1 // Copyright 2009 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
7 // This file contains the implementation of Go select statements.
14 const debugSelect = false
16 // Select case descriptor.
18 // Changes here must also be made in src/cmd/compile/internal/walk/select.go's scasetype.
21 elem unsafe.Pointer // data element
25 chansendpc = abi.FuncPCABIInternal(chansend)
26 chanrecvpc = abi.FuncPCABIInternal(chanrecv)
29 func selectsetpc(pc *uintptr) {
33 func sellock(scases []scase, lockorder []uint16) {
35 for _, o := range lockorder {
44 func selunlock(scases []scase, lockorder []uint16) {
45 // We must be very careful here to not touch sel after we have unlocked
46 // the last lock, because sel can be freed right after the last unlock.
47 // Consider the following situation.
48 // First M calls runtime·park() in runtime·selectgo() passing the sel.
49 // Once runtime·park() has unlocked the last lock, another M makes
50 // the G that calls select runnable again and schedules it for execution.
51 // When the G runs on another M, it locks all the locks and frees sel.
52 // Now if the first M touches sel, it will access freed memory.
53 for i := len(lockorder) - 1; i >= 0; i-- {
54 c := scases[lockorder[i]].c
55 if i > 0 && c == scases[lockorder[i-1]].c {
56 continue // will unlock it on the next iteration
62 func selparkcommit(gp *g, _ unsafe.Pointer) bool {
63 // There are unlocked sudogs that point into gp's stack. Stack
64 // copying must lock the channels of those sudogs.
65 // Set activeStackChans here instead of before we try parking
66 // because we could self-deadlock in stack growth on a
68 gp.activeStackChans = true
69 // Mark that it's safe for stack shrinking to occur now,
70 // because any thread acquiring this G's stack for shrinking
71 // is guaranteed to observe activeStackChans after this store.
72 gp.parkingOnChan.Store(false)
73 // Make sure we unlock after setting activeStackChans and
74 // unsetting parkingOnChan. The moment we unlock any of the
75 // channel locks we risk gp getting readied by a channel operation
76 // and so gp could continue running before everything before the
77 // unlock is visible (even to gp itself).
79 // This must not access gp's stack (see gopark). In
80 // particular, it must not access the *hselect. That's okay,
81 // because by the time this is called, gp.waiting has all
82 // channels in lock order.
84 for sg := gp.waiting; sg != nil; sg = sg.waitlink {
85 if sg.c != lastc && lastc != nil {
86 // As soon as we unlock the channel, fields in
87 // any sudog with that channel may change,
88 // including c and waitlink. Since multiple
89 // sudogs may have the same channel, we unlock
90 // only after we've passed the last instance
103 gopark(nil, nil, waitReasonSelectNoCases, traceBlockForever, 1) // forever
106 // selectgo implements the select statement.
108 // cas0 points to an array of type [ncases]scase, and order0 points to
109 // an array of type [2*ncases]uint16 where ncases must be <= 65536.
110 // Both reside on the goroutine's stack (regardless of any escaping in
113 // For race detector builds, pc0 points to an array of type
114 // [ncases]uintptr (also on the stack); for other builds, it's set to
117 // selectgo returns the index of the chosen scase, which matches the
118 // ordinal position of its respective select{recv,send,default} call.
119 // Also, if the chosen scase was a receive operation, it reports whether
120 // a value was received.
121 func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, block bool) (int, bool) {
123 print("select: cas0=", cas0, "\n")
126 // NOTE: In order to maintain a lean stack size, the number of scases
127 // is capped at 65536.
128 cas1 := (*[1 << 16]scase)(unsafe.Pointer(cas0))
129 order1 := (*[1 << 17]uint16)(unsafe.Pointer(order0))
131 ncases := nsends + nrecvs
132 scases := cas1[:ncases:ncases]
133 pollorder := order1[:ncases:ncases]
134 lockorder := order1[ncases:][:ncases:ncases]
135 // NOTE: pollorder/lockorder's underlying array was not zero-initialized by compiler.
137 // Even when raceenabled is true, there might be select
138 // statements in packages compiled without -race (e.g.,
139 // ensureSigM in runtime/signal_unix.go).
141 if raceenabled && pc0 != nil {
142 pc1 := (*[1 << 16]uintptr)(unsafe.Pointer(pc0))
143 pcs = pc1[:ncases:ncases]
145 casePC := func(casi int) uintptr {
153 if blockprofilerate > 0 {
157 // The compiler rewrites selects that statically have
158 // only 0 or 1 cases plus default into simpler constructs.
159 // The only way we can end up with such small sel.ncase
160 // values here is for a larger select in which most channels
161 // have been nilled out. The general code handles those
162 // cases correctly, and they are rare enough not to bother
163 // optimizing (and needing to test).
165 // generate permuted order
167 for i := range scases {
170 // Omit cases without channels from the poll and lock orders.
172 cas.elem = nil // allow GC
176 j := fastrandn(uint32(norder + 1))
177 pollorder[norder] = pollorder[j]
178 pollorder[j] = uint16(i)
181 pollorder = pollorder[:norder]
182 lockorder = lockorder[:norder]
184 // sort the cases by Hchan address to get the locking order.
185 // simple heap sort, to guarantee n log n time and constant stack footprint.
186 for i := range lockorder {
188 // Start with the pollorder to permute cases on the same channel.
189 c := scases[pollorder[i]].c
190 for j > 0 && scases[lockorder[(j-1)/2]].c.sortkey() < c.sortkey() {
192 lockorder[j] = lockorder[k]
195 lockorder[j] = pollorder[i]
197 for i := len(lockorder) - 1; i >= 0; i-- {
200 lockorder[i] = lockorder[0]
207 if k+1 < i && scases[lockorder[k]].c.sortkey() < scases[lockorder[k+1]].c.sortkey() {
210 if c.sortkey() < scases[lockorder[k]].c.sortkey() {
211 lockorder[j] = lockorder[k]
221 for i := 0; i+1 < len(lockorder); i++ {
222 if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() {
223 print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
224 throw("select: broken sort")
229 // lock all the channels involved in the select
230 sellock(scases, lockorder)
243 // pass 1 - look for something already waiting
247 var caseReleaseTime int64 = -1
249 for _, casei := range pollorder {
255 sg = c.sendq.dequeue()
267 racereadpc(c.raceaddr(), casePC(casi), chansendpc)
272 sg = c.recvq.dequeue()
276 if c.qcount < c.dataqsiz {
283 selunlock(scases, lockorder)
288 // pass 2 - enqueue on all chans
290 if gp.waiting != nil {
291 throw("gp.waiting != nil")
294 for _, casei := range lockorder {
301 // No stack splits between assigning elem and enqueuing
302 // sg on gp.waiting where copystack can find it.
309 // Construct waiting list in lock order.
320 // wait for someone to wake us up
322 // Signal to anyone trying to shrink our stack that we're about
323 // to park on a channel. The window between when this G's status
324 // changes and when we set gp.activeStackChans is not safe for
326 gp.parkingOnChan.Store(true)
327 gopark(selparkcommit, nil, waitReasonSelect, traceBlockSelect, 1)
328 gp.activeStackChans = false
330 sellock(scases, lockorder)
332 gp.selectDone.Store(0)
333 sg = (*sudog)(gp.param)
336 // pass 3 - dequeue from unsuccessful chans
337 // otherwise they stack up on quiet channels
338 // record the successful case, if any.
339 // We singly-linked up the SudoGs in lock order.
344 // Clear all elem before unlinking from gp.waiting.
345 for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
352 for _, casei := range lockorder {
355 // sg has already been dequeued by the G that woke us up.
358 caseSuccess = sglist.success
359 if sglist.releasetime > 0 {
360 caseReleaseTime = sglist.releasetime
364 if int(casei) < nsends {
365 c.sendq.dequeueSudoG(sglist)
367 c.recvq.dequeueSudoG(sglist)
370 sgnext = sglist.waitlink
371 sglist.waitlink = nil
377 throw("selectgo: bad wakeup")
383 print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " send=", casi < nsends, "\n")
396 raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
397 } else if cas.elem != nil {
398 raceWriteObjectPC(c.elemtype, cas.elem, casePC(casi), chanrecvpc)
403 msanread(cas.elem, c.elemtype.Size_)
404 } else if cas.elem != nil {
405 msanwrite(cas.elem, c.elemtype.Size_)
410 asanread(cas.elem, c.elemtype.Size_)
411 } else if cas.elem != nil {
412 asanwrite(cas.elem, c.elemtype.Size_)
416 selunlock(scases, lockorder)
420 // can receive from buffer
423 raceWriteObjectPC(c.elemtype, cas.elem, casePC(casi), chanrecvpc)
425 racenotify(c, c.recvx, nil)
427 if msanenabled && cas.elem != nil {
428 msanwrite(cas.elem, c.elemtype.Size_)
430 if asanenabled && cas.elem != nil {
431 asanwrite(cas.elem, c.elemtype.Size_)
434 qp = chanbuf(c, c.recvx)
436 typedmemmove(c.elemtype, cas.elem, qp)
438 typedmemclr(c.elemtype, qp)
440 if c.recvx == c.dataqsiz {
444 selunlock(scases, lockorder)
448 // can send to buffer
450 racenotify(c, c.sendx, nil)
451 raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
454 msanread(cas.elem, c.elemtype.Size_)
457 asanread(cas.elem, c.elemtype.Size_)
459 typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem)
461 if c.sendx == c.dataqsiz {
465 selunlock(scases, lockorder)
469 // can receive from sleeping sender (sg)
470 recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
472 print("syncrecv: cas0=", cas0, " c=", c, "\n")
478 // read at end of closed channel
479 selunlock(scases, lockorder)
482 typedmemclr(c.elemtype, cas.elem)
485 raceacquire(c.raceaddr())
490 // can send to a sleeping receiver (sg)
492 raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
495 msanread(cas.elem, c.elemtype.Size_)
498 asanread(cas.elem, c.elemtype.Size_)
500 send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
502 print("syncsend: cas0=", cas0, " c=", c, "\n")
507 if caseReleaseTime > 0 {
508 blockevent(caseReleaseTime-t0, 1)
513 // send on closed channel
514 selunlock(scases, lockorder)
515 panic(plainError("send on closed channel"))
518 func (c *hchan) sortkey() uintptr {
519 return uintptr(unsafe.Pointer(c))
522 // A runtimeSelect is a single case passed to rselect.
523 // This must match ../reflect/value.go:/runtimeSelect
524 type runtimeSelect struct {
526 typ unsafe.Pointer // channel type (not used here)
528 val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
531 // These values must match ../reflect/value.go:/SelectDir.
536 selectSend // case Chan <- Send
537 selectRecv // case <-Chan:
538 selectDefault // default
541 //go:linkname reflect_rselect reflect.rselect
542 func reflect_rselect(cases []runtimeSelect) (int, bool) {
546 sel := make([]scase, len(cases))
547 orig := make([]int, len(cases))
548 nsends, nrecvs := 0, 0
550 for i, rc := range cases {
561 j = len(cases) - nrecvs
564 sel[j] = scase{c: rc.ch, elem: rc.val}
568 // Only a default case.
569 if nsends+nrecvs == 0 {
573 // Compact sel and orig if necessary.
574 if nsends+nrecvs < len(cases) {
575 copy(sel[nsends:], sel[len(cases)-nrecvs:])
576 copy(orig[nsends:], orig[len(cases)-nrecvs:])
579 order := make([]uint16, 2*(nsends+nrecvs))
582 pcs := make([]uintptr, nsends+nrecvs)
589 chosen, recvOK := selectgo(&sel[0], &order[0], pc0, nsends, nrecvs, dflt == -1)
591 // Translate chosen back to caller's ordering.
595 chosen = orig[chosen]
597 return chosen, recvOK
600 func (q *waitq) dequeueSudoG(sgp *sudog) {
626 // x==y==nil. Either sgp is the only element in the queue,
627 // or it has already been removed. Use q.first to disambiguate.