src/runtime/select.go

   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.
   4
   5 package runtime
   6
   7 // This file contains the implementation of Go select statements.
   8
   9 import "unsafe"
  10
  11 const (
  12         debugSelect = false
  13 )
  14
  15 var (
  16         chansendpc = funcPC(chansend)
  17         chanrecvpc = funcPC(chanrecv)
  18 )
  19
  20 func selectsize(size uintptr) uintptr {
  21         selsize := unsafe.Sizeof(_select{}) +
  22                 (size-1)*unsafe.Sizeof(_select{}.scase[0]) +
  23                 size*unsafe.Sizeof(*_select{}.lockorder) +
  24                 size*unsafe.Sizeof(*_select{}.pollorder)
  25         return round(selsize, _Int64Align)
  26 }
  27
  28 func newselect(sel *_select, selsize int64, size int32) {
  29         if selsize != int64(selectsize(uintptr(size))) {
  30                 print("runtime: bad select size ", selsize, ", want ", selectsize(uintptr(size)), "\n")
  31                 gothrow("bad select size")
  32         }
  33         sel.tcase = uint16(size)
  34         sel.ncase = 0
  35         sel.lockorder = (**hchan)(add(unsafe.Pointer(&sel.scase), uintptr(size)*unsafe.Sizeof(_select{}.scase[0])))
  36         sel.pollorder = (*uint16)(add(unsafe.Pointer(sel.lockorder), uintptr(size)*unsafe.Sizeof(*_select{}.lockorder)))
  37
  38         if debugSelect {
  39                 print("newselect s=", sel, " size=", size, "\n")
  40         }
  41 }
  42
  43 //go:nosplit
  44 func selectsend(sel *_select, c *hchan, elem unsafe.Pointer) (selected bool) {
  45         // nil cases do not compete
  46         if c != nil {
  47                 selectsendImpl(sel, c, getcallerpc(unsafe.Pointer(&sel)), elem, uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
  48         }
  49         return
  50 }
  51
  52 // cut in half to give stack a chance to split
  53 func selectsendImpl(sel *_select, c *hchan, pc uintptr, elem unsafe.Pointer, so uintptr) {
  54         i := sel.ncase
  55         if i >= sel.tcase {
  56                 gothrow("selectsend: too many cases")
  57         }
  58         sel.ncase = i + 1
  59         cas := (*scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)*unsafe.Sizeof(sel.scase[0])))
  60
  61         cas.pc = pc
  62         cas._chan = c
  63         cas.so = uint16(so)
  64         cas.kind = _CaseSend
  65         cas.elem = elem
  66
  67         if debugSelect {
  68                 print("selectsend s=", sel, " pc=", hex(cas.pc), " chan=", cas._chan, " so=", cas.so, "\n")
  69         }
  70 }
  71
  72 //go:nosplit
  73 func selectrecv(sel *_select, c *hchan, elem unsafe.Pointer) (selected bool) {
  74         // nil cases do not compete
  75         if c != nil {
  76                 selectrecvImpl(sel, c, getcallerpc(unsafe.Pointer(&sel)), elem, nil, uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
  77         }
  78         return
  79 }
  80
  81 //go:nosplit
  82 func selectrecv2(sel *_select, c *hchan, elem unsafe.Pointer, received *bool) (selected bool) {
  83         // nil cases do not compete
  84         if c != nil {
  85                 selectrecvImpl(sel, c, getcallerpc(unsafe.Pointer(&sel)), elem, received, uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
  86         }
  87         return
  88 }
  89
  90 func selectrecvImpl(sel *_select, c *hchan, pc uintptr, elem unsafe.Pointer, received *bool, so uintptr) {
  91         i := sel.ncase
  92         if i >= sel.tcase {
  93                 gothrow("selectrecv: too many cases")
  94         }
  95         sel.ncase = i + 1
  96         cas := (*scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)*unsafe.Sizeof(sel.scase[0])))
  97         cas.pc = pc
  98         cas._chan = c
  99         cas.so = uint16(so)
 100         cas.kind = _CaseRecv
 101         cas.elem = elem
 102         cas.receivedp = received
 103
 104         if debugSelect {
 105                 print("selectrecv s=", sel, " pc=", hex(cas.pc), " chan=", cas._chan, " so=", cas.so, "\n")
 106         }
 107 }
 108
 109 //go:nosplit
 110 func selectdefault(sel *_select) (selected bool) {
 111         selectdefaultImpl(sel, getcallerpc(unsafe.Pointer(&sel)), uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
 112         return
 113 }
 114
 115 func selectdefaultImpl(sel *_select, callerpc uintptr, so uintptr) {
 116         i := sel.ncase
 117         if i >= sel.tcase {
 118                 gothrow("selectdefault: too many cases")
 119         }
 120         sel.ncase = i + 1
 121         cas := (*scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)*unsafe.Sizeof(sel.scase[0])))
 122         cas.pc = callerpc
 123         cas._chan = nil
 124         cas.so = uint16(so)
 125         cas.kind = _CaseDefault
 126
 127         if debugSelect {
 128                 print("selectdefault s=", sel, " pc=", hex(cas.pc), " so=", cas.so, "\n")
 129         }
 130 }
 131
 132 func sellock(sel *_select) {
 133         lockslice := sliceStruct{unsafe.Pointer(sel.lockorder), int(sel.ncase), int(sel.ncase)}
 134         lockorder := *(*[]*hchan)(unsafe.Pointer(&lockslice))
 135         var c *hchan
 136         for _, c0 := range lockorder {
 137                 if c0 != nil && c0 != c {
 138                         c = c0
 139                         lock(&c.lock)
 140                 }
 141         }
 142 }
 143
 144 func selunlock(sel *_select) {
 145         // We must be very careful here to not touch sel after we have unlocked
 146         // the last lock, because sel can be freed right after the last unlock.
 147         // Consider the following situation.
 148         // First M calls runtime·park() in runtime·selectgo() passing the sel.
 149         // Once runtime·park() has unlocked the last lock, another M makes
 150         // the G that calls select runnable again and schedules it for execution.
 151         // When the G runs on another M, it locks all the locks and frees sel.
 152         // Now if the first M touches sel, it will access freed memory.
 153         n := int(sel.ncase)
 154         r := 0
 155         lockslice := sliceStruct{unsafe.Pointer(sel.lockorder), n, n}
 156         lockorder := *(*[]*hchan)(unsafe.Pointer(&lockslice))
 157         // skip the default case
 158         if n > 0 && lockorder[0] == nil {
 159                 r = 1
 160         }
 161         for i := n - 1; i >= r; i-- {
 162                 c := lockorder[i]
 163                 if i > 0 && c == lockorder[i-1] {
 164                         continue // will unlock it on the next iteration
 165                 }
 166                 unlock(&c.lock)
 167         }
 168 }
 169
 170 func selparkcommit(gp *g, sel unsafe.Pointer) bool {
 171         selunlock((*_select)(sel))
 172         return true
 173 }
 174
 175 func block() {
 176         gopark(nil, nil, "select (no cases)") // forever
 177 }
 178
 179 // overwrites return pc on stack to signal which case of the select
 180 // to run, so cannot appear at the top of a split stack.
 181 //go:nosplit
 182 func selectgo(sel *_select) {
 183         pc, offset := selectgoImpl(sel)
 184         *(*bool)(add(unsafe.Pointer(&sel), uintptr(offset))) = true
 185         setcallerpc(unsafe.Pointer(&sel), pc)
 186 }
 187
 188 // selectgoImpl returns scase.pc and scase.so for the select
 189 // case which fired.
 190 func selectgoImpl(sel *_select) (uintptr, uint16) {
 191         if debugSelect {
 192                 print("select: sel=", sel, "\n")
 193         }
 194
 195         scaseslice := sliceStruct{unsafe.Pointer(&sel.scase), int(sel.ncase), int(sel.ncase)}
 196         scases := *(*[]scase)(unsafe.Pointer(&scaseslice))
 197
 198         var t0 int64
 199         if blockprofilerate > 0 {
 200                 t0 = cputicks()
 201                 for i := 0; i < int(sel.ncase); i++ {
 202                         scases[i].releasetime = -1
 203                 }
 204         }
 205
 206         // The compiler rewrites selects that statically have
 207         // only 0 or 1 cases plus default into simpler constructs.
 208         // The only way we can end up with such small sel.ncase
 209         // values here is for a larger select in which most channels
 210         // have been nilled out.  The general code handles those
 211         // cases correctly, and they are rare enough not to bother
 212         // optimizing (and needing to test).
 213
 214         // generate permuted order
 215         pollslice := sliceStruct{unsafe.Pointer(sel.pollorder), int(sel.ncase), int(sel.ncase)}
 216         pollorder := *(*[]uint16)(unsafe.Pointer(&pollslice))
 217         for i := 0; i < int(sel.ncase); i++ {
 218                 pollorder[i] = uint16(i)
 219         }
 220         for i := 1; i < int(sel.ncase); i++ {
 221                 o := pollorder[i]
 222                 j := int(fastrand1()) % (i + 1)
 223                 pollorder[i] = pollorder[j]
 224                 pollorder[j] = o
 225         }
 226
 227         // sort the cases by Hchan address to get the locking order.
 228         // simple heap sort, to guarantee n log n time and constant stack footprint.
 229         lockslice := sliceStruct{unsafe.Pointer(sel.lockorder), int(sel.ncase), int(sel.ncase)}
 230         lockorder := *(*[]*hchan)(unsafe.Pointer(&lockslice))
 231         for i := 0; i < int(sel.ncase); i++ {
 232                 j := i
 233                 c := scases[j]._chan
 234                 for j > 0 && lockorder[(j-1)/2].sortkey() < c.sortkey() {
 235                         k := (j - 1) / 2
 236                         lockorder[j] = lockorder[k]
 237                         j = k
 238                 }
 239                 lockorder[j] = c
 240         }
 241         for i := int(sel.ncase) - 1; i >= 0; i-- {
 242                 c := lockorder[i]
 243                 lockorder[i] = lockorder[0]
 244                 j := 0
 245                 for {
 246                         k := j*2 + 1
 247                         if k >= i {
 248                                 break
 249                         }
 250                         if k+1 < i && lockorder[k].sortkey() < lockorder[k+1].sortkey() {
 251                                 k++
 252                         }
 253                         if c.sortkey() < lockorder[k].sortkey() {
 254                                 lockorder[j] = lockorder[k]
 255                                 j = k
 256                                 continue
 257                         }
 258                         break
 259                 }
 260                 lockorder[j] = c
 261         }
 262         /*
 263                 for i := 0; i+1 < int(sel.ncase); i++ {
 264                         if lockorder[i].sortkey() > lockorder[i+1].sortkey() {
 265                                 print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
 266                                 gothrow("select: broken sort")
 267                         }
 268                 }
 269         */
 270
 271         // lock all the channels involved in the select
 272         sellock(sel)
 273
 274         var (
 275                 gp     *g
 276                 done   uint32
 277                 sg     *sudog
 278                 c      *hchan
 279                 k      *scase
 280                 sglist *sudog
 281                 sgnext *sudog
 282         )
 283
 284 loop:
 285         // pass 1 - look for something already waiting
 286         var dfl *scase
 287         var cas *scase
 288         for i := 0; i < int(sel.ncase); i++ {
 289                 cas = &scases[pollorder[i]]
 290                 c = cas._chan
 291
 292                 switch cas.kind {
 293                 case _CaseRecv:
 294                         if c.dataqsiz > 0 {
 295                                 if c.qcount > 0 {
 296                                         goto asyncrecv
 297                                 }
 298                         } else {
 299                                 sg = c.sendq.dequeue()
 300                                 if sg != nil {
 301                                         goto syncrecv
 302                                 }
 303                         }
 304                         if c.closed != 0 {
 305                                 goto rclose
 306                         }
 307
 308                 case _CaseSend:
 309                         if raceenabled {
 310                                 racereadpc(unsafe.Pointer(c), cas.pc, chansendpc)
 311                         }
 312                         if c.closed != 0 {
 313                                 goto sclose
 314                         }
 315                         if c.dataqsiz > 0 {
 316                                 if c.qcount < c.dataqsiz {
 317                                         goto asyncsend
 318                                 }
 319                         } else {
 320                                 sg = c.recvq.dequeue()
 321                                 if sg != nil {
 322                                         goto syncsend
 323                                 }
 324                         }
 325
 326                 case _CaseDefault:
 327                         dfl = cas
 328                 }
 329         }
 330
 331         if dfl != nil {
 332                 selunlock(sel)
 333                 cas = dfl
 334                 goto retc
 335         }
 336
 337         // pass 2 - enqueue on all chans
 338         gp = getg()
 339         done = 0
 340         for i := 0; i < int(sel.ncase); i++ {
 341                 cas = &scases[pollorder[i]]
 342                 c = cas._chan
 343                 sg := acquireSudog()
 344                 sg.g = gp
 345                 // Note: selectdone is adjusted for stack copies in stack.c:adjustsudogs
 346                 sg.selectdone = (*uint32)(noescape(unsafe.Pointer(&done)))
 347                 sg.elem = cas.elem
 348                 sg.releasetime = 0
 349                 if t0 != 0 {
 350                         sg.releasetime = -1
 351                 }
 352                 sg.waitlink = gp.waiting
 353                 gp.waiting = sg
 354
 355                 switch cas.kind {
 356                 case _CaseRecv:
 357                         c.recvq.enqueue(sg)
 358
 359                 case _CaseSend:
 360                         c.sendq.enqueue(sg)
 361                 }
 362         }
 363
 364         // wait for someone to wake us up
 365         gp.param = nil
 366         gopark(selparkcommit, unsafe.Pointer(sel), "select")
 367
 368         // someone woke us up
 369         sellock(sel)
 370         sg = (*sudog)(gp.param)
 371         gp.param = nil
 372
 373         // pass 3 - dequeue from unsuccessful chans
 374         // otherwise they stack up on quiet channels
 375         // record the successful case, if any.
 376         // We singly-linked up the SudoGs in case order, so when
 377         // iterating through the linked list they are in reverse order.
 378         cas = nil
 379         sglist = gp.waiting
 380         // Clear all elem before unlinking from gp.waiting.
 381         for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
 382                 sg1.selectdone = nil
 383                 sg1.elem = nil
 384         }
 385         gp.waiting = nil
 386         for i := int(sel.ncase) - 1; i >= 0; i-- {
 387                 k = &scases[pollorder[i]]
 388                 if sglist.releasetime > 0 {
 389                         k.releasetime = sglist.releasetime
 390                 }
 391                 if sg == sglist {
 392                         cas = k
 393                 } else {
 394                         c = k._chan
 395                         if k.kind == _CaseSend {
 396                                 c.sendq.dequeueSudoG(sglist)
 397                         } else {
 398                                 c.recvq.dequeueSudoG(sglist)
 399                         }
 400                 }
 401                 sgnext = sglist.waitlink
 402                 sglist.waitlink = nil
 403                 releaseSudog(sglist)
 404                 sglist = sgnext
 405         }
 406
 407         if cas == nil {
 408                 goto loop
 409         }
 410
 411         c = cas._chan
 412
 413         if c.dataqsiz > 0 {
 414                 gothrow("selectgo: shouldn't happen")
 415         }
 416
 417         if debugSelect {
 418                 print("wait-return: sel=", sel, " c=", c, " cas=", cas, " kind=", cas.kind, "\n")
 419         }
 420
 421         if cas.kind == _CaseRecv {
 422                 if cas.receivedp != nil {
 423                         *cas.receivedp = true
 424                 }
 425         }
 426
 427         if raceenabled {
 428                 if cas.kind == _CaseRecv && cas.elem != nil {
 429                         raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
 430                 } else if cas.kind == _CaseSend {
 431                         raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
 432                 }
 433         }
 434
 435         selunlock(sel)
 436         goto retc
 437
 438 asyncrecv:
 439         // can receive from buffer
 440         if raceenabled {
 441                 if cas.elem != nil {
 442                         raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
 443                 }
 444                 raceacquire(chanbuf(c, c.recvx))
 445                 racerelease(chanbuf(c, c.recvx))
 446         }
 447         if cas.receivedp != nil {
 448                 *cas.receivedp = true
 449         }
 450         if cas.elem != nil {
 451                 memmove(cas.elem, chanbuf(c, c.recvx), uintptr(c.elemsize))
 452         }
 453         memclr(chanbuf(c, c.recvx), uintptr(c.elemsize))
 454         c.recvx++
 455         if c.recvx == c.dataqsiz {
 456                 c.recvx = 0
 457         }
 458         c.qcount--
 459         sg = c.sendq.dequeue()
 460         if sg != nil {
 461                 gp = sg.g
 462                 selunlock(sel)
 463                 if sg.releasetime != 0 {
 464                         sg.releasetime = cputicks()
 465                 }
 466                 goready(gp)
 467         } else {
 468                 selunlock(sel)
 469         }
 470         goto retc
 471
 472 asyncsend:
 473         // can send to buffer
 474         if raceenabled {
 475                 raceacquire(chanbuf(c, c.sendx))
 476                 racerelease(chanbuf(c, c.sendx))
 477                 raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
 478         }
 479         memmove(chanbuf(c, c.sendx), cas.elem, uintptr(c.elemsize))
 480         c.sendx++
 481         if c.sendx == c.dataqsiz {
 482                 c.sendx = 0
 483         }
 484         c.qcount++
 485         sg = c.recvq.dequeue()
 486         if sg != nil {
 487                 gp = sg.g
 488                 selunlock(sel)
 489                 if sg.releasetime != 0 {
 490                         sg.releasetime = cputicks()
 491                 }
 492                 goready(gp)
 493         } else {
 494                 selunlock(sel)
 495         }
 496         goto retc
 497
 498 syncrecv:
 499         // can receive from sleeping sender (sg)
 500         if raceenabled {
 501                 if cas.elem != nil {
 502                         raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
 503                 }
 504                 racesync(c, sg)
 505         }
 506         selunlock(sel)
 507         if debugSelect {
 508                 print("syncrecv: sel=", sel, " c=", c, "\n")
 509         }
 510         if cas.receivedp != nil {
 511                 *cas.receivedp = true
 512         }
 513         if cas.elem != nil {
 514                 memmove(cas.elem, sg.elem, uintptr(c.elemsize))
 515         }
 516         sg.elem = nil
 517         gp = sg.g
 518         gp.param = unsafe.Pointer(sg)
 519         if sg.releasetime != 0 {
 520                 sg.releasetime = cputicks()
 521         }
 522         goready(gp)
 523         goto retc
 524
 525 rclose:
 526         // read at end of closed channel
 527         selunlock(sel)
 528         if cas.receivedp != nil {
 529                 *cas.receivedp = false
 530         }
 531         if cas.elem != nil {
 532                 memclr(cas.elem, uintptr(c.elemsize))
 533         }
 534         if raceenabled {
 535                 raceacquire(unsafe.Pointer(c))
 536         }
 537         goto retc
 538
 539 syncsend:
 540         // can send to sleeping receiver (sg)
 541         if raceenabled {
 542                 raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
 543                 racesync(c, sg)
 544         }
 545         selunlock(sel)
 546         if debugSelect {
 547                 print("syncsend: sel=", sel, " c=", c, "\n")
 548         }
 549         if sg.elem != nil {
 550                 memmove(sg.elem, cas.elem, uintptr(c.elemsize))
 551         }
 552         sg.elem = nil
 553         gp = sg.g
 554         gp.param = unsafe.Pointer(sg)
 555         if sg.releasetime != 0 {
 556                 sg.releasetime = cputicks()
 557         }
 558         goready(gp)
 559
 560 retc:
 561         if cas.releasetime > 0 {
 562                 blockevent(cas.releasetime-t0, 2)
 563         }
 564         return cas.pc, cas.so
 565
 566 sclose:
 567         // send on closed channel
 568         selunlock(sel)
 569         panic("send on closed channel")
 570 }
 571
 572 func (c *hchan) sortkey() uintptr {
 573         // TODO(khr): if we have a moving garbage collector, we'll need to
 574         // change this function.
 575         return uintptr(unsafe.Pointer(c))
 576 }
 577
 578 // A runtimeSelect is a single case passed to rselect.
 579 // This must match ../reflect/value.go:/runtimeSelect
 580 type runtimeSelect struct {
 581         dir selectDir
 582         typ unsafe.Pointer // channel type (not used here)
 583         ch  *hchan         // channel
 584         val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
 585 }
 586
 587 // These values must match ../reflect/value.go:/SelectDir.
 588 type selectDir int
 589
 590 const (
 591         _             selectDir = iota
 592         selectSend              // case Chan <- Send
 593         selectRecv              // case <-Chan:
 594         selectDefault           // default
 595 )
 596
 597 func reflect_rselect(cases []runtimeSelect) (chosen int, recvOK bool) {
 598         // flagNoScan is safe here, because all objects are also referenced from cases.
 599         size := selectsize(uintptr(len(cases)))
 600         sel := (*_select)(mallocgc(size, nil, flagNoScan))
 601         newselect(sel, int64(size), int32(len(cases)))
 602         r := new(bool)
 603         for i := range cases {
 604                 rc := &cases[i]
 605                 switch rc.dir {
 606                 case selectDefault:
 607                         selectdefaultImpl(sel, uintptr(i), 0)
 608                 case selectSend:
 609                         if rc.ch == nil {
 610                                 break
 611                         }
 612                         selectsendImpl(sel, rc.ch, uintptr(i), rc.val, 0)
 613                 case selectRecv:
 614                         if rc.ch == nil {
 615                                 break
 616                         }
 617                         selectrecvImpl(sel, rc.ch, uintptr(i), rc.val, r, 0)
 618                 }
 619         }
 620
 621         pc, _ := selectgoImpl(sel)
 622         chosen = int(pc)
 623         recvOK = *r
 624         return
 625 }
 626
 627 func (q *waitq) dequeueSudoG(s *sudog) {
 628         var prevsgp *sudog
 629         l := &q.first
 630         for {
 631                 sgp := *l
 632                 if sgp == nil {
 633                         return
 634                 }
 635                 if sgp == s {
 636                         *l = sgp.next
 637                         if q.last == sgp {
 638                                 q.last = prevsgp
 639                         }
 640                         s.next = nil
 641                         return
 642                 }
 643                 l = &sgp.next
 644                 prevsgp = sgp
 645         }
 646 }