1 // Copyright 2014 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 channels. 8 9 // Invariants: 10 // At least one of c.sendq and c.recvq is empty, 11 // except for the case of an unbuffered channel with a single goroutine 12 // blocked on it for both sending and receiving using a select statement, 13 // in which case the length of c.sendq and c.recvq is limited only by the 14 // size of the select statement. 15 // 16 // For buffered channels, also: 17 // c.qcount > 0 implies that c.recvq is empty. 18 // c.qcount < c.dataqsiz implies that c.sendq is empty. 19 20 import ( 21 "runtime/internal/atomic" 22 "runtime/internal/math" 23 "unsafe" 24 ) 25 26 const ( 27 maxAlign = 8 28 hchanSize = unsafe.Sizeof(hchan{}) + uintptr(-int(unsafe.Sizeof(hchan{}))&(maxAlign-1)) 29 debugChan = false 30 ) 31 32 type hchan struct { 33 qcount uint // total data in the queue 34 dataqsiz uint // size of the circular queue 35 buf unsafe.Pointer // points to an array of dataqsiz elements 36 elemsize uint16 37 closed uint32 38 elemtype *_type // element type 39 sendx uint // send index 40 recvx uint // receive index 41 recvq waitq // list of recv waiters 42 sendq waitq // list of send waiters 43 44 // lock protects all fields in hchan, as well as several 45 // fields in sudogs blocked on this channel. 46 // 47 // Do not change another G's status while holding this lock 48 // (in particular, do not ready a G), as this can deadlock 49 // with stack shrinking. 50 lock mutex 51 } 52 53 type waitq struct { 54 first *sudog 55 last *sudog 56 } 57 58 //go:linkname reflect_makechan reflect.makechan 59 func reflect_makechan(t *chantype, size int) *hchan { 60 return makechan(t, size) 61 } 62 63 func makechan64(t *chantype, size int64) *hchan { 64 if int64(int(size)) != size { 65 panic(plainError("makechan: size out of range")) 66 } 67 68 return makechan(t, int(size)) 69 } 70 71 func makechan(t *chantype, size int) *hchan { 72 elem := t.elem 73 74 // compiler checks this but be safe. 75 if elem.size >= 1<<16 { 76 throw("makechan: invalid channel element type") 77 } 78 if hchanSize%maxAlign != 0 || elem.align > maxAlign { 79 throw("makechan: bad alignment") 80 } 81 82 mem, overflow := math.MulUintptr(elem.size, uintptr(size)) 83 if overflow || mem > maxAlloc-hchanSize || size < 0 { 84 panic(plainError("makechan: size out of range")) 85 } 86 87 // Hchan does not contain pointers interesting for GC when elements stored in buf do not contain pointers. 88 // buf points into the same allocation, elemtype is persistent. 89 // SudoG's are referenced from their owning thread so they can't be collected. 90 // TODO(dvyukov,rlh): Rethink when collector can move allocated objects. 91 var c *hchan 92 switch { 93 case mem == 0: 94 // Queue or element size is zero. 95 c = (*hchan)(mallocgc(hchanSize, nil, true)) 96 // Race detector uses this location for synchronization. 97 c.buf = c.raceaddr() 98 case elem.ptrdata == 0: 99 // Elements do not contain pointers. 100 // Allocate hchan and buf in one call. 101 c = (*hchan)(mallocgc(hchanSize+mem, nil, true)) 102 c.buf = add(unsafe.Pointer(c), hchanSize) 103 default: 104 // Elements contain pointers. 105 c = new(hchan) 106 c.buf = mallocgc(mem, elem, true) 107 } 108 109 c.elemsize = uint16(elem.size) 110 c.elemtype = elem 111 c.dataqsiz = uint(size) 112 113 if debugChan { 114 print("makechan: chan=", c, "; elemsize=", elem.size, "; elemalg=", elem.alg, "; dataqsiz=", size, "\n") 115 } 116 return c 117 } 118 119 // chanbuf(c, i) is pointer to the i'th slot in the buffer. 120 func chanbuf(c *hchan, i uint) unsafe.Pointer { 121 return add(c.buf, uintptr(i)*uintptr(c.elemsize)) 122 } 123 124 // entry point for c <- x from compiled code 125 //go:nosplit 126 func chansend1(c *hchan, elem unsafe.Pointer) { 127 chansend(c, elem, true, getcallerpc()) 128 } 129 130 /* 131 * generic single channel send/recv 132 * If block is not nil, 133 * then the protocol will not 134 * sleep but return if it could 135 * not complete. 136 * 137 * sleep can wake up with g.param == nil 138 * when a channel involved in the sleep has 139 * been closed. it is easiest to loop and re-run 140 * the operation; we'll see that it's now closed. 141 */ 142 func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool { 143 if c == nil { 144 if !block { 145 return false 146 } 147 gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2) 148 throw("unreachable") 149 } 150 151 if debugChan { 152 print("chansend: chan=", c, "\n") 153 } 154 155 if raceenabled { 156 racereadpc(c.raceaddr(), callerpc, funcPC(chansend)) 157 } 158 159 // Fast path: check for failed non-blocking operation without acquiring the lock. 160 // 161 // After observing that the channel is not closed, we observe that the channel is 162 // not ready for sending. Each of these observations is a single word-sized read 163 // (first c.closed and second c.recvq.first or c.qcount depending on kind of channel). 164 // Because a closed channel cannot transition from 'ready for sending' to 165 // 'not ready for sending', even if the channel is closed between the two observations, 166 // they imply a moment between the two when the channel was both not yet closed 167 // and not ready for sending. We behave as if we observed the channel at that moment, 168 // and report that the send cannot proceed. 169 // 170 // It is okay if the reads are reordered here: if we observe that the channel is not 171 // ready for sending and then observe that it is not closed, that implies that the 172 // channel wasn't closed during the first observation. 173 if !block && c.closed == 0 && ((c.dataqsiz == 0 && c.recvq.first == nil) || 174 (c.dataqsiz > 0 && c.qcount == c.dataqsiz)) { 175 return false 176 } 177 178 var t0 int64 179 if blockprofilerate > 0 { 180 t0 = cputicks() 181 } 182 183 lock(&c.lock) 184 185 if c.closed != 0 { 186 unlock(&c.lock) 187 panic(plainError("send on closed channel")) 188 } 189 190 if sg := c.recvq.dequeue(); sg != nil { 191 // Found a waiting receiver. We pass the value we want to send 192 // directly to the receiver, bypassing the channel buffer (if any). 193 send(c, sg, ep, func() { unlock(&c.lock) }, 3) 194 return true 195 } 196 197 if c.qcount < c.dataqsiz { 198 // Space is available in the channel buffer. Enqueue the element to send. 199 qp := chanbuf(c, c.sendx) 200 if raceenabled { 201 raceacquire(qp) 202 racerelease(qp) 203 } 204 typedmemmove(c.elemtype, qp, ep) 205 c.sendx++ 206 if c.sendx == c.dataqsiz { 207 c.sendx = 0 208 } 209 c.qcount++ 210 unlock(&c.lock) 211 return true 212 } 213 214 if !block { 215 unlock(&c.lock) 216 return false 217 } 218 219 // Block on the channel. Some receiver will complete our operation for us. 220 gp := getg() 221 mysg := acquireSudog() 222 mysg.releasetime = 0 223 if t0 != 0 { 224 mysg.releasetime = -1 225 } 226 // No stack splits between assigning elem and enqueuing mysg 227 // on gp.waiting where copystack can find it. 228 mysg.elem = ep 229 mysg.waitlink = nil 230 mysg.g = gp 231 mysg.isSelect = false 232 mysg.c = c 233 gp.waiting = mysg 234 gp.param = nil 235 c.sendq.enqueue(mysg) 236 goparkunlock(&c.lock, waitReasonChanSend, traceEvGoBlockSend, 3) 237 // Ensure the value being sent is kept alive until the 238 // receiver copies it out. The sudog has a pointer to the 239 // stack object, but sudogs aren't considered as roots of the 240 // stack tracer. 241 KeepAlive(ep) 242 243 // someone woke us up. 244 if mysg != gp.waiting { 245 throw("G waiting list is corrupted") 246 } 247 gp.waiting = nil 248 if gp.param == nil { 249 if c.closed == 0 { 250 throw("chansend: spurious wakeup") 251 } 252 panic(plainError("send on closed channel")) 253 } 254 gp.param = nil 255 if mysg.releasetime > 0 { 256 blockevent(mysg.releasetime-t0, 2) 257 } 258 mysg.c = nil 259 releaseSudog(mysg) 260 return true 261 } 262 263 // send processes a send operation on an empty channel c. 264 // The value ep sent by the sender is copied to the receiver sg. 265 // The receiver is then woken up to go on its merry way. 266 // Channel c must be empty and locked. send unlocks c with unlockf. 267 // sg must already be dequeued from c. 268 // ep must be non-nil and point to the heap or the caller's stack. 269 func send(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) { 270 if raceenabled { 271 if c.dataqsiz == 0 { 272 racesync(c, sg) 273 } else { 274 // Pretend we go through the buffer, even though 275 // we copy directly. Note that we need to increment 276 // the head/tail locations only when raceenabled. 277 qp := chanbuf(c, c.recvx) 278 raceacquire(qp) 279 racerelease(qp) 280 raceacquireg(sg.g, qp) 281 racereleaseg(sg.g, qp) 282 c.recvx++ 283 if c.recvx == c.dataqsiz { 284 c.recvx = 0 285 } 286 c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz 287 } 288 } 289 if sg.elem != nil { 290 sendDirect(c.elemtype, sg, ep) 291 sg.elem = nil 292 } 293 gp := sg.g 294 unlockf() 295 gp.param = unsafe.Pointer(sg) 296 if sg.releasetime != 0 { 297 sg.releasetime = cputicks() 298 } 299 goready(gp, skip+1) 300 } 301 302 // Sends and receives on unbuffered or empty-buffered channels are the 303 // only operations where one running goroutine writes to the stack of 304 // another running goroutine. The GC assumes that stack writes only 305 // happen when the goroutine is running and are only done by that 306 // goroutine. Using a write barrier is sufficient to make up for 307 // violating that assumption, but the write barrier has to work. 308 // typedmemmove will call bulkBarrierPreWrite, but the target bytes 309 // are not in the heap, so that will not help. We arrange to call 310 // memmove and typeBitsBulkBarrier instead. 311 312 func sendDirect(t *_type, sg *sudog, src unsafe.Pointer) { 313 // src is on our stack, dst is a slot on another stack. 314 315 // Once we read sg.elem out of sg, it will no longer 316 // be updated if the destination's stack gets copied (shrunk). 317 // So make sure that no preemption points can happen between read & use. 318 dst := sg.elem 319 typeBitsBulkBarrier(t, uintptr(dst), uintptr(src), t.size) 320 // No need for cgo write barrier checks because dst is always 321 // Go memory. 322 memmove(dst, src, t.size) 323 } 324 325 func recvDirect(t *_type, sg *sudog, dst unsafe.Pointer) { 326 // dst is on our stack or the heap, src is on another stack. 327 // The channel is locked, so src will not move during this 328 // operation. 329 src := sg.elem 330 typeBitsBulkBarrier(t, uintptr(dst), uintptr(src), t.size) 331 memmove(dst, src, t.size) 332 } 333 334 func closechan(c *hchan) { 335 if c == nil { 336 panic(plainError("close of nil channel")) 337 } 338 339 lock(&c.lock) 340 if c.closed != 0 { 341 unlock(&c.lock) 342 panic(plainError("close of closed channel")) 343 } 344 345 if raceenabled { 346 callerpc := getcallerpc() 347 racewritepc(c.raceaddr(), callerpc, funcPC(closechan)) 348 racerelease(c.raceaddr()) 349 } 350 351 c.closed = 1 352 353 var glist gList 354 355 // release all readers 356 for { 357 sg := c.recvq.dequeue() 358 if sg == nil { 359 break 360 } 361 if sg.elem != nil { 362 typedmemclr(c.elemtype, sg.elem) 363 sg.elem = nil 364 } 365 if sg.releasetime != 0 { 366 sg.releasetime = cputicks() 367 } 368 gp := sg.g 369 gp.param = nil 370 if raceenabled { 371 raceacquireg(gp, c.raceaddr()) 372 } 373 glist.push(gp) 374 } 375 376 // release all writers (they will panic) 377 for { 378 sg := c.sendq.dequeue() 379 if sg == nil { 380 break 381 } 382 sg.elem = nil 383 if sg.releasetime != 0 { 384 sg.releasetime = cputicks() 385 } 386 gp := sg.g 387 gp.param = nil 388 if raceenabled { 389 raceacquireg(gp, c.raceaddr()) 390 } 391 glist.push(gp) 392 } 393 unlock(&c.lock) 394 395 // Ready all Gs now that we've dropped the channel lock. 396 for !glist.empty() { 397 gp := glist.pop() 398 gp.schedlink = 0 399 goready(gp, 3) 400 } 401 } 402 403 // entry points for <- c from compiled code 404 //go:nosplit 405 func chanrecv1(c *hchan, elem unsafe.Pointer) { 406 chanrecv(c, elem, true) 407 } 408 409 //go:nosplit 410 func chanrecv2(c *hchan, elem unsafe.Pointer) (received bool) { 411 _, received = chanrecv(c, elem, true) 412 return 413 } 414 415 // chanrecv receives on channel c and writes the received data to ep. 416 // ep may be nil, in which case received data is ignored. 417 // If block == false and no elements are available, returns (false, false). 418 // Otherwise, if c is closed, zeros *ep and returns (true, false). 419 // Otherwise, fills in *ep with an element and returns (true, true). 420 // A non-nil ep must point to the heap or the caller's stack. 421 func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) { 422 // raceenabled: don't need to check ep, as it is always on the stack 423 // or is new memory allocated by reflect. 424 425 if debugChan { 426 print("chanrecv: chan=", c, "\n") 427 } 428 429 if c == nil { 430 if !block { 431 return 432 } 433 gopark(nil, nil, waitReasonChanReceiveNilChan, traceEvGoStop, 2) 434 throw("unreachable") 435 } 436 437 // Fast path: check for failed non-blocking operation without acquiring the lock. 438 // 439 // After observing that the channel is not ready for receiving, we observe that the 440 // channel is not closed. Each of these observations is a single word-sized read 441 // (first c.sendq.first or c.qcount, and second c.closed). 442 // Because a channel cannot be reopened, the later observation of the channel 443 // being not closed implies that it was also not closed at the moment of the 444 // first observation. We behave as if we observed the channel at that moment 445 // and report that the receive cannot proceed. 446 // 447 // The order of operations is important here: reversing the operations can lead to 448 // incorrect behavior when racing with a close. 449 if !block && (c.dataqsiz == 0 && c.sendq.first == nil || 450 c.dataqsiz > 0 && atomic.Loaduint(&c.qcount) == 0) && 451 atomic.Load(&c.closed) == 0 { 452 return 453 } 454 455 var t0 int64 456 if blockprofilerate > 0 { 457 t0 = cputicks() 458 } 459 460 lock(&c.lock) 461 462 if c.closed != 0 && c.qcount == 0 { 463 if raceenabled { 464 raceacquire(c.raceaddr()) 465 } 466 unlock(&c.lock) 467 if ep != nil { 468 typedmemclr(c.elemtype, ep) 469 } 470 return true, false 471 } 472 473 if sg := c.sendq.dequeue(); sg != nil { 474 // Found a waiting sender. If buffer is size 0, receive value 475 // directly from sender. Otherwise, receive from head of queue 476 // and add sender's value to the tail of the queue (both map to 477 // the same buffer slot because the queue is full). 478 recv(c, sg, ep, func() { unlock(&c.lock) }, 3) 479 return true, true 480 } 481 482 if c.qcount > 0 { 483 // Receive directly from queue 484 qp := chanbuf(c, c.recvx) 485 if raceenabled { 486 raceacquire(qp) 487 racerelease(qp) 488 } 489 if ep != nil { 490 typedmemmove(c.elemtype, ep, qp) 491 } 492 typedmemclr(c.elemtype, qp) 493 c.recvx++ 494 if c.recvx == c.dataqsiz { 495 c.recvx = 0 496 } 497 c.qcount-- 498 unlock(&c.lock) 499 return true, true 500 } 501 502 if !block { 503 unlock(&c.lock) 504 return false, false 505 } 506 507 // no sender available: block on this channel. 508 gp := getg() 509 mysg := acquireSudog() 510 mysg.releasetime = 0 511 if t0 != 0 { 512 mysg.releasetime = -1 513 } 514 // No stack splits between assigning elem and enqueuing mysg 515 // on gp.waiting where copystack can find it. 516 mysg.elem = ep 517 mysg.waitlink = nil 518 gp.waiting = mysg 519 mysg.g = gp 520 mysg.isSelect = false 521 mysg.c = c 522 gp.param = nil 523 c.recvq.enqueue(mysg) 524 goparkunlock(&c.lock, waitReasonChanReceive, traceEvGoBlockRecv, 3) 525 526 // someone woke us up 527 if mysg != gp.waiting { 528 throw("G waiting list is corrupted") 529 } 530 gp.waiting = nil 531 if mysg.releasetime > 0 { 532 blockevent(mysg.releasetime-t0, 2) 533 } 534 closed := gp.param == nil 535 gp.param = nil 536 mysg.c = nil 537 releaseSudog(mysg) 538 return true, !closed 539 } 540 541 // recv processes a receive operation on a full channel c. 542 // There are 2 parts: 543 // 1) The value sent by the sender sg is put into the channel 544 // and the sender is woken up to go on its merry way. 545 // 2) The value received by the receiver (the current G) is 546 // written to ep. 547 // For synchronous channels, both values are the same. 548 // For asynchronous channels, the receiver gets its data from 549 // the channel buffer and the sender's data is put in the 550 // channel buffer. 551 // Channel c must be full and locked. recv unlocks c with unlockf. 552 // sg must already be dequeued from c. 553 // A non-nil ep must point to the heap or the caller's stack. 554 func recv(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) { 555 if c.dataqsiz == 0 { 556 if raceenabled { 557 racesync(c, sg) 558 } 559 if ep != nil { 560 // copy data from sender 561 recvDirect(c.elemtype, sg, ep) 562 } 563 } else { 564 // Queue is full. Take the item at the 565 // head of the queue. Make the sender enqueue 566 // its item at the tail of the queue. Since the 567 // queue is full, those are both the same slot. 568 qp := chanbuf(c, c.recvx) 569 if raceenabled { 570 raceacquire(qp) 571 racerelease(qp) 572 raceacquireg(sg.g, qp) 573 racereleaseg(sg.g, qp) 574 } 575 // copy data from queue to receiver 576 if ep != nil { 577 typedmemmove(c.elemtype, ep, qp) 578 } 579 // copy data from sender to queue 580 typedmemmove(c.elemtype, qp, sg.elem) 581 c.recvx++ 582 if c.recvx == c.dataqsiz { 583 c.recvx = 0 584 } 585 c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz 586 } 587 sg.elem = nil 588 gp := sg.g 589 unlockf() 590 gp.param = unsafe.Pointer(sg) 591 if sg.releasetime != 0 { 592 sg.releasetime = cputicks() 593 } 594 goready(gp, skip+1) 595 } 596 597 // compiler implements 598 // 599 // select { 600 // case c <- v: 601 // ... foo 602 // default: 603 // ... bar 604 // } 605 // 606 // as 607 // 608 // if selectnbsend(c, v) { 609 // ... foo 610 // } else { 611 // ... bar 612 // } 613 // 614 func selectnbsend(c *hchan, elem unsafe.Pointer) (selected bool) { 615 return chansend(c, elem, false, getcallerpc()) 616 } 617 618 // compiler implements 619 // 620 // select { 621 // case v = <-c: 622 // ... foo 623 // default: 624 // ... bar 625 // } 626 // 627 // as 628 // 629 // if selectnbrecv(&v, c) { 630 // ... foo 631 // } else { 632 // ... bar 633 // } 634 // 635 func selectnbrecv(elem unsafe.Pointer, c *hchan) (selected bool) { 636 selected, _ = chanrecv(c, elem, false) 637 return 638 } 639 640 // compiler implements 641 // 642 // select { 643 // case v, ok = <-c: 644 // ... foo 645 // default: 646 // ... bar 647 // } 648 // 649 // as 650 // 651 // if c != nil && selectnbrecv2(&v, &ok, c) { 652 // ... foo 653 // } else { 654 // ... bar 655 // } 656 // 657 func selectnbrecv2(elem unsafe.Pointer, received *bool, c *hchan) (selected bool) { 658 // TODO(khr): just return 2 values from this function, now that it is in Go. 659 selected, *received = chanrecv(c, elem, false) 660 return 661 } 662 663 //go:linkname reflect_chansend reflect.chansend 664 func reflect_chansend(c *hchan, elem unsafe.Pointer, nb bool) (selected bool) { 665 return chansend(c, elem, !nb, getcallerpc()) 666 } 667 668 //go:linkname reflect_chanrecv reflect.chanrecv 669 func reflect_chanrecv(c *hchan, nb bool, elem unsafe.Pointer) (selected bool, received bool) { 670 return chanrecv(c, elem, !nb) 671 } 672 673 //go:linkname reflect_chanlen reflect.chanlen 674 func reflect_chanlen(c *hchan) int { 675 if c == nil { 676 return 0 677 } 678 return int(c.qcount) 679 } 680 681 //go:linkname reflectlite_chanlen internal/reflectlite.chanlen 682 func reflectlite_chanlen(c *hchan) int { 683 if c == nil { 684 return 0 685 } 686 return int(c.qcount) 687 } 688 689 //go:linkname reflect_chancap reflect.chancap 690 func reflect_chancap(c *hchan) int { 691 if c == nil { 692 return 0 693 } 694 return int(c.dataqsiz) 695 } 696 697 //go:linkname reflect_chanclose reflect.chanclose 698 func reflect_chanclose(c *hchan) { 699 closechan(c) 700 } 701 702 func (q *waitq) enqueue(sgp *sudog) { 703 sgp.next = nil 704 x := q.last 705 if x == nil { 706 sgp.prev = nil 707 q.first = sgp 708 q.last = sgp 709 return 710 } 711 sgp.prev = x 712 x.next = sgp 713 q.last = sgp 714 } 715 716 func (q *waitq) dequeue() *sudog { 717 for { 718 sgp := q.first 719 if sgp == nil { 720 return nil 721 } 722 y := sgp.next 723 if y == nil { 724 q.first = nil 725 q.last = nil 726 } else { 727 y.prev = nil 728 q.first = y 729 sgp.next = nil // mark as removed (see dequeueSudog) 730 } 731 732 // if a goroutine was put on this queue because of a 733 // select, there is a small window between the goroutine 734 // being woken up by a different case and it grabbing the 735 // channel locks. Once it has the lock 736 // it removes itself from the queue, so we won't see it after that. 737 // We use a flag in the G struct to tell us when someone 738 // else has won the race to signal this goroutine but the goroutine 739 // hasn't removed itself from the queue yet. 740 if sgp.isSelect && !atomic.Cas(&sgp.g.selectDone, 0, 1) { 741 continue 742 } 743 744 return sgp 745 } 746 } 747 748 func (c *hchan) raceaddr() unsafe.Pointer { 749 // Treat read-like and write-like operations on the channel to 750 // happen at this address. Avoid using the address of qcount 751 // or dataqsiz, because the len() and cap() builtins read 752 // those addresses, and we don't want them racing with 753 // operations like close(). 754 return unsafe.Pointer(&c.buf) 755 } 756 757 func racesync(c *hchan, sg *sudog) { 758 racerelease(chanbuf(c, 0)) 759 raceacquireg(sg.g, chanbuf(c, 0)) 760 racereleaseg(sg.g, chanbuf(c, 0)) 761 raceacquire(chanbuf(c, 0)) 762 } 763
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