select.go

Documentation: runtime

     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  const debugSelect = false
    14  
    15  // scase.kind values.
    16  // Known to compiler.
    17  // Changes here must also be made in src/cmd/compile/internal/gc/select.go's walkselect.
    18  const (
    19  	caseNil = iota
    20  	caseRecv
    21  	caseSend
    22  	caseDefault
    23  )
    24  
    25  // Select case descriptor.
    26  // Known to compiler.
    27  // Changes here must also be made in src/cmd/internal/gc/select.go's scasetype.
    28  type scase struct {
    29  	c           *hchan         // chan
    30  	elem        unsafe.Pointer // data element
    31  	kind        uint16
    32  	pc          uintptr // race pc (for race detector / msan)
    33  	releasetime int64
    34  }
    35  
    36  var (
    37  	chansendpc = funcPC(chansend)
    38  	chanrecvpc = funcPC(chanrecv)
    39  )
    40  
    41  func selectsetpc(cas *scase) {
    42  	cas.pc = getcallerpc()
    43  }
    44  
    45  func sellock(scases []scase, lockorder []uint16) {
    46  	var c *hchan
    47  	for _, o := range lockorder {
    48  		c0 := scases[o].c
    49  		if c0 != nil && c0 != c {
    50  			c = c0
    51  			lock(&c.lock)
    52  		}
    53  	}
    54  }
    55  
    56  func selunlock(scases []scase, lockorder []uint16) {
    57  	// We must be very careful here to not touch sel after we have unlocked
    58  	// the last lock, because sel can be freed right after the last unlock.
    59  	// Consider the following situation.
    60  	// First M calls runtime·park() in runtime·selectgo() passing the sel.
    61  	// Once runtime·park() has unlocked the last lock, another M makes
    62  	// the G that calls select runnable again and schedules it for execution.
    63  	// When the G runs on another M, it locks all the locks and frees sel.
    64  	// Now if the first M touches sel, it will access freed memory.
    65  	for i := len(scases) - 1; i >= 0; i-- {
    66  		c := scases[lockorder[i]].c
    67  		if c == nil {
    68  			break
    69  		}
    70  		if i > 0 && c == scases[lockorder[i-1]].c {
    71  			continue // will unlock it on the next iteration
    72  		}
    73  		unlock(&c.lock)
    74  	}
    75  }
    76  
    77  func selparkcommit(gp *g, _ unsafe.Pointer) bool {
    78  	// This must not access gp's stack (see gopark). In
    79  	// particular, it must not access the *hselect. That's okay,
    80  	// because by the time this is called, gp.waiting has all
    81  	// channels in lock order.
    82  	var lastc *hchan
    83  	for sg := gp.waiting; sg != nil; sg = sg.waitlink {
    84  		if sg.c != lastc && lastc != nil {
    85  			// As soon as we unlock the channel, fields in
    86  			// any sudog with that channel may change,
    87  			// including c and waitlink. Since multiple
    88  			// sudogs may have the same channel, we unlock
    89  			// only after we've passed the last instance
    90  			// of a channel.
    91  			unlock(&lastc.lock)
    92  		}
    93  		lastc = sg.c
    94  	}
    95  	if lastc != nil {
    96  		unlock(&lastc.lock)
    97  	}
    98  	return true
    99  }
   100  
   101  func block() {
   102  	gopark(nil, nil, waitReasonSelectNoCases, traceEvGoStop, 1) // forever
   103  }
   104  
   105  // selectgo implements the select statement.
   106  //
   107  // cas0 points to an array of type [ncases]scase, and order0 points to
   108  // an array of type [2*ncases]uint16. Both reside on the goroutine's
   109  // stack (regardless of any escaping in selectgo).
   110  //
   111  // selectgo returns the index of the chosen scase, which matches the
   112  // ordinal position of its respective select{recv,send,default} call.
   113  // Also, if the chosen scase was a receive operation, it returns whether
   114  // a value was received.
   115  func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
   116  	if debugSelect {
   117  		print("select: cas0=", cas0, "\n")
   118  	}
   119  
   120  	cas1 := (*[1 << 16]scase)(unsafe.Pointer(cas0))
   121  	order1 := (*[1 << 17]uint16)(unsafe.Pointer(order0))
   122  
   123  	scases := cas1[:ncases:ncases]
   124  	pollorder := order1[:ncases:ncases]
   125  	lockorder := order1[ncases:][:ncases:ncases]
   126  
   127  	// Replace send/receive cases involving nil channels with
   128  	// caseNil so logic below can assume non-nil channel.
   129  	for i := range scases {
   130  		cas := &scases[i]
   131  		if cas.c == nil && cas.kind != caseDefault {
   132  			*cas = scase{}
   133  		}
   134  	}
   135  
   136  	var t0 int64
   137  	if blockprofilerate > 0 {
   138  		t0 = cputicks()
   139  		for i := 0; i < ncases; i++ {
   140  			scases[i].releasetime = -1
   141  		}
   142  	}
   143  
   144  	// The compiler rewrites selects that statically have
   145  	// only 0 or 1 cases plus default into simpler constructs.
   146  	// The only way we can end up with such small sel.ncase
   147  	// values here is for a larger select in which most channels
   148  	// have been nilled out. The general code handles those
   149  	// cases correctly, and they are rare enough not to bother
   150  	// optimizing (and needing to test).
   151  
   152  	// generate permuted order
   153  	for i := 1; i < ncases; i++ {
   154  		j := fastrandn(uint32(i + 1))
   155  		pollorder[i] = pollorder[j]
   156  		pollorder[j] = uint16(i)
   157  	}
   158  
   159  	// sort the cases by Hchan address to get the locking order.
   160  	// simple heap sort, to guarantee n log n time and constant stack footprint.
   161  	for i := 0; i < ncases; i++ {
   162  		j := i
   163  		// Start with the pollorder to permute cases on the same channel.
   164  		c := scases[pollorder[i]].c
   165  		for j > 0 && scases[lockorder[(j-1)/2]].c.sortkey() < c.sortkey() {
   166  			k := (j - 1) / 2
   167  			lockorder[j] = lockorder[k]
   168  			j = k
   169  		}
   170  		lockorder[j] = pollorder[i]
   171  	}
   172  	for i := ncases - 1; i >= 0; i-- {
   173  		o := lockorder[i]
   174  		c := scases[o].c
   175  		lockorder[i] = lockorder[0]
   176  		j := 0
   177  		for {
   178  			k := j*2 + 1
   179  			if k >= i {
   180  				break
   181  			}
   182  			if k+1 < i && scases[lockorder[k]].c.sortkey() < scases[lockorder[k+1]].c.sortkey() {
   183  				k++
   184  			}
   185  			if c.sortkey() < scases[lockorder[k]].c.sortkey() {
   186  				lockorder[j] = lockorder[k]
   187  				j = k
   188  				continue
   189  			}
   190  			break
   191  		}
   192  		lockorder[j] = o
   193  	}
   194  
   195  	if debugSelect {
   196  		for i := 0; i+1 < ncases; i++ {
   197  			if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() {
   198  				print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
   199  				throw("select: broken sort")
   200  			}
   201  		}
   202  	}
   203  
   204  	// lock all the channels involved in the select
   205  	sellock(scases, lockorder)
   206  
   207  	var (
   208  		gp     *g
   209  		sg     *sudog
   210  		c      *hchan
   211  		k      *scase
   212  		sglist *sudog
   213  		sgnext *sudog
   214  		qp     unsafe.Pointer
   215  		nextp  **sudog
   216  	)
   217  
   218  loop:
   219  	// pass 1 - look for something already waiting
   220  	var dfli int
   221  	var dfl *scase
   222  	var casi int
   223  	var cas *scase
   224  	var recvOK bool
   225  	for i := 0; i < ncases; i++ {
   226  		casi = int(pollorder[i])
   227  		cas = &scases[casi]
   228  		c = cas.c
   229  
   230  		switch cas.kind {
   231  		case caseNil:
   232  			continue
   233  
   234  		case caseRecv:
   235  			sg = c.sendq.dequeue()
   236  			if sg != nil {
   237  				goto recv
   238  			}
   239  			if c.qcount > 0 {
   240  				goto bufrecv
   241  			}
   242  			if c.closed != 0 {
   243  				goto rclose
   244  			}
   245  
   246  		case caseSend:
   247  			if raceenabled {
   248  				racereadpc(c.raceaddr(), cas.pc, chansendpc)
   249  			}
   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  
   315  	sellock(scases, lockorder)
   316  
   317  	gp.selectDone = 0
   318  	sg = (*sudog)(gp.param)
   319  	gp.param = nil
   320  
   321  	// pass 3 - dequeue from unsuccessful chans
   322  	// otherwise they stack up on quiet channels
   323  	// record the successful case, if any.
   324  	// We singly-linked up the SudoGs in lock order.
   325  	casi = -1
   326  	cas = nil
   327  	sglist = gp.waiting
   328  	// Clear all elem before unlinking from gp.waiting.
   329  	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
   330  		sg1.isSelect = false
   331  		sg1.elem = nil
   332  		sg1.c = nil
   333  	}
   334  	gp.waiting = nil
   335  
   336  	for _, casei := range lockorder {
   337  		k = &scases[casei]
   338  		if k.kind == caseNil {
   339  			continue
   340  		}
   341  		if sglist.releasetime > 0 {
   342  			k.releasetime = sglist.releasetime
   343  		}
   344  		if sg == sglist {
   345  			// sg has already been dequeued by the G that woke us up.
   346  			casi = int(casei)
   347  			cas = k
   348  		} else {
   349  			c = k.c
   350  			if k.kind == caseSend {
   351  				c.sendq.dequeueSudoG(sglist)
   352  			} else {
   353  				c.recvq.dequeueSudoG(sglist)
   354  			}
   355  		}
   356  		sgnext = sglist.waitlink
   357  		sglist.waitlink = nil
   358  		releaseSudog(sglist)
   359  		sglist = sgnext
   360  	}
   361  
   362  	if cas == nil {
   363  		// We can wake up with gp.param == nil (so cas == nil)
   364  		// when a channel involved in the select has been closed.
   365  		// It is easiest to loop and re-run the operation;
   366  		// we'll see that it's now closed.
   367  		// Maybe some day we can signal the close explicitly,
   368  		// but we'd have to distinguish close-on-reader from close-on-writer.
   369  		// It's easiest not to duplicate the code and just recheck above.
   370  		// We know that something closed, and things never un-close,
   371  		// so we won't block again.
   372  		goto loop
   373  	}
   374  
   375  	c = cas.c
   376  
   377  	if debugSelect {
   378  		print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " kind=", cas.kind, "\n")
   379  	}
   380  
   381  	if cas.kind == caseRecv {
   382  		recvOK = true
   383  	}
   384  
   385  	if raceenabled {
   386  		if cas.kind == caseRecv && cas.elem != nil {
   387  			raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
   388  		} else if cas.kind == caseSend {
   389  			raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
   390  		}
   391  	}
   392  	if msanenabled {
   393  		if cas.kind == caseRecv && cas.elem != nil {
   394  			msanwrite(cas.elem, c.elemtype.size)
   395  		} else if cas.kind == caseSend {
   396  			msanread(cas.elem, c.elemtype.size)
   397  		}
   398  	}
   399  
   400  	selunlock(scases, lockorder)
   401  	goto retc
   402  
   403  bufrecv:
   404  	// can receive from buffer
   405  	if raceenabled {
   406  		if cas.elem != nil {
   407  			raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
   408  		}
   409  		raceacquire(chanbuf(c, c.recvx))
   410  		racerelease(chanbuf(c, c.recvx))
   411  	}
   412  	if msanenabled && cas.elem != nil {
   413  		msanwrite(cas.elem, c.elemtype.size)
   414  	}
   415  	recvOK = true
   416  	qp = chanbuf(c, c.recvx)
   417  	if cas.elem != nil {
   418  		typedmemmove(c.elemtype, cas.elem, qp)
   419  	}
   420  	typedmemclr(c.elemtype, qp)
   421  	c.recvx++
   422  	if c.recvx == c.dataqsiz {
   423  		c.recvx = 0
   424  	}
   425  	c.qcount--
   426  	selunlock(scases, lockorder)
   427  	goto retc
   428  
   429  bufsend:
   430  	// can send to buffer
   431  	if raceenabled {
   432  		raceacquire(chanbuf(c, c.sendx))
   433  		racerelease(chanbuf(c, c.sendx))
   434  		raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
   435  	}
   436  	if msanenabled {
   437  		msanread(cas.elem, c.elemtype.size)
   438  	}
   439  	typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem)
   440  	c.sendx++
   441  	if c.sendx == c.dataqsiz {
   442  		c.sendx = 0
   443  	}
   444  	c.qcount++
   445  	selunlock(scases, lockorder)
   446  	goto retc
   447  
   448  recv:
   449  	// can receive from sleeping sender (sg)
   450  	recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
   451  	if debugSelect {
   452  		print("syncrecv: cas0=", cas0, " c=", c, "\n")
   453  	}
   454  	recvOK = true
   455  	goto retc
   456  
   457  rclose:
   458  	// read at end of closed channel
   459  	selunlock(scases, lockorder)
   460  	recvOK = false
   461  	if cas.elem != nil {
   462  		typedmemclr(c.elemtype, cas.elem)
   463  	}
   464  	if raceenabled {
   465  		raceacquire(c.raceaddr())
   466  	}
   467  	goto retc
   468  
   469  send:
   470  	// can send to a sleeping receiver (sg)
   471  	if raceenabled {
   472  		raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
   473  	}
   474  	if msanenabled {
   475  		msanread(cas.elem, c.elemtype.size)
   476  	}
   477  	send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
   478  	if debugSelect {
   479  		print("syncsend: cas0=", cas0, " c=", c, "\n")
   480  	}
   481  	goto retc
   482  
   483  retc:
   484  	if cas.releasetime > 0 {
   485  		blockevent(cas.releasetime-t0, 1)
   486  	}
   487  	return casi, recvOK
   488  
   489  sclose:
   490  	// send on closed channel
   491  	selunlock(scases, lockorder)
   492  	panic(plainError("send on closed channel"))
   493  }
   494  
   495  func (c *hchan) sortkey() uintptr {
   496  	// TODO(khr): if we have a moving garbage collector, we'll need to
   497  	// change this function.
   498  	return uintptr(unsafe.Pointer(c))
   499  }
   500  
   501  // A runtimeSelect is a single case passed to rselect.
   502  // This must match ../reflect/value.go:/runtimeSelect
   503  type runtimeSelect struct {
   504  	dir selectDir
   505  	typ unsafe.Pointer // channel type (not used here)
   506  	ch  *hchan         // channel
   507  	val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
   508  }
   509  
   510  // These values must match ../reflect/value.go:/SelectDir.
   511  type selectDir int
   512  
   513  const (
   514  	_             selectDir = iota
   515  	selectSend              // case Chan <- Send
   516  	selectRecv              // case <-Chan:
   517  	selectDefault           // default
   518  )
   519  
   520  //go:linkname reflect_rselect reflect.rselect
   521  func reflect_rselect(cases []runtimeSelect) (int, bool) {
   522  	if len(cases) == 0 {
   523  		block()
   524  	}
   525  	sel := make([]scase, len(cases))
   526  	order := make([]uint16, 2*len(cases))
   527  	for i := range cases {
   528  		rc := &cases[i]
   529  		switch rc.dir {
   530  		case selectDefault:
   531  			sel[i] = scase{kind: caseDefault}
   532  		case selectSend:
   533  			sel[i] = scase{kind: caseSend, c: rc.ch, elem: rc.val}
   534  		case selectRecv:
   535  			sel[i] = scase{kind: caseRecv, c: rc.ch, elem: rc.val}
   536  		}
   537  		if raceenabled || msanenabled {
   538  			selectsetpc(&sel[i])
   539  		}
   540  	}
   541  
   542  	return selectgo(&sel[0], &order[0], len(cases))
   543  }
   544  
   545  func (q *waitq) dequeueSudoG(sgp *sudog) {
   546  	x := sgp.prev
   547  	y := sgp.next
   548  	if x != nil {
   549  		if y != nil {
   550  			// middle of queue
   551  			x.next = y
   552  			y.prev = x
   553  			sgp.next = nil
   554  			sgp.prev = nil
   555  			return
   556  		}
   557  		// end of queue
   558  		x.next = nil
   559  		q.last = x
   560  		sgp.prev = nil
   561  		return
   562  	}
   563  	if y != nil {
   564  		// start of queue
   565  		y.prev = nil
   566  		q.first = y
   567  		sgp.next = nil
   568  		return
   569  	}
   570  
   571  	// x==y==nil. Either sgp is the only element in the queue,
   572  	// or it has already been removed. Use q.first to disambiguate.
   573  	if q.first == sgp {
   574  		q.first = nil
   575  		q.last = nil
   576  	}
   577  }
   578
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