is ready.
// In general the situation can cascade until something notices the
// problem and causes a crash.
//
// A stack shrink does not have this problem, because it locks
// all the channels that are involved first, blocking out the
// possibility of a cas on selectdone.
//
// A stack growth before gopark above does not have this
// problem, because we hold those channel locks (released by
// selparkcommit).
//
// A stack growth after sellock below does not have this
// problem, because again we hold those channel locks.
//
// The only problem is a stack growth during sellock.
// To keep that from happening, run sellock on the system stack.
//
// It might be that we could avoid this if copystack copied the
// stack before calling adjustsudogs. In that case,
// syncadjustsudogs would need to recopy the tiny part that
// it copies today, resulting in a little bit of extra copying.
//
// An even better fix, not for the week before a release candidate,
// would be to put space in every sudog and make selectdone
// point at (say) the space in the first sudog.
systemstack(func() {
sellock(scases, lockorder)
})
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.
cas = nil
sglist = gp.waiting
// Clear all elem before unlinking from gp.waiting.
for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
sg1.selectdone = nil
sg1.elem = nil
sg1.c = nil
}
gp.waiting = nil
for _, casei := range lockorder {
k = &scases[casei]
if sglist.releasetime > 0 {
k.releasetime = sglist.releasetime
}
if sg == sglist {
// sg has already been dequeued by the G that woke us up.
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: sel=", sel, " c=", c, " cas=", cas, " kind=", cas.kind, "\n")
}
if cas.kind == caseRecv {
if cas.receivedp != nil {
*cas.receivedp = 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)
}
if cas.receivedp != nil {
*cas.receivedp = true
}
qp = chanbuf(c, c.recvx)
if cas.elem != nil {
typedmemmove(c.elemtype, cas.elem, qp)
}
memclr(qp, uintptr(c.elemsize))
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) })
if debugSelect {
print("syncrecv: sel=", sel, " c=", c, "\n")
}
if cas.receivedp != nil {
*cas.receivedp = true
}
goto retc
rclose:
// read at end of closed channel
selunlock(scases, lockorder)
if cas.receivedp != nil {
*cas.receivedp = false
}
if cas.elem != nil {
memclr(cas.elem, uintptr(c.elemsize))
}
if raceenabled {
raceacquire(unsafe.Pointer(c))
}
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) })
if debugSelect {
print("syncsend: sel=", sel, " c=", c, "\n")
}
goto retc
retc:
if cas.releasetime > 0 {
blockevent(cas.releasetime-t0, 2)
}
return cas.pc, cas.so
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) (chosen int, recvOK bool) {
// flagNoScan is safe here, because all objects are also referenced from cases.
size := selectsize(uintptr(len(cases)))
sel := (*hselect)(mallocgc(size, nil, true))
newselect(sel, int64(size), int32(len(cases)))
r := new(bool)
for i := range cases {
rc := &cases[i]
switch rc.dir {
case selectDefault:
selectdefaultImpl(sel, uintptr(i), 0)
case selectSend:
if rc.ch == nil {
break
}
selectsendImpl(sel, rc.ch, uintptr(i), rc.val, 0)
case selectRecv:
if rc.ch == nil {
break
}
selectrecvImpl(sel, rc.ch, uintptr(i), rc.val, r, 0)
}
}
pc, _ := selectgoImpl(sel)
chosen = int(pc)
recvOK = *r
return
}
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
}
}