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 (
    10  	"internal/abi"
    11  	"internal/runtime/sys"
    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 = sys.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  	gp.parkingOnChan.Store(false)
    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, traceBlockForever, 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  		if cas.c.timer != nil {
   178  			cas.c.timer.maybeRunChan()
   179  		}
   180  
   181  		j := cheaprandn(uint32(norder + 1))
   182  		pollorder[norder] = pollorder[j]
   183  		pollorder[j] = uint16(i)
   184  		norder++
   185  	}
   186  	pollorder = pollorder[:norder]
   187  	lockorder = lockorder[:norder]
   188  
   189  	// sort the cases by Hchan address to get the locking order.
   190  	// simple heap sort, to guarantee n log n time and constant stack footprint.
   191  	for i := range lockorder {
   192  		j := i
   193  		// Start with the pollorder to permute cases on the same channel.
   194  		c := scases[pollorder[i]].c
   195  		for j > 0 && scases[lockorder[(j-1)/2]].c.sortkey() < c.sortkey() {
   196  			k := (j - 1) / 2
   197  			lockorder[j] = lockorder[k]
   198  			j = k
   199  		}
   200  		lockorder[j] = pollorder[i]
   201  	}
   202  	for i := len(lockorder) - 1; i >= 0; i-- {
   203  		o := lockorder[i]
   204  		c := scases[o].c
   205  		lockorder[i] = lockorder[0]
   206  		j := 0
   207  		for {
   208  			k := j*2 + 1
   209  			if k >= i {
   210  				break
   211  			}
   212  			if k+1 < i && scases[lockorder[k]].c.sortkey() < scases[lockorder[k+1]].c.sortkey() {
   213  				k++
   214  			}
   215  			if c.sortkey() < scases[lockorder[k]].c.sortkey() {
   216  				lockorder[j] = lockorder[k]
   217  				j = k
   218  				continue
   219  			}
   220  			break
   221  		}
   222  		lockorder[j] = o
   223  	}
   224  
   225  	if debugSelect {
   226  		for i := 0; i+1 < len(lockorder); i++ {
   227  			if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() {
   228  				print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
   229  				throw("select: broken sort")
   230  			}
   231  		}
   232  	}
   233  
   234  	// lock all the channels involved in the select
   235  	sellock(scases, lockorder)
   236  
   237  	var (
   238  		gp     *g
   239  		sg     *sudog
   240  		c      *hchan
   241  		k      *scase
   242  		sglist *sudog
   243  		sgnext *sudog
   244  		qp     unsafe.Pointer
   245  		nextp  **sudog
   246  	)
   247  
   248  	// pass 1 - look for something already waiting
   249  	var casi int
   250  	var cas *scase
   251  	var caseSuccess bool
   252  	var caseReleaseTime int64 = -1
   253  	var recvOK bool
   254  	for _, casei := range pollorder {
   255  		casi = int(casei)
   256  		cas = &scases[casi]
   257  		c = cas.c
   258  
   259  		if casi >= nsends {
   260  			sg = c.sendq.dequeue()
   261  			if sg != nil {
   262  				goto recv
   263  			}
   264  			if c.qcount > 0 {
   265  				goto bufrecv
   266  			}
   267  			if c.closed != 0 {
   268  				goto rclose
   269  			}
   270  		} else {
   271  			if raceenabled {
   272  				racereadpc(c.raceaddr(), casePC(casi), chansendpc)
   273  			}
   274  			if c.closed != 0 {
   275  				goto sclose
   276  			}
   277  			sg = c.recvq.dequeue()
   278  			if sg != nil {
   279  				goto send
   280  			}
   281  			if c.qcount < c.dataqsiz {
   282  				goto bufsend
   283  			}
   284  		}
   285  	}
   286  
   287  	if !block {
   288  		selunlock(scases, lockorder)
   289  		casi = -1
   290  		goto retc
   291  	}
   292  
   293  	// pass 2 - enqueue on all chans
   294  	gp = getg()
   295  	if gp.waiting != nil {
   296  		throw("gp.waiting != nil")
   297  	}
   298  	nextp = &gp.waiting
   299  	for _, casei := range lockorder {
   300  		casi = int(casei)
   301  		cas = &scases[casi]
   302  		c = cas.c
   303  		sg := acquireSudog()
   304  		sg.g = gp
   305  		sg.isSelect = true
   306  		// No stack splits between assigning elem and enqueuing
   307  		// sg on gp.waiting where copystack can find it.
   308  		sg.elem = cas.elem
   309  		sg.releasetime = 0
   310  		if t0 != 0 {
   311  			sg.releasetime = -1
   312  		}
   313  		sg.c = c
   314  		// Construct waiting list in lock order.
   315  		*nextp = sg
   316  		nextp = &sg.waitlink
   317  
   318  		if casi < nsends {
   319  			c.sendq.enqueue(sg)
   320  		} else {
   321  			c.recvq.enqueue(sg)
   322  		}
   323  
   324  		if c.timer != nil {
   325  			blockTimerChan(c)
   326  		}
   327  	}
   328  
   329  	// wait for someone to wake us up
   330  	gp.param = nil
   331  	// Signal to anyone trying to shrink our stack that we're about
   332  	// to park on a channel. The window between when this G's status
   333  	// changes and when we set gp.activeStackChans is not safe for
   334  	// stack shrinking.
   335  	gp.parkingOnChan.Store(true)
   336  	gopark(selparkcommit, nil, waitReasonSelect, traceBlockSelect, 1)
   337  	gp.activeStackChans = false
   338  
   339  	sellock(scases, lockorder)
   340  
   341  	gp.selectDone.Store(0)
   342  	sg = (*sudog)(gp.param)
   343  	gp.param = nil
   344  
   345  	// pass 3 - dequeue from unsuccessful chans
   346  	// otherwise they stack up on quiet channels
   347  	// record the successful case, if any.
   348  	// We singly-linked up the SudoGs in lock order.
   349  	casi = -1
   350  	cas = nil
   351  	caseSuccess = false
   352  	sglist = gp.waiting
   353  	// Clear all elem before unlinking from gp.waiting.
   354  	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
   355  		sg1.isSelect = false
   356  		sg1.elem = nil
   357  		sg1.c = nil
   358  	}
   359  	gp.waiting = nil
   360  
   361  	for _, casei := range lockorder {
   362  		k = &scases[casei]
   363  		if k.c.timer != nil {
   364  			unblockTimerChan(k.c)
   365  		}
   366  		if sg == sglist {
   367  			// sg has already been dequeued by the G that woke us up.
   368  			casi = int(casei)
   369  			cas = k
   370  			caseSuccess = sglist.success
   371  			if sglist.releasetime > 0 {
   372  				caseReleaseTime = sglist.releasetime
   373  			}
   374  		} else {
   375  			c = k.c
   376  			if int(casei) < nsends {
   377  				c.sendq.dequeueSudoG(sglist)
   378  			} else {
   379  				c.recvq.dequeueSudoG(sglist)
   380  			}
   381  		}
   382  		sgnext = sglist.waitlink
   383  		sglist.waitlink = nil
   384  		releaseSudog(sglist)
   385  		sglist = sgnext
   386  	}
   387  
   388  	if cas == nil {
   389  		throw("selectgo: bad wakeup")
   390  	}
   391  
   392  	c = cas.c
   393  
   394  	if debugSelect {
   395  		print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " send=", casi < nsends, "\n")
   396  	}
   397  
   398  	if casi < nsends {
   399  		if !caseSuccess {
   400  			goto sclose
   401  		}
   402  	} else {
   403  		recvOK = caseSuccess
   404  	}
   405  
   406  	if raceenabled {
   407  		if casi < nsends {
   408  			raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
   409  		} else if cas.elem != nil {
   410  			raceWriteObjectPC(c.elemtype, cas.elem, casePC(casi), chanrecvpc)
   411  		}
   412  	}
   413  	if msanenabled {
   414  		if casi < nsends {
   415  			msanread(cas.elem, c.elemtype.Size_)
   416  		} else if cas.elem != nil {
   417  			msanwrite(cas.elem, c.elemtype.Size_)
   418  		}
   419  	}
   420  	if asanenabled {
   421  		if casi < nsends {
   422  			asanread(cas.elem, c.elemtype.Size_)
   423  		} else if cas.elem != nil {
   424  			asanwrite(cas.elem, c.elemtype.Size_)
   425  		}
   426  	}
   427  
   428  	selunlock(scases, lockorder)
   429  	goto retc
   430  
   431  bufrecv:
   432  	// can receive from buffer
   433  	if raceenabled {
   434  		if cas.elem != nil {
   435  			raceWriteObjectPC(c.elemtype, cas.elem, casePC(casi), chanrecvpc)
   436  		}
   437  		racenotify(c, c.recvx, nil)
   438  	}
   439  	if msanenabled && cas.elem != nil {
   440  		msanwrite(cas.elem, c.elemtype.Size_)
   441  	}
   442  	if asanenabled && cas.elem != nil {
   443  		asanwrite(cas.elem, c.elemtype.Size_)
   444  	}
   445  	recvOK = true
   446  	qp = chanbuf(c, c.recvx)
   447  	if cas.elem != nil {
   448  		typedmemmove(c.elemtype, cas.elem, qp)
   449  	}
   450  	typedmemclr(c.elemtype, qp)
   451  	c.recvx++
   452  	if c.recvx == c.dataqsiz {
   453  		c.recvx = 0
   454  	}
   455  	c.qcount--
   456  	selunlock(scases, lockorder)
   457  	goto retc
   458  
   459  bufsend:
   460  	// can send to buffer
   461  	if raceenabled {
   462  		racenotify(c, c.sendx, nil)
   463  		raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
   464  	}
   465  	if msanenabled {
   466  		msanread(cas.elem, c.elemtype.Size_)
   467  	}
   468  	if asanenabled {
   469  		asanread(cas.elem, c.elemtype.Size_)
   470  	}
   471  	typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem)
   472  	c.sendx++
   473  	if c.sendx == c.dataqsiz {
   474  		c.sendx = 0
   475  	}
   476  	c.qcount++
   477  	selunlock(scases, lockorder)
   478  	goto retc
   479  
   480  recv:
   481  	// can receive from sleeping sender (sg)
   482  	recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
   483  	if debugSelect {
   484  		print("syncrecv: cas0=", cas0, " c=", c, "\n")
   485  	}
   486  	recvOK = true
   487  	goto retc
   488  
   489  rclose:
   490  	// read at end of closed channel
   491  	selunlock(scases, lockorder)
   492  	recvOK = false
   493  	if cas.elem != nil {
   494  		typedmemclr(c.elemtype, cas.elem)
   495  	}
   496  	if raceenabled {
   497  		raceacquire(c.raceaddr())
   498  	}
   499  	goto retc
   500  
   501  send:
   502  	// can send to a sleeping receiver (sg)
   503  	if raceenabled {
   504  		raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
   505  	}
   506  	if msanenabled {
   507  		msanread(cas.elem, c.elemtype.Size_)
   508  	}
   509  	if asanenabled {
   510  		asanread(cas.elem, c.elemtype.Size_)
   511  	}
   512  	send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
   513  	if debugSelect {
   514  		print("syncsend: cas0=", cas0, " c=", c, "\n")
   515  	}
   516  	goto retc
   517  
   518  retc:
   519  	if caseReleaseTime > 0 {
   520  		blockevent(caseReleaseTime-t0, 1)
   521  	}
   522  	return casi, recvOK
   523  
   524  sclose:
   525  	// send on closed channel
   526  	selunlock(scases, lockorder)
   527  	panic(plainError("send on closed channel"))
   528  }
   529  
   530  func (c *hchan) sortkey() uintptr {
   531  	return uintptr(unsafe.Pointer(c))
   532  }
   533  
   534  // A runtimeSelect is a single case passed to rselect.
   535  // This must match ../reflect/value.go:/runtimeSelect
   536  type runtimeSelect struct {
   537  	dir selectDir
   538  	typ unsafe.Pointer // channel type (not used here)
   539  	ch  *hchan         // channel
   540  	val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
   541  }
   542  
   543  // These values must match ../reflect/value.go:/SelectDir.
   544  type selectDir int
   545  
   546  const (
   547  	_             selectDir = iota
   548  	selectSend              // case Chan <- Send
   549  	selectRecv              // case <-Chan:
   550  	selectDefault           // default
   551  )
   552  
   553  //go:linkname reflect_rselect reflect.rselect
   554  func reflect_rselect(cases []runtimeSelect) (int, bool) {
   555  	if len(cases) == 0 {
   556  		block()
   557  	}
   558  	sel := make([]scase, len(cases))
   559  	orig := make([]int, len(cases))
   560  	nsends, nrecvs := 0, 0
   561  	dflt := -1
   562  	for i, rc := range cases {
   563  		var j int
   564  		switch rc.dir {
   565  		case selectDefault:
   566  			dflt = i
   567  			continue
   568  		case selectSend:
   569  			j = nsends
   570  			nsends++
   571  		case selectRecv:
   572  			nrecvs++
   573  			j = len(cases) - nrecvs
   574  		}
   575  
   576  		sel[j] = scase{c: rc.ch, elem: rc.val}
   577  		orig[j] = i
   578  	}
   579  
   580  	// Only a default case.
   581  	if nsends+nrecvs == 0 {
   582  		return dflt, false
   583  	}
   584  
   585  	// Compact sel and orig if necessary.
   586  	if nsends+nrecvs < len(cases) {
   587  		copy(sel[nsends:], sel[len(cases)-nrecvs:])
   588  		copy(orig[nsends:], orig[len(cases)-nrecvs:])
   589  	}
   590  
   591  	order := make([]uint16, 2*(nsends+nrecvs))
   592  	var pc0 *uintptr
   593  	if raceenabled {
   594  		pcs := make([]uintptr, nsends+nrecvs)
   595  		for i := range pcs {
   596  			selectsetpc(&pcs[i])
   597  		}
   598  		pc0 = &pcs[0]
   599  	}
   600  
   601  	chosen, recvOK := selectgo(&sel[0], &order[0], pc0, nsends, nrecvs, dflt == -1)
   602  
   603  	// Translate chosen back to caller's ordering.
   604  	if chosen < 0 {
   605  		chosen = dflt
   606  	} else {
   607  		chosen = orig[chosen]
   608  	}
   609  	return chosen, recvOK
   610  }
   611  
   612  func (q *waitq) dequeueSudoG(sgp *sudog) {
   613  	x := sgp.prev
   614  	y := sgp.next
   615  	if x != nil {
   616  		if y != nil {
   617  			// middle of queue
   618  			x.next = y
   619  			y.prev = x
   620  			sgp.next = nil
   621  			sgp.prev = nil
   622  			return
   623  		}
   624  		// end of queue
   625  		x.next = nil
   626  		q.last = x
   627  		sgp.prev = nil
   628  		return
   629  	}
   630  	if y != nil {
   631  		// start of queue
   632  		y.prev = nil
   633  		q.first = y
   634  		sgp.next = nil
   635  		return
   636  	}
   637  
   638  	// x==y==nil. Either sgp is the only element in the queue,
   639  	// or it has already been removed. Use q.first to disambiguate.
   640  	if q.first == sgp {
   641  		q.first = nil
   642  		q.last = nil
   643  	}
   644  }
   645
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