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  	"runtime/internal/atomic"
    11  	"unsafe"
    12  )
    13  
    14  const debugSelect = false
    15  
    16  // Select case descriptor.
    17  // Known to compiler.
    18  // Changes here must also be made in src/cmd/internal/gc/select.go's scasetype.
    19  type scase struct {
    20  	c    *hchan         // chan
    21  	elem unsafe.Pointer // data element
    22  }
    23  
    24  var (
    25  	chansendpc = funcPC(chansend)
    26  	chanrecvpc = funcPC(chanrecv)
    27  )
    28  
    29  func selectsetpc(pc *uintptr) {
    30  	*pc = getcallerpc()
    31  }
    32  
    33  func sellock(scases []scase, lockorder []uint16) {
    34  	var c *hchan
    35  	for _, o := range lockorder {
    36  		c0 := scases[o].c
    37  		if c0 != c {
    38  			c = c0
    39  			lock(&c.lock)
    40  		}
    41  	}
    42  }
    43  
    44  func selunlock(scases []scase, lockorder []uint16) {
    45  	// We must be very careful here to not touch sel after we have unlocked
    46  	// the last lock, because sel can be freed right after the last unlock.
    47  	// Consider the following situation.
    48  	// First M calls runtime·park() in runtime·selectgo() passing the sel.
    49  	// Once runtime·park() has unlocked the last lock, another M makes
    50  	// the G that calls select runnable again and schedules it for execution.
    51  	// When the G runs on another M, it locks all the locks and frees sel.
    52  	// Now if the first M touches sel, it will access freed memory.
    53  	for i := len(lockorder) - 1; i >= 0; i-- {
    54  		c := scases[lockorder[i]].c
    55  		if i > 0 && c == scases[lockorder[i-1]].c {
    56  			continue // will unlock it on the next iteration
    57  		}
    58  		unlock(&c.lock)
    59  	}
    60  }
    61  
    62  func selparkcommit(gp *g, _ unsafe.Pointer) bool {
    63  	// There are unlocked sudogs that point into gp's stack. Stack
    64  	// copying must lock the channels of those sudogs.
    65  	// Set activeStackChans here instead of before we try parking
    66  	// because we could self-deadlock in stack growth on a
    67  	// channel lock.
    68  	gp.activeStackChans = true
    69  	// Mark that it's safe for stack shrinking to occur now,
    70  	// because any thread acquiring this G's stack for shrinking
    71  	// is guaranteed to observe activeStackChans after this store.
    72  	atomic.Store8(&gp.parkingOnChan, 0)
    73  	// Make sure we unlock after setting activeStackChans and
    74  	// unsetting parkingOnChan. The moment we unlock any of the
    75  	// channel locks we risk gp getting readied by a channel operation
    76  	// and so gp could continue running before everything before the
    77  	// unlock is visible (even to gp itself).
    78  
    79  	// This must not access gp's stack (see gopark). In
    80  	// particular, it must not access the *hselect. That's okay,
    81  	// because by the time this is called, gp.waiting has all
    82  	// channels in lock order.
    83  	var lastc *hchan
    84  	for sg := gp.waiting; sg != nil; sg = sg.waitlink {
    85  		if sg.c != lastc && lastc != nil {
    86  			// As soon as we unlock the channel, fields in
    87  			// any sudog with that channel may change,
    88  			// including c and waitlink. Since multiple
    89  			// sudogs may have the same channel, we unlock
    90  			// only after we've passed the last instance
    91  			// of a channel.
    92  			unlock(&lastc.lock)
    93  		}
    94  		lastc = sg.c
    95  	}
    96  	if lastc != nil {
    97  		unlock(&lastc.lock)
    98  	}
    99  	return true
   100  }
   101  
   102  func block() {
   103  	gopark(nil, nil, waitReasonSelectNoCases, traceEvGoStop, 1) // forever
   104  }
   105  
   106  // selectgo implements the select statement.
   107  //
   108  // cas0 points to an array of type [ncases]scase, and order0 points to
   109  // an array of type [2*ncases]uint16 where ncases must be <= 65536.
   110  // Both reside on the goroutine's stack (regardless of any escaping in
   111  // selectgo).
   112  //
   113  // For race detector builds, pc0 points to an array of type
   114  // [ncases]uintptr (also on the stack); for other builds, it's set to
   115  // nil.
   116  //
   117  // selectgo returns the index of the chosen scase, which matches the
   118  // ordinal position of its respective select{recv,send,default} call.
   119  // Also, if the chosen scase was a receive operation, it reports whether
   120  // a value was received.
   121  func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, block bool) (int, bool) {
   122  	if debugSelect {
   123  		print("select: cas0=", cas0, "\n")
   124  	}
   125  
   126  	// NOTE: In order to maintain a lean stack size, the number of scases
   127  	// is capped at 65536.
   128  	cas1 := (*[1 << 16]scase)(unsafe.Pointer(cas0))
   129  	order1 := (*[1 << 17]uint16)(unsafe.Pointer(order0))
   130  
   131  	ncases := nsends + nrecvs
   132  	scases := cas1[:ncases:ncases]
   133  	pollorder := order1[:ncases:ncases]
   134  	lockorder := order1[ncases:][:ncases:ncases]
   135  	// NOTE: pollorder/lockorder's underlying array was not zero-initialized by compiler.
   136  
   137  	// Even when raceenabled is true, there might be select
   138  	// statements in packages compiled without -race (e.g.,
   139  	// ensureSigM in runtime/signal_unix.go).
   140  	var pcs []uintptr
   141  	if raceenabled && pc0 != nil {
   142  		pc1 := (*[1 << 16]uintptr)(unsafe.Pointer(pc0))
   143  		pcs = pc1[:ncases:ncases]
   144  	}
   145  	casePC := func(casi int) uintptr {
   146  		if pcs == nil {
   147  			return 0
   148  		}
   149  		return pcs[casi]
   150  	}
   151  
   152  	var t0 int64
   153  	if blockprofilerate > 0 {
   154  		t0 = cputicks()
   155  	}
   156  
   157  	// The compiler rewrites selects that statically have
   158  	// only 0 or 1 cases plus default into simpler constructs.
   159  	// The only way we can end up with such small sel.ncase
   160  	// values here is for a larger select in which most channels
   161  	// have been nilled out. The general code handles those
   162  	// cases correctly, and they are rare enough not to bother
   163  	// optimizing (and needing to test).
   164  
   165  	// generate permuted order
   166  	norder := 0
   167  	for i := range scases {
   168  		cas := &scases[i]
   169  
   170  		// Omit cases without channels from the poll and lock orders.
   171  		if cas.c == nil {
   172  			cas.elem = nil // allow GC
   173  			continue
   174  		}
   175  
   176  		j := fastrandn(uint32(norder + 1))
   177  		pollorder[norder] = pollorder[j]
   178  		pollorder[j] = uint16(i)
   179  		norder++
   180  	}
   181  	pollorder = pollorder[:norder]
   182  	lockorder = lockorder[:norder]
   183  
   184  	// sort the cases by Hchan address to get the locking order.
   185  	// simple heap sort, to guarantee n log n time and constant stack footprint.
   186  	for i := range lockorder {
   187  		j := i
   188  		// Start with the pollorder to permute cases on the same channel.
   189  		c := scases[pollorder[i]].c
   190  		for j > 0 && scases[lockorder[(j-1)/2]].c.sortkey() < c.sortkey() {
   191  			k := (j - 1) / 2
   192  			lockorder[j] = lockorder[k]
   193  			j = k
   194  		}
   195  		lockorder[j] = pollorder[i]
   196  	}
   197  	for i := len(lockorder) - 1; i >= 0; i-- {
   198  		o := lockorder[i]
   199  		c := scases[o].c
   200  		lockorder[i] = lockorder[0]
   201  		j := 0
   202  		for {
   203  			k := j*2 + 1
   204  			if k >= i {
   205  				break
   206  			}
   207  			if k+1 < i && scases[lockorder[k]].c.sortkey() < scases[lockorder[k+1]].c.sortkey() {
   208  				k++
   209  			}
   210  			if c.sortkey() < scases[lockorder[k]].c.sortkey() {
   211  				lockorder[j] = lockorder[k]
   212  				j = k
   213  				continue
   214  			}
   215  			break
   216  		}
   217  		lockorder[j] = o
   218  	}
   219  
   220  	if debugSelect {
   221  		for i := 0; i+1 < len(lockorder); i++ {
   222  			if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() {
   223  				print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
   224  				throw("select: broken sort")
   225  			}
   226  		}
   227  	}
   228  
   229  	// lock all the channels involved in the select
   230  	sellock(scases, lockorder)
   231  
   232  	var (
   233  		gp     *g
   234  		sg     *sudog
   235  		c      *hchan
   236  		k      *scase
   237  		sglist *sudog
   238  		sgnext *sudog
   239  		qp     unsafe.Pointer
   240  		nextp  **sudog
   241  	)
   242  
   243  	// pass 1 - look for something already waiting
   244  	var casi int
   245  	var cas *scase
   246  	var caseSuccess bool
   247  	var caseReleaseTime int64 = -1
   248  	var recvOK bool
   249  	for _, casei := range pollorder {
   250  		casi = int(casei)
   251  		cas = &scases[casi]
   252  		c = cas.c
   253  
   254  		if casi >= nsends {
   255  			sg = c.sendq.dequeue()
   256  			if sg != nil {
   257  				goto recv
   258  			}
   259  			if c.qcount > 0 {
   260  				goto bufrecv
   261  			}
   262  			if c.closed != 0 {
   263  				goto rclose
   264  			}
   265  		} else {
   266  			if raceenabled {
   267  				racereadpc(c.raceaddr(), casePC(casi), chansendpc)
   268  			}
   269  			if c.closed != 0 {
   270  				goto sclose
   271  			}
   272  			sg = c.recvq.dequeue()
   273  			if sg != nil {
   274  				goto send
   275  			}
   276  			if c.qcount < c.dataqsiz {
   277  				goto bufsend
   278  			}
   279  		}
   280  	}
   281  
   282  	if !block {
   283  		selunlock(scases, lockorder)
   284  		casi = -1
   285  		goto retc
   286  	}
   287  
   288  	// pass 2 - enqueue on all chans
   289  	gp = getg()
   290  	if gp.waiting != nil {
   291  		throw("gp.waiting != nil")
   292  	}
   293  	nextp = &gp.waiting
   294  	for _, casei := range lockorder {
   295  		casi = int(casei)
   296  		cas = &scases[casi]
   297  		c = cas.c
   298  		sg := acquireSudog()
   299  		sg.g = gp
   300  		sg.isSelect = true
   301  		// No stack splits between assigning elem and enqueuing
   302  		// sg on gp.waiting where copystack can find it.
   303  		sg.elem = cas.elem
   304  		sg.releasetime = 0
   305  		if t0 != 0 {
   306  			sg.releasetime = -1
   307  		}
   308  		sg.c = c
   309  		// Construct waiting list in lock order.
   310  		*nextp = sg
   311  		nextp = &sg.waitlink
   312  
   313  		if casi < nsends {
   314  			c.sendq.enqueue(sg)
   315  		} else {
   316  			c.recvq.enqueue(sg)
   317  		}
   318  	}
   319  
   320  	// wait for someone to wake us up
   321  	gp.param = nil
   322  	// Signal to anyone trying to shrink our stack that we're about
   323  	// to park on a channel. The window between when this G's status
   324  	// changes and when we set gp.activeStackChans is not safe for
   325  	// stack shrinking.
   326  	atomic.Store8(&gp.parkingOnChan, 1)
   327  	gopark(selparkcommit, nil, waitReasonSelect, traceEvGoBlockSelect, 1)
   328  	gp.activeStackChans = false
   329  
   330  	sellock(scases, lockorder)
   331  
   332  	gp.selectDone = 0
   333  	sg = (*sudog)(gp.param)
   334  	gp.param = nil
   335  
   336  	// pass 3 - dequeue from unsuccessful chans
   337  	// otherwise they stack up on quiet channels
   338  	// record the successful case, if any.
   339  	// We singly-linked up the SudoGs in lock order.
   340  	casi = -1
   341  	cas = nil
   342  	caseSuccess = false
   343  	sglist = gp.waiting
   344  	// Clear all elem before unlinking from gp.waiting.
   345  	for sg1 := gp.waiting; sg1 != nil; sg1 = sg1.waitlink {
   346  		sg1.isSelect = false
   347  		sg1.elem = nil
   348  		sg1.c = nil
   349  	}
   350  	gp.waiting = nil
   351  
   352  	for _, casei := range lockorder {
   353  		k = &scases[casei]
   354  		if sg == sglist {
   355  			// sg has already been dequeued by the G that woke us up.
   356  			casi = int(casei)
   357  			cas = k
   358  			caseSuccess = sglist.success
   359  			if sglist.releasetime > 0 {
   360  				caseReleaseTime = sglist.releasetime
   361  			}
   362  		} else {
   363  			c = k.c
   364  			if int(casei) < nsends {
   365  				c.sendq.dequeueSudoG(sglist)
   366  			} else {
   367  				c.recvq.dequeueSudoG(sglist)
   368  			}
   369  		}
   370  		sgnext = sglist.waitlink
   371  		sglist.waitlink = nil
   372  		releaseSudog(sglist)
   373  		sglist = sgnext
   374  	}
   375  
   376  	if cas == nil {
   377  		throw("selectgo: bad wakeup")
   378  	}
   379  
   380  	c = cas.c
   381  
   382  	if debugSelect {
   383  		print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " send=", casi < nsends, "\n")
   384  	}
   385  
   386  	if casi < nsends {
   387  		if !caseSuccess {
   388  			goto sclose
   389  		}
   390  	} else {
   391  		recvOK = caseSuccess
   392  	}
   393  
   394  	if raceenabled {
   395  		if casi < nsends {
   396  			raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
   397  		} else if cas.elem != nil {
   398  			raceWriteObjectPC(c.elemtype, cas.elem, casePC(casi), chanrecvpc)
   399  		}
   400  	}
   401  	if msanenabled {
   402  		if casi < nsends {
   403  			msanread(cas.elem, c.elemtype.size)
   404  		} else if cas.elem != nil {
   405  			msanwrite(cas.elem, c.elemtype.size)
   406  		}
   407  	}
   408  
   409  	selunlock(scases, lockorder)
   410  	goto retc
   411  
   412  bufrecv:
   413  	// can receive from buffer
   414  	if raceenabled {
   415  		if cas.elem != nil {
   416  			raceWriteObjectPC(c.elemtype, cas.elem, casePC(casi), chanrecvpc)
   417  		}
   418  		racenotify(c, c.recvx, nil)
   419  	}
   420  	if msanenabled && cas.elem != nil {
   421  		msanwrite(cas.elem, c.elemtype.size)
   422  	}
   423  	recvOK = true
   424  	qp = chanbuf(c, c.recvx)
   425  	if cas.elem != nil {
   426  		typedmemmove(c.elemtype, cas.elem, qp)
   427  	}
   428  	typedmemclr(c.elemtype, qp)
   429  	c.recvx++
   430  	if c.recvx == c.dataqsiz {
   431  		c.recvx = 0
   432  	}
   433  	c.qcount--
   434  	selunlock(scases, lockorder)
   435  	goto retc
   436  
   437  bufsend:
   438  	// can send to buffer
   439  	if raceenabled {
   440  		racenotify(c, c.sendx, nil)
   441  		raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
   442  	}
   443  	if msanenabled {
   444  		msanread(cas.elem, c.elemtype.size)
   445  	}
   446  	typedmemmove(c.elemtype, chanbuf(c, c.sendx), cas.elem)
   447  	c.sendx++
   448  	if c.sendx == c.dataqsiz {
   449  		c.sendx = 0
   450  	}
   451  	c.qcount++
   452  	selunlock(scases, lockorder)
   453  	goto retc
   454  
   455  recv:
   456  	// can receive from sleeping sender (sg)
   457  	recv(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
   458  	if debugSelect {
   459  		print("syncrecv: cas0=", cas0, " c=", c, "\n")
   460  	}
   461  	recvOK = true
   462  	goto retc
   463  
   464  rclose:
   465  	// read at end of closed channel
   466  	selunlock(scases, lockorder)
   467  	recvOK = false
   468  	if cas.elem != nil {
   469  		typedmemclr(c.elemtype, cas.elem)
   470  	}
   471  	if raceenabled {
   472  		raceacquire(c.raceaddr())
   473  	}
   474  	goto retc
   475  
   476  send:
   477  	// can send to a sleeping receiver (sg)
   478  	if raceenabled {
   479  		raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
   480  	}
   481  	if msanenabled {
   482  		msanread(cas.elem, c.elemtype.size)
   483  	}
   484  	send(c, sg, cas.elem, func() { selunlock(scases, lockorder) }, 2)
   485  	if debugSelect {
   486  		print("syncsend: cas0=", cas0, " c=", c, "\n")
   487  	}
   488  	goto retc
   489  
   490  retc:
   491  	if caseReleaseTime > 0 {
   492  		blockevent(caseReleaseTime-t0, 1)
   493  	}
   494  	return casi, recvOK
   495  
   496  sclose:
   497  	// send on closed channel
   498  	selunlock(scases, lockorder)
   499  	panic(plainError("send on closed channel"))
   500  }
   501  
   502  func (c *hchan) sortkey() uintptr {
   503  	return uintptr(unsafe.Pointer(c))
   504  }
   505  
   506  // A runtimeSelect is a single case passed to rselect.
   507  // This must match ../reflect/value.go:/runtimeSelect
   508  type runtimeSelect struct {
   509  	dir selectDir
   510  	typ unsafe.Pointer // channel type (not used here)
   511  	ch  *hchan         // channel
   512  	val unsafe.Pointer // ptr to data (SendDir) or ptr to receive buffer (RecvDir)
   513  }
   514  
   515  // These values must match ../reflect/value.go:/SelectDir.
   516  type selectDir int
   517  
   518  const (
   519  	_             selectDir = iota
   520  	selectSend              // case Chan <- Send
   521  	selectRecv              // case <-Chan:
   522  	selectDefault           // default
   523  )
   524  
   525  //go:linkname reflect_rselect reflect.rselect
   526  func reflect_rselect(cases []runtimeSelect) (int, bool) {
   527  	if len(cases) == 0 {
   528  		block()
   529  	}
   530  	sel := make([]scase, len(cases))
   531  	orig := make([]int, len(cases))
   532  	nsends, nrecvs := 0, 0
   533  	dflt := -1
   534  	for i, rc := range cases {
   535  		var j int
   536  		switch rc.dir {
   537  		case selectDefault:
   538  			dflt = i
   539  			continue
   540  		case selectSend:
   541  			j = nsends
   542  			nsends++
   543  		case selectRecv:
   544  			nrecvs++
   545  			j = len(cases) - nrecvs
   546  		}
   547  
   548  		sel[j] = scase{c: rc.ch, elem: rc.val}
   549  		orig[j] = i
   550  	}
   551  
   552  	// Only a default case.
   553  	if nsends+nrecvs == 0 {
   554  		return dflt, false
   555  	}
   556  
   557  	// Compact sel and orig if necessary.
   558  	if nsends+nrecvs < len(cases) {
   559  		copy(sel[nsends:], sel[len(cases)-nrecvs:])
   560  		copy(orig[nsends:], orig[len(cases)-nrecvs:])
   561  	}
   562  
   563  	order := make([]uint16, 2*(nsends+nrecvs))
   564  	var pc0 *uintptr
   565  	if raceenabled {
   566  		pcs := make([]uintptr, nsends+nrecvs)
   567  		for i := range pcs {
   568  			selectsetpc(&pcs[i])
   569  		}
   570  		pc0 = &pcs[0]
   571  	}
   572  
   573  	chosen, recvOK := selectgo(&sel[0], &order[0], pc0, nsends, nrecvs, dflt == -1)
   574  
   575  	// Translate chosen back to caller's ordering.
   576  	if chosen < 0 {
   577  		chosen = dflt
   578  	} else {
   579  		chosen = orig[chosen]
   580  	}
   581  	return chosen, recvOK
   582  }
   583  
   584  func (q *waitq) dequeueSudoG(sgp *sudog) {
   585  	x := sgp.prev
   586  	y := sgp.next
   587  	if x != nil {
   588  		if y != nil {
   589  			// middle of queue
   590  			x.next = y
   591  			y.prev = x
   592  			sgp.next = nil
   593  			sgp.prev = nil
   594  			return
   595  		}
   596  		// end of queue
   597  		x.next = nil
   598  		q.last = x
   599  		sgp.prev = nil
   600  		return
   601  	}
   602  	if y != nil {
   603  		// start of queue
   604  		y.prev = nil
   605  		q.first = y
   606  		sgp.next = nil
   607  		return
   608  	}
   609  
   610  	// x==y==nil. Either sgp is the only element in the queue,
   611  	// or it has already been removed. Use q.first to disambiguate.
   612  	if q.first == sgp {
   613  		q.first = nil
   614  		q.last = nil
   615  	}
   616  }
   617
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