github.com/guyezi/gofrontend@v0.0.0-20200228202240-7a62a49e62c0/libgo/go/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 "unsafe" 11 ) 12 13 // For gccgo, use go:linkname to export compiler-called functions. 14 // 15 //go:linkname selectgo 16 //go:linkname block 17 18 const debugSelect = false 19 20 // scase.kind values. 21 // Known to compiler. 22 // Changes here must also be made in src/cmd/compile/internal/gc/select.go's walkselectcases. 23 const ( 24 caseNil = iota 25 caseRecv 26 caseSend 27 caseDefault 28 ) 29 30 // Select case descriptor. 31 // Known to compiler. 32 // Changes here must also be made in src/cmd/internal/gc/select.go's scasetype. 33 type scase struct { 34 c *hchan // chan 35 elem unsafe.Pointer // data element 36 kind uint16 37 releasetime int64 38 } 39 40 func sellock(scases []scase, lockorder []uint16) { 41 var c *hchan 42 for _, o := range lockorder { 43 c0 := scases[o].c 44 if c0 != nil && c0 != c { 45 c = c0 46 lock(&c.lock) 47 } 48 } 49 } 50 51 func selunlock(scases []scase, lockorder []uint16) { 52 // We must be very careful here to not touch sel after we have unlocked 53 // the last lock, because sel can be freed right after the last unlock. 54 // Consider the following situation. 55 // First M calls runtime·park() in runtime·selectgo() passing the sel. 56 // Once runtime·park() has unlocked the last lock, another M makes 57 // the G that calls select runnable again and schedules it for execution. 58 // When the G runs on another M, it locks all the locks and frees sel. 59 // Now if the first M touches sel, it will access freed memory. 60 for i := len(scases) - 1; i >= 0; i-- { 61 c := scases[lockorder[i]].c 62 if c == nil { 63 break 64 } 65 if i > 0 && c == scases[lockorder[i-1]].c { 66 continue // will unlock it on the next iteration 67 } 68 unlock(&c.lock) 69 } 70 } 71 72 func selparkcommit(gp *g, _ unsafe.Pointer) bool { 73 // There are unlocked sudogs that point into gp's stack. Stack 74 // copying must lock the channels of those sudogs. 75 gp.activeStackChans = true 76 // This must not access gp's stack (see gopark). In 77 // particular, it must not access the *hselect. That's okay, 78 // because by the time this is called, gp.waiting has all 79 // channels in lock order. 80 var lastc *hchan 81 for sg := gp.waiting; sg != nil; sg = sg.waitlink { 82 if sg.c != lastc && lastc != nil { 83 // As soon as we unlock the channel, fields in 84 // any sudog with that channel may change, 85 // including c and waitlink. Since multiple 86 // sudogs may have the same channel, we unlock 87 // only after we've passed the last instance 88 // of a channel. 89 unlock(&lastc.lock) 90 } 91 lastc = sg.c 92 } 93 if lastc != nil { 94 unlock(&lastc.lock) 95 } 96 return true 97 } 98 99 func block() { 100 gopark(nil, nil, waitReasonSelectNoCases, traceEvGoStop, 1) // forever 101 } 102 103 // selectgo implements the select statement. 104 // 105 // cas0 points to an array of type [ncases]scase, and order0 points to 106 // an array of type [2*ncases]uint16. Both reside on the goroutine's 107 // stack (regardless of any escaping in selectgo). 108 // 109 // selectgo returns the index of the chosen scase, which matches the 110 // ordinal position of its respective select{recv,send,default} call. 111 // Also, if the chosen scase was a receive operation, it reports whether 112 // a value was received. 113 func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) { 114 if debugSelect { 115 print("select: cas0=", cas0, "\n") 116 } 117 118 cas1 := (*[1 << 16]scase)(unsafe.Pointer(cas0)) 119 order1 := (*[1 << 17]uint16)(unsafe.Pointer(order0)) 120 121 scases := cas1[:ncases:ncases] 122 pollorder := order1[:ncases:ncases] 123 lockorder := order1[ncases:][:ncases:ncases] 124 125 // Replace send/receive cases involving nil channels with 126 // caseNil so logic below can assume non-nil channel. 127 for i := range scases { 128 cas := &scases[i] 129 if cas.c == nil && cas.kind != caseDefault { 130 *cas = scase{} 131 } 132 } 133 134 var t0 int64 135 if blockprofilerate > 0 { 136 t0 = cputicks() 137 for i := 0; i < ncases; i++ { 138 scases[i].releasetime = -1 139 } 140 } 141 142 // The compiler rewrites selects that statically have 143 // only 0 or 1 cases plus default into simpler constructs. 144 // The only way we can end up with such small sel.ncase 145 // values here is for a larger select in which most channels 146 // have been nilled out. The general code handles those 147 // cases correctly, and they are rare enough not to bother 148 // optimizing (and needing to test). 149 150 // needed for gccgo, which doesn't zero pollorder 151 if ncases > 0 { 152 pollorder[0] = 0 153 } 154 155 // generate permuted order 156 for i := 1; i < ncases; i++ { 157 j := fastrandn(uint32(i + 1)) 158 pollorder[i] = pollorder[j] 159 pollorder[j] = uint16(i) 160 } 161 162 // sort the cases by Hchan address to get the locking order. 163 // simple heap sort, to guarantee n log n time and constant stack footprint. 164 for i := 0; i < ncases; i++ { 165 j := i 166 // Start with the pollorder to permute cases on the same channel. 167 c := scases[pollorder[i]].c 168 for j > 0 && scases[lockorder[(j-1)/2]].c.sortkey() < c.sortkey() { 169 k := (j - 1) / 2 170 lockorder[j] = lockorder[k] 171 j = k 172 } 173 lockorder[j] = pollorder[i] 174 } 175 for i := ncases - 1; i >= 0; i-- { 176 o := lockorder[i] 177 c := scases[o].c 178 lockorder[i] = lockorder[0] 179 j := 0 180 for { 181 k := j*2 + 1 182 if k >= i { 183 break 184 } 185 if k+1 < i && scases[lockorder[k]].c.sortkey() < scases[lockorder[k+1]].c.sortkey() { 186 k++ 187 } 188 if c.sortkey() < scases[lockorder[k]].c.sortkey() { 189 lockorder[j] = lockorder[k] 190 j = k 191 continue 192 } 193 break 194 } 195 lockorder[j] = o 196 } 197 198 if debugSelect { 199 for i := 0; i+1 < ncases; i++ { 200 if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() { 201 print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n") 202 throw("select: broken sort") 203 } 204 } 205 } 206 207 // lock all the channels involved in the select 208 sellock(scases, lockorder) 209 210 var ( 211 gp *g 212 sg *sudog 213 c *hchan 214 k *scase 215 sglist *sudog 216 sgnext *sudog 217 qp unsafe.Pointer 218 nextp **sudog 219 ) 220 221 loop: 222 // pass 1 - look for something already waiting 223 var dfli int 224 var dfl *scase 225 var casi int 226 var cas *scase 227 var recvOK bool 228 for i := 0; i < ncases; i++ { 229 casi = int(pollorder[i]) 230 cas = &scases[casi] 231 c = cas.c 232 233 switch cas.kind { 234 case caseNil: 235 continue 236 237 case caseRecv: 238 sg = c.sendq.dequeue() 239 if sg != nil { 240 goto recv 241 } 242 if c.qcount > 0 { 243 goto bufrecv 244 } 245 if c.closed != 0 { 246 goto rclose 247 } 248 249 case caseSend: 250 if c.closed != 0 { 251 goto sclose 252 } 253 sg = c.recvq.dequeue() 254 if sg != nil { 255 goto send 256 } 257 if c.qcount < c.dataqsiz { 258 goto bufsend 259 } 260 261 case caseDefault: 262 dfli = casi 263 dfl = cas 264 } 265 } 266 267 if dfl != nil { 268 selunlock(scases, lockorder) 269 casi = dfli 270 cas = dfl 271 goto retc 272 } 273 274 // pass 2 - enqueue on all chans 275 gp = getg() 276 if gp.waiting != nil { 277 throw("gp.waiting != nil") 278 } 279 nextp = &gp.waiting 280 for _, casei := range lockorder { 281 casi = int(casei) 282 cas = &scases[casi] 283 if cas.kind == caseNil { 284 continue 285 } 286 c = cas.c 287 sg := acquireSudog() 288 sg.g = gp 289 sg.isSelect = true 290 // No stack splits between assigning elem and enqueuing 291 // sg on gp.waiting where copystack can find it. 292 sg.elem = cas.elem 293 sg.releasetime = 0 294 if t0 != 0 { 295 sg.releasetime = -1 296 } 297 sg.c = c 298 // Construct waiting list in lock order. 299 *nextp = sg 300 nextp = &sg.waitlink 301 302 switch cas.kind { 303 case caseRecv: 304 c.recvq.enqueue(sg) 305 306 case caseSend: 307 c.sendq.enqueue(sg) 308 } 309 } 310 311 // wait for someone to wake us up 312 gp.param = nil 313 gopark(selparkcommit, nil, waitReasonSelect, traceEvGoBlockSelect, 1) 314 gp.activeStackChans = false 315 316 sellock(scases, lockorder) 317 318 gp.selectDone = 0 319 sg = (*sudog)(gp.param) 320 gp.param = nil 321 322 // pass 3 - dequeue from unsuccessful chans 323 // otherwise they stack up on quiet channels 324 // record the successful case, if any. 325 // We singly-linked up the SudoGs in lock order. 326 casi = -1 327 cas = nil 328 sglist = gp.waiting 329 // Clear all elem before unlinking from gp.waiting. 330 for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink { 331 sg1.isSelect = false 332 sg1.elem = nil 333 sg1.c = nil 334 } 335 gp.waiting = nil 336 337 for _, casei := range lockorder { 338 k = &scases[casei] 339 if k.kind == caseNil { 340 continue 341 } 342 if sglist.releasetime > 0 { 343 k.releasetime = sglist.releasetime 344 } 345 if sg == sglist { 346 // sg has already been dequeued by the G that woke us up. 347 casi = int(casei) 348 cas = k 349 } else { 350 c = k.c 351 if k.kind == caseSend { 352 c.sendq.dequeueSudoG(sglist) 353 } else { 354 c.recvq.dequeueSudoG(sglist) 355 } 356 } 357 sgnext = sglist.waitlink 358 sglist.waitlink = nil 359 releaseSudog(sglist) 360 sglist = sgnext 361 } 362 363 if cas == nil { 364 // We can wake up with gp.param == nil (so cas == nil) 365 // when a channel involved in the select has been closed. 366 // It is easiest to loop and re-run the operation; 367 // we'll see that it's now closed. 368 // Maybe some day we can signal the close explicitly, 369 // but we'd have to distinguish close-on-reader from close-on-writer. 370 // It's easiest not to duplicate the code and just recheck above. 371 // We know that something closed, and things never un-close, 372 // so we won't block again. 373 goto loop 374 } 375 376 c = cas.c 377 378 if debugSelect { 379 print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " kind=", cas.kind, "\n") 380 } 381 382 if cas.kind == caseRecv { 383 recvOK = true 384 } 385 386 selunlock(scases, lockorder) 387 goto retc 388 389 bufrecv: 390 // can receive from buffer 391 recvOK = true 392 qp = chanbuf(c, c.recvx) 393 if cas.elem != nil { 394 typedmemmove(c.elemtype, cas.elem, qp) 395 } 396 typedmemclr(c.elemtype, qp) 397 c.recvx++ 398 if c.recvx == c.dataqsiz { 399 c.recvx = 0 400 } 401 c.qcount-- 402 selunlock(scases, lockorder) 403 goto retc 404 405 bufsend: 406 // can send to buffer 407 typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem) 408 c.sendx++ 409 if c.sendx == c.dataqsiz { 410 c.sendx = 0 411 } 412 c.qcount++ 413 selunlock(scases, lockorder) 414 goto retc 415 416 recv: 417 // can receive from sleeping sender (sg) 418 recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2) 419 if debugSelect { 420 print("syncrecv: cas0=", cas0, " c=", c, "\n") 421 } 422 recvOK = true 423 goto retc 424 425 rclose: 426 // read at end of closed channel 427 selunlock(scases, lockorder) 428 recvOK = false 429 if cas.elem != nil { 430 typedmemclr(c.elemtype, cas.elem) 431 } 432 if raceenabled { 433 raceacquire(c.raceaddr()) 434 } 435 goto retc 436 437 send: 438 // can send to a sleeping receiver (sg) 439 send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2) 440 if debugSelect { 441 print("syncsend: cas0=", cas0, " c=", c, "\n") 442 } 443 goto retc 444 445 retc: 446 if cas.releasetime > 0 { 447 blockevent(cas.releasetime-t0, 1) 448 } 449 450 // Check preemption, since unlike gc we don't check on every call. 451 // A test case for this one is BenchmarkPingPongHog in proc_test.go. 452 if dfl != nil && getg().preempt { 453 checkPreempt() 454 } 455 456 return casi, recvOK 457 458 sclose: 459 // send on closed channel 460 selunlock(scases, lockorder) 461 panic(plainError("send on closed channel")) 462 } 463 464 func (c *hchan) sortkey() uintptr { 465 return uintptr(unsafe.Pointer(c)) 466 } 467 468 // A runtimeSelect is a single case passed to rselect. 469 // This must match ../reflect/value.go:/runtimeSelect 470 type runtimeSelect struct { 471 dir selectDir 472 typ unsafe.Pointer // channel type (not used here) 473 ch *hchan // channel 474 val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir) 475 } 476 477 // These values must match ../reflect/value.go:/SelectDir. 478 type selectDir int 479 480 const ( 481 _ selectDir = iota 482 selectSend // case Chan <- Send 483 selectRecv // case <-Chan: 484 selectDefault // default 485 ) 486 487 //go:linkname reflect_rselect reflect.rselect 488 func reflect_rselect(cases []runtimeSelect) (int, bool) { 489 if len(cases) == 0 { 490 block() 491 } 492 sel := make([]scase, len(cases)) 493 order := make([]uint16, 2*len(cases)) 494 for i := range cases { 495 rc := &cases[i] 496 switch rc.dir { 497 case selectDefault: 498 sel[i] = scase{kind: caseDefault} 499 case selectSend: 500 sel[i] = scase{kind: caseSend, c: rc.ch, elem: rc.val} 501 case selectRecv: 502 sel[i] = scase{kind: caseRecv, c: rc.ch, elem: rc.val} 503 } 504 } 505 506 return selectgo(&sel[0], &order[0], len(cases)) 507 } 508 509 func (q *waitq) dequeueSudoG(sgp *sudog) { 510 x := sgp.prev 511 y := sgp.next 512 if x != nil { 513 if y != nil { 514 // middle of queue 515 x.next = y 516 y.prev = x 517 sgp.next = nil 518 sgp.prev = nil 519 return 520 } 521 // end of queue 522 x.next = nil 523 q.last = x 524 sgp.prev = nil 525 return 526 } 527 if y != nil { 528 // start of queue 529 y.prev = nil 530 q.first = y 531 sgp.next = nil 532 return 533 } 534 535 // x==y==nil. Either sgp is the only element in the queue, 536 // or it has already been removed. Use q.first to disambiguate. 537 if q.first == sgp { 538 q.first = nil 539 q.last = nil 540 } 541 }