github.com/m10x/go/src@v0.0.0-20220112094212-ba61592315da/runtime/select.go (about) 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 ( 10 "internal/abi" 11 "runtime/internal/atomic" 12 "unsafe" 13 ) 14 15 const debugSelect = false 16 17 // Select case descriptor. 18 // Known to compiler. 19 // Changes here must also be made in src/cmd/compile/internal/walk/select.go's scasetype. 20 type scase struct { 21 c *hchan // chan 22 elem unsafe.Pointer // data element 23 } 24 25 var ( 26 chansendpc = abi.FuncPCABIInternal(chansend) 27 chanrecvpc = abi.FuncPCABIInternal(chanrecv) 28 ) 29 30 func selectsetpc(pc *uintptr) { 31 *pc = getcallerpc() 32 } 33 34 func sellock(scases []scase, lockorder []uint16) { 35 var c *hchan 36 for _, o := range lockorder { 37 c0 := scases[o].c 38 if c0 != c { 39 c = c0 40 lock(&c.lock) 41 } 42 } 43 } 44 45 func selunlock(scases []scase, lockorder []uint16) { 46 // We must be very careful here to not touch sel after we have unlocked 47 // the last lock, because sel can be freed right after the last unlock. 48 // Consider the following situation. 49 // First M calls runtime·park() in runtime·selectgo() passing the sel. 50 // Once runtime·park() has unlocked the last lock, another M makes 51 // the G that calls select runnable again and schedules it for execution. 52 // When the G runs on another M, it locks all the locks and frees sel. 53 // Now if the first M touches sel, it will access freed memory. 54 for i := len(lockorder) - 1; i >= 0; i-- { 55 c := scases[lockorder[i]].c 56 if i > 0 && c == scases[lockorder[i-1]].c { 57 continue // will unlock it on the next iteration 58 } 59 unlock(&c.lock) 60 } 61 } 62 63 func selparkcommit(gp *g, _ unsafe.Pointer) bool { 64 // There are unlocked sudogs that point into gp's stack. Stack 65 // copying must lock the channels of those sudogs. 66 // Set activeStackChans here instead of before we try parking 67 // because we could self-deadlock in stack growth on a 68 // channel lock. 69 gp.activeStackChans = true 70 // Mark that it's safe for stack shrinking to occur now, 71 // because any thread acquiring this G's stack for shrinking 72 // is guaranteed to observe activeStackChans after this store. 73 atomic.Store8(&gp.parkingOnChan, 0) 74 // Make sure we unlock after setting activeStackChans and 75 // unsetting parkingOnChan. The moment we unlock any of the 76 // channel locks we risk gp getting readied by a channel operation 77 // and so gp could continue running before everything before the 78 // unlock is visible (even to gp itself). 79 80 // This must not access gp's stack (see gopark). In 81 // particular, it must not access the *hselect. That's okay, 82 // because by the time this is called, gp.waiting has all 83 // channels in lock order. 84 var lastc *hchan 85 for sg := gp.waiting; sg != nil; sg = sg.waitlink { 86 if sg.c != lastc && lastc != nil { 87 // As soon as we unlock the channel, fields in 88 // any sudog with that channel may change, 89 // including c and waitlink. Since multiple 90 // sudogs may have the same channel, we unlock 91 // only after we've passed the last instance 92 // of a channel. 93 unlock(&lastc.lock) 94 } 95 lastc = sg.c 96 } 97 if lastc != nil { 98 unlock(&lastc.lock) 99 } 100 return true 101 } 102 103 func block() { 104 gopark(nil, nil, waitReasonSelectNoCases, traceEvGoStop, 1) // forever 105 } 106 107 // selectgo implements the select statement. 108 // 109 // cas0 points to an array of type [ncases]scase, and order0 points to 110 // an array of type [2*ncases]uint16 where ncases must be <= 65536. 111 // Both reside on the goroutine's stack (regardless of any escaping in 112 // selectgo). 113 // 114 // For race detector builds, pc0 points to an array of type 115 // [ncases]uintptr (also on the stack); for other builds, it's set to 116 // nil. 117 // 118 // selectgo returns the index of the chosen scase, which matches the 119 // ordinal position of its respective select{recv,send,default} call. 120 // Also, if the chosen scase was a receive operation, it reports whether 121 // a value was received. 122 func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, block bool) (int, bool) { 123 if debugSelect { 124 print("select: cas0=", cas0, "\n") 125 } 126 127 // NOTE: In order to maintain a lean stack size, the number of scases 128 // is capped at 65536. 129 cas1 := (*[1 << 16]scase)(unsafe.Pointer(cas0)) 130 order1 := (*[1 << 17]uint16)(unsafe.Pointer(order0)) 131 132 ncases := nsends + nrecvs 133 scases := cas1[:ncases:ncases] 134 pollorder := order1[:ncases:ncases] 135 lockorder := order1[ncases:][:ncases:ncases] 136 // NOTE: pollorder/lockorder's underlying array was not zero-initialized by compiler. 137 138 // Even when raceenabled is true, there might be select 139 // statements in packages compiled without -race (e.g., 140 // ensureSigM in runtime/signal_unix.go). 141 var pcs []uintptr 142 if raceenabled && pc0 != nil { 143 pc1 := (*[1 << 16]uintptr)(unsafe.Pointer(pc0)) 144 pcs = pc1[:ncases:ncases] 145 } 146 casePC := func(casi int) uintptr { 147 if pcs == nil { 148 return 0 149 } 150 return pcs[casi] 151 } 152 153 var t0 int64 154 if blockprofilerate > 0 { 155 t0 = cputicks() 156 } 157 158 // The compiler rewrites selects that statically have 159 // only 0 or 1 cases plus default into simpler constructs. 160 // The only way we can end up with such small sel.ncase 161 // values here is for a larger select in which most channels 162 // have been nilled out. The general code handles those 163 // cases correctly, and they are rare enough not to bother 164 // optimizing (and needing to test). 165 166 // generate permuted order 167 norder := 0 168 for i := range scases { 169 cas := &scases[i] 170 171 // Omit cases without channels from the poll and lock orders. 172 if cas.c == nil { 173 cas.elem = nil // allow GC 174 continue 175 } 176 177 j := fastrandn(uint32(norder + 1)) 178 pollorder[norder] = pollorder[j] 179 pollorder[j] = uint16(i) 180 norder++ 181 } 182 pollorder = pollorder[:norder] 183 lockorder = lockorder[:norder] 184 185 // sort the cases by Hchan address to get the locking order. 186 // simple heap sort, to guarantee n log n time and constant stack footprint. 187 for i := range lockorder { 188 j := i 189 // Start with the pollorder to permute cases on the same channel. 190 c := scases[pollorder[i]].c 191 for j > 0 && scases[lockorder[(j-1)/2]].c.sortkey() < c.sortkey() { 192 k := (j - 1) / 2 193 lockorder[j] = lockorder[k] 194 j = k 195 } 196 lockorder[j] = pollorder[i] 197 } 198 for i := len(lockorder) - 1; i >= 0; i-- { 199 o := lockorder[i] 200 c := scases[o].c 201 lockorder[i] = lockorder[0] 202 j := 0 203 for { 204 k := j*2 + 1 205 if k >= i { 206 break 207 } 208 if k+1 < i && scases[lockorder[k]].c.sortkey() < scases[lockorder[k+1]].c.sortkey() { 209 k++ 210 } 211 if c.sortkey() < scases[lockorder[k]].c.sortkey() { 212 lockorder[j] = lockorder[k] 213 j = k 214 continue 215 } 216 break 217 } 218 lockorder[j] = o 219 } 220 221 if debugSelect { 222 for i := 0; i+1 < len(lockorder); i++ { 223 if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() { 224 print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n") 225 throw("select: broken sort") 226 } 227 } 228 } 229 230 // lock all the channels involved in the select 231 sellock(scases, lockorder) 232 233 var ( 234 gp *g 235 sg *sudog 236 c *hchan 237 k *scase 238 sglist *sudog 239 sgnext *sudog 240 qp unsafe.Pointer 241 nextp **sudog 242 ) 243 244 // pass 1 - look for something already waiting 245 var casi int 246 var cas *scase 247 var caseSuccess bool 248 var caseReleaseTime int64 = -1 249 var recvOK bool 250 for _, casei := range pollorder { 251 casi = int(casei) 252 cas = &scases[casi] 253 c = cas.c 254 255 if casi >= nsends { 256 sg = c.sendq.dequeue() 257 if sg != nil { 258 goto recv 259 } 260 if c.qcount > 0 { 261 goto bufrecv 262 } 263 if c.closed != 0 { 264 goto rclose 265 } 266 } else { 267 if raceenabled { 268 racereadpc(c.raceaddr(), casePC(casi), chansendpc) 269 } 270 if c.closed != 0 { 271 goto sclose 272 } 273 sg = c.recvq.dequeue() 274 if sg != nil { 275 goto send 276 } 277 if c.qcount < c.dataqsiz { 278 goto bufsend 279 } 280 } 281 } 282 283 if !block { 284 selunlock(scases, lockorder) 285 casi = -1 286 goto retc 287 } 288 289 // pass 2 - enqueue on all chans 290 gp = getg() 291 if gp.waiting != nil { 292 throw("gp.waiting != nil") 293 } 294 nextp = &gp.waiting 295 for _, casei := range lockorder { 296 casi = int(casei) 297 cas = &scases[casi] 298 c = cas.c 299 sg := acquireSudog() 300 sg.g = gp 301 sg.isSelect = true 302 // No stack splits between assigning elem and enqueuing 303 // sg on gp.waiting where copystack can find it. 304 sg.elem = cas.elem 305 sg.releasetime = 0 306 if t0 != 0 { 307 sg.releasetime = -1 308 } 309 sg.c = c 310 // Construct waiting list in lock order. 311 *nextp = sg 312 nextp = &sg.waitlink 313 314 if casi < nsends { 315 c.sendq.enqueue(sg) 316 } else { 317 c.recvq.enqueue(sg) 318 } 319 } 320 321 // wait for someone to wake us up 322 gp.param = nil 323 // Signal to anyone trying to shrink our stack that we're about 324 // to park on a channel. The window between when this G's status 325 // changes and when we set gp.activeStackChans is not safe for 326 // stack shrinking. 327 atomic.Store8(&gp.parkingOnChan, 1) 328 gopark(selparkcommit, nil, waitReasonSelect, traceEvGoBlockSelect, 1) 329 gp.activeStackChans = false 330 331 sellock(scases, lockorder) 332 333 gp.selectDone = 0 334 sg = (*sudog)(gp.param) 335 gp.param = nil 336 337 // pass 3 - dequeue from unsuccessful chans 338 // otherwise they stack up on quiet channels 339 // record the successful case, if any. 340 // We singly-linked up the SudoGs in lock order. 341 casi = -1 342 cas = nil 343 caseSuccess = false 344 sglist = gp.waiting 345 // Clear all elem before unlinking from gp.waiting. 346 for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink { 347 sg1.isSelect = false 348 sg1.elem = nil 349 sg1.c = nil 350 } 351 gp.waiting = nil 352 353 for _, casei := range lockorder { 354 k = &scases[casei] 355 if sg == sglist { 356 // sg has already been dequeued by the G that woke us up. 357 casi = int(casei) 358 cas = k 359 caseSuccess = sglist.success 360 if sglist.releasetime > 0 { 361 caseReleaseTime = sglist.releasetime 362 } 363 } else { 364 c = k.c 365 if int(casei) < nsends { 366 c.sendq.dequeueSudoG(sglist) 367 } else { 368 c.recvq.dequeueSudoG(sglist) 369 } 370 } 371 sgnext = sglist.waitlink 372 sglist.waitlink = nil 373 releaseSudog(sglist) 374 sglist = sgnext 375 } 376 377 if cas == nil { 378 throw("selectgo: bad wakeup") 379 } 380 381 c = cas.c 382 383 if debugSelect { 384 print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " send=", casi < nsends, "\n") 385 } 386 387 if casi < nsends { 388 if !caseSuccess { 389 goto sclose 390 } 391 } else { 392 recvOK = caseSuccess 393 } 394 395 if raceenabled { 396 if casi < nsends { 397 raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc) 398 } else if cas.elem != nil { 399 raceWriteObjectPC(c.elemtype, cas.elem, casePC(casi), chanrecvpc) 400 } 401 } 402 if msanenabled { 403 if casi < nsends { 404 msanread(cas.elem, c.elemtype.size) 405 } else if cas.elem != nil { 406 msanwrite(cas.elem, c.elemtype.size) 407 } 408 } 409 if asanenabled { 410 if casi < nsends { 411 asanread(cas.elem, c.elemtype.size) 412 } else if cas.elem != nil { 413 asanwrite(cas.elem, c.elemtype.size) 414 } 415 } 416 417 selunlock(scases, lockorder) 418 goto retc 419 420 bufrecv: 421 // can receive from buffer 422 if raceenabled { 423 if cas.elem != nil { 424 raceWriteObjectPC(c.elemtype, cas.elem, casePC(casi), chanrecvpc) 425 } 426 racenotify(c, c.recvx, nil) 427 } 428 if msanenabled && cas.elem != nil { 429 msanwrite(cas.elem, c.elemtype.size) 430 } 431 if asanenabled && cas.elem != nil { 432 asanwrite(cas.elem, c.elemtype.size) 433 } 434 recvOK = true 435 qp = chanbuf(c, c.recvx) 436 if cas.elem != nil { 437 typedmemmove(c.elemtype, cas.elem, qp) 438 } 439 typedmemclr(c.elemtype, qp) 440 c.recvx++ 441 if c.recvx == c.dataqsiz { 442 c.recvx = 0 443 } 444 c.qcount-- 445 selunlock(scases, lockorder) 446 goto retc 447 448 bufsend: 449 // can send to buffer 450 if raceenabled { 451 racenotify(c, c.sendx, nil) 452 raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc) 453 } 454 if msanenabled { 455 msanread(cas.elem, c.elemtype.size) 456 } 457 if asanenabled { 458 asanread(cas.elem, c.elemtype.size) 459 } 460 typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem) 461 c.sendx++ 462 if c.sendx == c.dataqsiz { 463 c.sendx = 0 464 } 465 c.qcount++ 466 selunlock(scases, lockorder) 467 goto retc 468 469 recv: 470 // can receive from sleeping sender (sg) 471 recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2) 472 if debugSelect { 473 print("syncrecv: cas0=", cas0, " c=", c, "\n") 474 } 475 recvOK = true 476 goto retc 477 478 rclose: 479 // read at end of closed channel 480 selunlock(scases, lockorder) 481 recvOK = false 482 if cas.elem != nil { 483 typedmemclr(c.elemtype, cas.elem) 484 } 485 if raceenabled { 486 raceacquire(c.raceaddr()) 487 } 488 goto retc 489 490 send: 491 // can send to a sleeping receiver (sg) 492 if raceenabled { 493 raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc) 494 } 495 if msanenabled { 496 msanread(cas.elem, c.elemtype.size) 497 } 498 if asanenabled { 499 asanread(cas.elem, c.elemtype.size) 500 } 501 send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2) 502 if debugSelect { 503 print("syncsend: cas0=", cas0, " c=", c, "\n") 504 } 505 goto retc 506 507 retc: 508 if caseReleaseTime > 0 { 509 blockevent(caseReleaseTime-t0, 1) 510 } 511 return casi, recvOK 512 513 sclose: 514 // send on closed channel 515 selunlock(scases, lockorder) 516 panic(plainError("send on closed channel")) 517 } 518 519 func (c *hchan) sortkey() uintptr { 520 return uintptr(unsafe.Pointer(c)) 521 } 522 523 // A runtimeSelect is a single case passed to rselect. 524 // This must match ../reflect/value.go:/runtimeSelect 525 type runtimeSelect struct { 526 dir selectDir 527 typ unsafe.Pointer // channel type (not used here) 528 ch *hchan // channel 529 val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir) 530 } 531 532 // These values must match ../reflect/value.go:/SelectDir. 533 type selectDir int 534 535 const ( 536 _ selectDir = iota 537 selectSend // case Chan <- Send 538 selectRecv // case <-Chan: 539 selectDefault // default 540 ) 541 542 //go:linkname reflect_rselect reflect.rselect 543 func reflect_rselect(cases []runtimeSelect) (int, bool) { 544 if len(cases) == 0 { 545 block() 546 } 547 sel := make([]scase, len(cases)) 548 orig := make([]int, len(cases)) 549 nsends, nrecvs := 0, 0 550 dflt := -1 551 for i, rc := range cases { 552 var j int 553 switch rc.dir { 554 case selectDefault: 555 dflt = i 556 continue 557 case selectSend: 558 j = nsends 559 nsends++ 560 case selectRecv: 561 nrecvs++ 562 j = len(cases) - nrecvs 563 } 564 565 sel[j] = scase{c: rc.ch, elem: rc.val} 566 orig[j] = i 567 } 568 569 // Only a default case. 570 if nsends+nrecvs == 0 { 571 return dflt, false 572 } 573 574 // Compact sel and orig if necessary. 575 if nsends+nrecvs < len(cases) { 576 copy(sel[nsends:], sel[len(cases)-nrecvs:]) 577 copy(orig[nsends:], orig[len(cases)-nrecvs:]) 578 } 579 580 order := make([]uint16, 2*(nsends+nrecvs)) 581 var pc0 *uintptr 582 if raceenabled { 583 pcs := make([]uintptr, nsends+nrecvs) 584 for i := range pcs { 585 selectsetpc(&pcs[i]) 586 } 587 pc0 = &pcs[0] 588 } 589 590 chosen, recvOK := selectgo(&sel[0], &order[0], pc0, nsends, nrecvs, dflt == -1) 591 592 // Translate chosen back to caller's ordering. 593 if chosen < 0 { 594 chosen = dflt 595 } else { 596 chosen = orig[chosen] 597 } 598 return chosen, recvOK 599 } 600 601 func (q *waitq) dequeueSudoG(sgp *sudog) { 602 x := sgp.prev 603 y := sgp.next 604 if x != nil { 605 if y != nil { 606 // middle of queue 607 x.next = y 608 y.prev = x 609 sgp.next = nil 610 sgp.prev = nil 611 return 612 } 613 // end of queue 614 x.next = nil 615 q.last = x 616 sgp.prev = nil 617 return 618 } 619 if y != nil { 620 // start of queue 621 y.prev = nil 622 q.first = y 623 sgp.next = nil 624 return 625 } 626 627 // x==y==nil. Either sgp is the only element in the queue, 628 // or it has already been removed. Use q.first to disambiguate. 629 if q.first == sgp { 630 q.first = nil 631 q.last = nil 632 } 633 }