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 walkselectcases.
    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  	// There are unlocked sudogs that point into gp's stack. Stack
    79  	// copying must lock the channels of those sudogs.
    80  	gp.activeStackChans = true
    81  	// This must not access gp's stack (see gopark). In
    82  	// particular, it must not access the *hselect. That's okay,
    83  	// because by the time this is called, gp.waiting has all
    84  	// channels in lock order.
    85  	var lastc *hchan
    86  	for sg := gp.waiting; sg != nil; sg = sg.waitlink {
    87  		if sg.c != lastc && lastc != nil {
    88  			// As soon as we unlock the channel, fields in
    89  			// any sudog with that channel may change,
    90  			// including c and waitlink. Since multiple
    91  			// sudogs may have the same channel, we unlock
    92  			// only after we've passed the last instance
    93  			// of a channel.
    94  			unlock(&lastc.lock)
    95  		}
    96  		lastc = sg.c
    97  	}
    98  	if lastc != nil {
    99  		unlock(&lastc.lock)
   100  	}
   101  	return true
   102  }
   103  
   104  func block() {
   105  	gopark(nil, nil, waitReasonSelectNoCases, traceEvGoStop, 1) // forever
   106  }
   107  
   108  // selectgo implements the select statement.
   109  //
   110  // cas0 points to an array of type [ncases]scase, and order0 points to
   111  // an array of type [2*ncases]uint16. Both reside on the goroutine's
   112  // stack (regardless of any escaping in selectgo).
   113  //
   114  // selectgo returns the index of the chosen scase, which matches the
   115  // ordinal position of its respective select{recv,send,default} call.
   116  // Also, if the chosen scase was a receive operation, it reports whether
   117  // a value was received.
   118  func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
   119  	if debugSelect {
   120  		print("select: cas0=", cas0, "\n")
   121  	}
   122  
   123  	cas1 := (*[1 << 16]scase)(unsafe.Pointer(cas0))
   124  	order1 := (*[1 << 17]uint16)(unsafe.Pointer(order0))
   125  
   126  	scases := cas1[:ncases:ncases]
   127  	pollorder := order1[:ncases:ncases]
   128  	lockorder := order1[ncases:][:ncases:ncases]
   129  
   130  	// Replace send/receive cases involving nil channels with
   131  	// caseNil so logic below can assume non-nil channel.
   132  	for i := range scases {
   133  		cas := &scases[i]
   134  		if cas.c == nil && cas.kind != caseDefault {
   135  			*cas = scase{}
   136  		}
   137  	}
   138  
   139  	var t0 int64
   140  	if blockprofilerate > 0 {
   141  		t0 = cputicks()
   142  		for i := 0; i < ncases; i++ {
   143  			scases[i].releasetime = -1
   144  		}
   145  	}
   146  
   147  	// The compiler rewrites selects that statically have
   148  	// only 0 or 1 cases plus default into simpler constructs.
   149  	// The only way we can end up with such small sel.ncase
   150  	// values here is for a larger select in which most channels
   151  	// have been nilled out. The general code handles those
   152  	// cases correctly, and they are rare enough not to bother
   153  	// optimizing (and needing to test).
   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 raceenabled {
   251  				racereadpc(c.raceaddr(), cas.pc, chansendpc)
   252  			}
   253  			if c.closed != 0 {
   254  				goto sclose
   255  			}
   256  			sg = c.recvq.dequeue()
   257  			if sg != nil {
   258  				goto send
   259  			}
   260  			if c.qcount < c.dataqsiz {
   261  				goto bufsend
   262  			}
   263  
   264  		case caseDefault:
   265  			dfli = casi
   266  			dfl = cas
   267  		}
   268  	}
   269  
   270  	if dfl != nil {
   271  		selunlock(scases, lockorder)
   272  		casi = dfli
   273  		cas = dfl
   274  		goto retc
   275  	}
   276  
   277  	// pass 2 - enqueue on all chans
   278  	gp = getg()
   279  	if gp.waiting != nil {
   280  		throw("gp.waiting != nil")
   281  	}
   282  	nextp = &gp.waiting
   283  	for _, casei := range lockorder {
   284  		casi = int(casei)
   285  		cas = &scases[casi]
   286  		if cas.kind == caseNil {
   287  			continue
   288  		}
   289  		c = cas.c
   290  		sg := acquireSudog()
   291  		sg.g = gp
   292  		sg.isSelect = true
   293  		// No stack splits between assigning elem and enqueuing
   294  		// sg on gp.waiting where copystack can find it.
   295  		sg.elem = cas.elem
   296  		sg.releasetime = 0
   297  		if t0 != 0 {
   298  			sg.releasetime = -1
   299  		}
   300  		sg.c = c
   301  		// Construct waiting list in lock order.
   302  		*nextp = sg
   303  		nextp = &sg.waitlink
   304  
   305  		switch cas.kind {
   306  		case caseRecv:
   307  			c.recvq.enqueue(sg)
   308  
   309  		case caseSend:
   310  			c.sendq.enqueue(sg)
   311  		}
   312  	}
   313  
   314  	// wait for someone to wake us up
   315  	gp.param = nil
   316  	gopark(selparkcommit, nil, waitReasonSelect, traceEvGoBlockSelect, 1)
   317  	gp.activeStackChans = false
   318  
   319  	sellock(scases, lockorder)
   320  
   321  	gp.selectDone = 0
   322  	sg = (*sudog)(gp.param)
   323  	gp.param = nil
   324  
   325  	// pass 3 - dequeue from unsuccessful chans
   326  	// otherwise they stack up on quiet channels
   327  	// record the successful case, if any.
   328  	// We singly-linked up the SudoGs in lock order.
   329  	casi = -1
   330  	cas = nil
   331  	sglist = gp.waiting
   332  	// Clear all elem before unlinking from gp.waiting.
   333  	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
   334  		sg1.isSelect = false
   335  		sg1.elem = nil
   336  		sg1.c = nil
   337  	}
   338  	gp.waiting = nil
   339  
   340  	for _, casei := range lockorder {
   341  		k = &scases[casei]
   342  		if k.kind == caseNil {
   343  			continue
   344  		}
   345  		if sglist.releasetime > 0 {
   346  			k.releasetime = sglist.releasetime
   347  		}
   348  		if sg == sglist {
   349  			// sg has already been dequeued by the G that woke us up.
   350  			casi = int(casei)
   351  			cas = k
   352  		} else {
   353  			c = k.c
   354  			if k.kind == caseSend {
   355  				c.sendq.dequeueSudoG(sglist)
   356  			} else {
   357  				c.recvq.dequeueSudoG(sglist)
   358  			}
   359  		}
   360  		sgnext = sglist.waitlink
   361  		sglist.waitlink = nil
   362  		releaseSudog(sglist)
   363  		sglist = sgnext
   364  	}
   365  
   366  	if cas == nil {
   367  		// We can wake up with gp.param == nil (so cas == nil)
   368  		// when a channel involved in the select has been closed.
   369  		// It is easiest to loop and re-run the operation;
   370  		// we'll see that it's now closed.
   371  		// Maybe some day we can signal the close explicitly,
   372  		// but we'd have to distinguish close-on-reader from close-on-writer.
   373  		// It's easiest not to duplicate the code and just recheck above.
   374  		// We know that something closed, and things never un-close,
   375  		// so we won't block again.
   376  		goto loop
   377  	}
   378  
   379  	c = cas.c
   380  
   381  	if debugSelect {
   382  		print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " kind=", cas.kind, "\n")
   383  	}
   384  
   385  	if cas.kind == caseRecv {
   386  		recvOK = true
   387  	}
   388  
   389  	if raceenabled {
   390  		if cas.kind == caseRecv && cas.elem != nil {
   391  			raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
   392  		} else if cas.kind == caseSend {
   393  			raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
   394  		}
   395  	}
   396  	if msanenabled {
   397  		if cas.kind == caseRecv && cas.elem != nil {
   398  			msanwrite(cas.elem, c.elemtype.size)
   399  		} else if cas.kind == caseSend {
   400  			msanread(cas.elem, c.elemtype.size)
   401  		}
   402  	}
   403  
   404  	selunlock(scases, lockorder)
   405  	goto retc
   406  
   407  bufrecv:
   408  	// can receive from buffer
   409  	if raceenabled {
   410  		if cas.elem != nil {
   411  			raceWriteObjectPC(c.elemtype, cas.elem, cas.pc, chanrecvpc)
   412  		}
   413  		raceacquire(chanbuf(c, c.recvx))
   414  		racerelease(chanbuf(c, c.recvx))
   415  	}
   416  	if msanenabled && cas.elem != nil {
   417  		msanwrite(cas.elem, c.elemtype.size)
   418  	}
   419  	recvOK = true
   420  	qp = chanbuf(c, c.recvx)
   421  	if cas.elem != nil {
   422  		typedmemmove(c.elemtype, cas.elem, qp)
   423  	}
   424  	typedmemclr(c.elemtype, qp)
   425  	c.recvx++
   426  	if c.recvx == c.dataqsiz {
   427  		c.recvx = 0
   428  	}
   429  	c.qcount--
   430  	selunlock(scases, lockorder)
   431  	goto retc
   432  
   433  bufsend:
   434  	// can send to buffer
   435  	if raceenabled {
   436  		raceacquire(chanbuf(c, c.sendx))
   437  		racerelease(chanbuf(c, c.sendx))
   438  		raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
   439  	}
   440  	if msanenabled {
   441  		msanread(cas.elem, c.elemtype.size)
   442  	}
   443  	typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem)
   444  	c.sendx++
   445  	if c.sendx == c.dataqsiz {
   446  		c.sendx = 0
   447  	}
   448  	c.qcount++
   449  	selunlock(scases, lockorder)
   450  	goto retc
   451  
   452  recv:
   453  	// can receive from sleeping sender (sg)
   454  	recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
   455  	if debugSelect {
   456  		print("syncrecv: cas0=", cas0, " c=", c, "\n")
   457  	}
   458  	recvOK = true
   459  	goto retc
   460  
   461  rclose:
   462  	// read at end of closed channel
   463  	selunlock(scases, lockorder)
   464  	recvOK = false
   465  	if cas.elem != nil {
   466  		typedmemclr(c.elemtype, cas.elem)
   467  	}
   468  	if raceenabled {
   469  		raceacquire(c.raceaddr())
   470  	}
   471  	goto retc
   472  
   473  send:
   474  	// can send to a sleeping receiver (sg)
   475  	if raceenabled {
   476  		raceReadObjectPC(c.elemtype, cas.elem, cas.pc, chansendpc)
   477  	}
   478  	if msanenabled {
   479  		msanread(cas.elem, c.elemtype.size)
   480  	}
   481  	send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
   482  	if debugSelect {
   483  		print("syncsend: cas0=", cas0, " c=", c, "\n")
   484  	}
   485  	goto retc
   486  
   487  retc:
   488  	if cas.releasetime > 0 {
   489  		blockevent(cas.releasetime-t0, 1)
   490  	}
   491  	return casi, recvOK
   492  
   493  sclose:
   494  	// send on closed channel
   495  	selunlock(scases, lockorder)
   496  	panic(plainError("send on closed channel"))
   497  }
   498  
   499  func (c *hchan) sortkey() uintptr {
   500  	return uintptr(unsafe.Pointer(c))
   501  }
   502  
   503  // A runtimeSelect is a single case passed to rselect.
   504  // This must match ../reflect/value.go:/runtimeSelect
   505  type runtimeSelect struct {
   506  	dir selectDir
   507  	typ unsafe.Pointer // channel type (not used here)
   508  	ch  *hchan         // channel
   509  	val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
   510  }
   511  
   512  // These values must match ../reflect/value.go:/SelectDir.
   513  type selectDir int
   514  
   515  const (
   516  	_             selectDir = iota
   517  	selectSend              // case Chan <- Send
   518  	selectRecv              // case <-Chan:
   519  	selectDefault           // default
   520  )
   521  
   522  //go:linkname reflect_rselect reflect.rselect
   523  func reflect_rselect(cases []runtimeSelect) (int, bool) {
   524  	if len(cases) == 0 {
   525  		block()
   526  	}
   527  	sel := make([]scase, len(cases))
   528  	order := make([]uint16, 2*len(cases))
   529  	for i := range cases {
   530  		rc := &cases[i]
   531  		switch rc.dir {
   532  		case selectDefault:
   533  			sel[i] = scase{kind: caseDefault}
   534  		case selectSend:
   535  			sel[i] = scase{kind: caseSend, c: rc.ch, elem: rc.val}
   536  		case selectRecv:
   537  			sel[i] = scase{kind: caseRecv, c: rc.ch, elem: rc.val}
   538  		}
   539  		if raceenabled || msanenabled {
   540  			selectsetpc(&sel[i])
   541  		}
   542  	}
   543  
   544  	return selectgo(&sel[0], &order[0], len(cases))
   545  }
   546  
   547  func (q *waitq) dequeueSudoG(sgp *sudog) {
   548  	x := sgp.prev
   549  	y := sgp.next
   550  	if x != nil {
   551  		if y != nil {
   552  			// middle of queue
   553  			x.next = y
   554  			y.prev = x
   555  			sgp.next = nil
   556  			sgp.prev = nil
   557  			return
   558  		}
   559  		// end of queue
   560  		x.next = nil
   561  		q.last = x
   562  		sgp.prev = nil
   563  		return
   564  	}
   565  	if y != nil {
   566  		// start of queue
   567  		y.prev = nil
   568  		q.first = y
   569  		sgp.next = nil
   570  		return
   571  	}
   572  
   573  	// x==y==nil. Either sgp is the only element in the queue,
   574  	// or it has already been removed. Use q.first to disambiguate.
   575  	if q.first == sgp {
   576  		q.first = nil
   577  		q.last = nil
   578  	}
   579  }
   580
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