src/runtime/chan.go - go - Git at Google

 // Copyright 2014 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package runtime

 // This file contains the implementation of Go channels.

 // Invariants:
 //  At least one of c.sendq and c.recvq is empty,
 //  except for the case of an unbuffered channel with a single goroutine
 //  blocked on it for both sending and receiving using a select statement,
 //  in which case the length of c.sendq and c.recvq is limited only by the
 //  size of the select statement.
 //
 // For buffered channels, also:
 //  c.qcount > 0 implies that c.recvq is empty.
 //  c.qcount < c.dataqsiz implies that c.sendq is empty.

 import (
 	"runtime/internal/atomic"
 	"runtime/internal/math"
 	"unsafe"
 )

 const (
 	maxAlign  = 8
 	hchanSize = unsafe.Sizeof(hchan{}) + uintptr(-int(unsafe.Sizeof(hchan{}))&(maxAlign-1))
 	debugChan = false
 )

 type hchan struct {
 	qcount   uint           // total data in the queue
 	dataqsiz uint           // size of the circular queue
 	buf      unsafe.Pointer // points to an array of dataqsiz elements
 	elemsize uint16
 	closed   uint32
 	elemtype *_type // element type
 	sendx    uint   // send index
 	recvx    uint   // receive index
 	recvq    waitq  // list of recv waiters
 	sendq    waitq  // list of send waiters

 	// lock protects all fields in hchan, as well as several
 	// fields in sudogs blocked on this channel.
 	//
 	// Do not change another G's status while holding this lock
 	// (in particular, do not ready a G), as this can deadlock
 	// with stack shrinking.
 	lock mutex
 }

 type waitq struct {
 	first *sudog
 	last  *sudog
 }

 //go:linkname reflect_makechan reflect.makechan
 func reflect_makechan(t *chantype, size int) *hchan {
 	return makechan(t, size)
 }

 func makechan64(t *chantype, size int64) *hchan {
 	if int64(int(size)) != size {
 		panic(plainError("makechan: size out of range"))
 	}

 	return makechan(t, int(size))
 }

 func makechan(t *chantype, size int) *hchan {
 	elem := t.elem

 	// compiler checks this but be safe.
 	if elem.size >= 1<<16 {
 		throw("makechan: invalid channel element type")
 	}
 	if hchanSize%maxAlign != 0 || elem.align > maxAlign {
 		throw("makechan: bad alignment")
 	}

 	mem, overflow := math.MulUintptr(elem.size, uintptr(size))
 	if overflow || mem > maxAlloc-hchanSize || size < 0 {
 		panic(plainError("makechan: size out of range"))
 	}

 	// Hchan does not contain pointers interesting for GC when elements stored in buf do not contain pointers.
 	// buf points into the same allocation, elemtype is persistent.
 	// SudoG's are referenced from their owning thread so they can't be collected.
 	// TODO(dvyukov,rlh): Rethink when collector can move allocated objects.
 	var c *hchan
 	switch {
 	case mem == 0:
 		// Queue or element size is zero.
 		c = (*hchan)(mallocgc(hchanSize, nil, true))
 		// Race detector uses this location for synchronization.
 		c.buf = c.raceaddr()
 	case elem.kind&kindNoPointers != 0:
 		// Elements do not contain pointers.
 		// Allocate hchan and buf in one call.
 		c = (*hchan)(mallocgc(hchanSize+mem, nil, true))
 		c.buf = add(unsafe.Pointer(c), hchanSize)
 	default:
 		// Elements contain pointers.
 		c = new(hchan)
 		c.buf = mallocgc(mem, elem, true)
 	}

 	c.elemsize = uint16(elem.size)
 	c.elemtype = elem
 	c.dataqsiz = uint(size)

 	if debugChan {
 		print("makechan: chan=", c, "; elemsize=", elem.size, "; elemalg=", elem.alg, "; dataqsiz=", size, "\n")
 	}
 	return c
 }

 // chanbuf(c, i) is pointer to the i'th slot in the buffer.
 func chanbuf(c *hchan, i uint) unsafe.Pointer {
 	return add(c.buf, uintptr(i)*uintptr(c.elemsize))
 }

 // entry point for c <- x from compiled code
 //go:nosplit
 func chansend1(c *hchan, elem unsafe.Pointer) {
 	chansend(c, elem, true, getcallerpc())
 }

 /*
  * generic single channel send/recv
  * If block is not nil,
  * then the protocol will not
  * sleep but return if it could
  * not complete.
  *
  * sleep can wake up with g.param == nil
  * when a channel involved in the sleep has
  * been closed.  it is easiest to loop and re-run
  * the operation; we'll see that it's now closed.
  */
 func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
 	if c == nil {
 		if !block {
 			return false
 		}
 		gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2)
 		throw("unreachable")
 	}

 	if debugChan {
 		print("chansend: chan=", c, "\n")
 	}

 	if raceenabled {
 		racereadpc(c.raceaddr(), callerpc, funcPC(chansend))
 	}

 	// Fast path: check for failed non-blocking operation without acquiring the lock.
 	//
 	// After observing that the channel is not closed, we observe that the channel is
 	// not ready for sending. Each of these observations is a single word-sized read
 	// (first c.closed and second c.recvq.first or c.qcount depending on kind of channel).
 	// Because a closed channel cannot transition from 'ready for sending' to
 	// 'not ready for sending', even if the channel is closed between the two observations,
 	// they imply a moment between the two when the channel was both not yet closed
 	// and not ready for sending. We behave as if we observed the channel at that moment,
 	// and report that the send cannot proceed.
 	//
 	// It is okay if the reads are reordered here: if we observe that the channel is not
 	// ready for sending and then observe that it is not closed, that implies that the
 	// channel wasn't closed during the first observation.
 	if !block && c.closed == 0 && ((c.dataqsiz == 0 && c.recvq.first == nil) ||
 		(c.dataqsiz > 0 && c.qcount == c.dataqsiz)) {
 		return false
 	}

 	var t0 int64
 	if blockprofilerate > 0 {
 		t0 = cputicks()
 	}

 	lock(&c.lock)

 	if c.closed != 0 {
 		unlock(&c.lock)
 		panic(plainError("send on closed channel"))
 	}

 	if sg := c.recvq.dequeue(); sg != nil {
 		// Found a waiting receiver. We pass the value we want to send
 		// directly to the receiver, bypassing the channel buffer (if any).
 		send(c, sg, ep, func() { unlock(&c.lock) }, 3)
 		return true
 	}

 	if c.qcount < c.dataqsiz {
 		// Space is available in the channel buffer. Enqueue the element to send.
 		qp := chanbuf(c, c.sendx)
 		if raceenabled {
 			raceacquire(qp)
 			racerelease(qp)
 		}
 		typedmemmove(c.elemtype, qp, ep)
 		c.sendx++
 		if c.sendx == c.dataqsiz {
 			c.sendx = 0
 		}
 		c.qcount++
 		unlock(&c.lock)
 		return true
 	}

 	if !block {
 		unlock(&c.lock)
 		return false
 	}

 	// Block on the channel. Some receiver will complete our operation for us.
 	gp := getg()
 	mysg := acquireSudog()
 	mysg.releasetime = 0
 	if t0 != 0 {
 		mysg.releasetime = -1
 	}
 	// No stack splits between assigning elem and enqueuing mysg
 	// on gp.waiting where copystack can find it.
 	mysg.elem = ep
 	mysg.waitlink = nil
 	mysg.g = gp
 	mysg.isSelect = false
 	mysg.c = c
 	gp.waiting = mysg
 	gp.param = nil
 	c.sendq.enqueue(mysg)
 	goparkunlock(&c.lock, waitReasonChanSend, traceEvGoBlockSend, 3)
 	// Ensure the value being sent is kept alive until the
 	// receiver copies it out. The sudog has a pointer to the
 	// stack object, but sudogs aren't considered as roots of the
 	// stack tracer.
 	KeepAlive(ep)

 	// someone woke us up.
 	if mysg != gp.waiting {
 		throw("G waiting list is corrupted")
 	}
 	gp.waiting = nil
 	if gp.param == nil {
 		if c.closed == 0 {
 			throw("chansend: spurious wakeup")
 		}
 		panic(plainError("send on closed channel"))
 	}
 	gp.param = nil
 	if mysg.releasetime > 0 {
 		blockevent(mysg.releasetime-t0, 2)
 	}
 	mysg.c = nil
 	releaseSudog(mysg)
 	return true
 }

 // send processes a send operation on an empty channel c.
 // The value ep sent by the sender is copied to the receiver sg.
 // The receiver is then woken up to go on its merry way.
 // Channel c must be empty and locked.  send unlocks c with unlockf.
 // sg must already be dequeued from c.
 // ep must be non-nil and point to the heap or the caller's stack.
 func send(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
 	if raceenabled {
 		if c.dataqsiz == 0 {
 			racesync(c, sg)
 		} else {
 			// Pretend we go through the buffer, even though
 			// we copy directly. Note that we need to increment
 			// the head/tail locations only when raceenabled.
 			qp := chanbuf(c, c.recvx)
 			raceacquire(qp)
 			racerelease(qp)
 			raceacquireg(sg.g, qp)
 			racereleaseg(sg.g, qp)
 			c.recvx++
 			if c.recvx == c.dataqsiz {
 				c.recvx = 0
 			}
 			c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz
 		}
 	}
 	if sg.elem != nil {
 		sendDirect(c.elemtype, sg, ep)
 		sg.elem = nil
 	}
 	gp := sg.g
 	unlockf()
 	gp.param = unsafe.Pointer(sg)
 	if sg.releasetime != 0 {
 		sg.releasetime = cputicks()
 	}
 	goready(gp, skip+1)
 }

 // Sends and receives on unbuffered or empty-buffered channels are the
 // only operations where one running goroutine writes to the stack of
 // another running goroutine. The GC assumes that stack writes only
 // happen when the goroutine is running and are only done by that
 // goroutine. Using a write barrier is sufficient to make up for
 // violating that assumption, but the write barrier has to work.
 // typedmemmove will call bulkBarrierPreWrite, but the target bytes
 // are not in the heap, so that will not help. We arrange to call
 // memmove and typeBitsBulkBarrier instead.

 func sendDirect(t *_type, sg *sudog, src unsafe.Pointer) {
 	// src is on our stack, dst is a slot on another stack.

 	// Once we read sg.elem out of sg, it will no longer
 	// be updated if the destination's stack gets copied (shrunk).
 	// So make sure that no preemption points can happen between read & use.
 	dst := sg.elem
 	typeBitsBulkBarrier(t, uintptr(dst), uintptr(src), t.size)
 	// No need for cgo write barrier checks because dst is always
 	// Go memory.
 	memmove(dst, src, t.size)
 }

 func recvDirect(t *_type, sg *sudog, dst unsafe.Pointer) {
 	// dst is on our stack or the heap, src is on another stack.
 	// The channel is locked, so src will not move during this
 	// operation.
 	src := sg.elem
 	typeBitsBulkBarrier(t, uintptr(dst), uintptr(src), t.size)
 	memmove(dst, src, t.size)
 }

 func closechan(c *hchan) {
 	if c == nil {
 		panic(plainError("close of nil channel"))
 	}

 	lock(&c.lock)
 	if c.closed != 0 {
 		unlock(&c.lock)
 		panic(plainError("close of closed channel"))
 	}

 	if raceenabled {
 		callerpc := getcallerpc()
 		racewritepc(c.raceaddr(), callerpc, funcPC(closechan))
 		racerelease(c.raceaddr())
 	}

 	c.closed = 1

 	var glist gList

 	// release all readers
 	for {
 		sg := c.recvq.dequeue()
 		if sg == nil {
 			break
 		}
 		if sg.elem != nil {
 			typedmemclr(c.elemtype, sg.elem)
 			sg.elem = nil
 		}
 		if sg.releasetime != 0 {
 			sg.releasetime = cputicks()
 		}
 		gp := sg.g
 		gp.param = nil
 		if raceenabled {
 			raceacquireg(gp, c.raceaddr())
 		}
 		glist.push(gp)
 	}

 	// release all writers (they will panic)
 	for {
 		sg := c.sendq.dequeue()
 		if sg == nil {
 			break
 		}
 		sg.elem = nil
 		if sg.releasetime != 0 {
 			sg.releasetime = cputicks()
 		}
 		gp := sg.g
 		gp.param = nil
 		if raceenabled {
 			raceacquireg(gp, c.raceaddr())
 		}
 		glist.push(gp)
 	}
 	unlock(&c.lock)

 	// Ready all Gs now that we've dropped the channel lock.
 	for !glist.empty() {
 		gp := glist.pop()
 		gp.schedlink = 0
 		goready(gp, 3)
 	}
 }

 // entry points for <- c from compiled code
 //go:nosplit
 func chanrecv1(c *hchan, elem unsafe.Pointer) {
 	chanrecv(c, elem, true)
 }

 //go:nosplit
 func chanrecv2(c *hchan, elem unsafe.Pointer) (received bool) {
 	_, received = chanrecv(c, elem, true)
 	return
 }

 // chanrecv receives on channel c and writes the received data to ep.
 // ep may be nil, in which case received data is ignored.
 // If block == false and no elements are available, returns (false, false).
 // Otherwise, if c is closed, zeros *ep and returns (true, false).
 // Otherwise, fills in *ep with an element and returns (true, true).
 // A non-nil ep must point to the heap or the caller's stack.
 func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) {
 	// raceenabled: don't need to check ep, as it is always on the stack
 	// or is new memory allocated by reflect.

 	if debugChan {
 		print("chanrecv: chan=", c, "\n")
 	}

 	if c == nil {
 		if !block {
 			return
 		}
 		gopark(nil, nil, waitReasonChanReceiveNilChan, traceEvGoStop, 2)
 		throw("unreachable")
 	}

 	// Fast path: check for failed non-blocking operation without acquiring the lock.
 	//
 	// After observing that the channel is not ready for receiving, we observe that the
 	// channel is not closed. Each of these observations is a single word-sized read
 	// (first c.sendq.first or c.qcount, and second c.closed).
 	// Because a channel cannot be reopened, the later observation of the channel
 	// being not closed implies that it was also not closed at the moment of the
 	// first observation. We behave as if we observed the channel at that moment
 	// and report that the receive cannot proceed.
 	//
 	// The order of operations is important here: reversing the operations can lead to
 	// incorrect behavior when racing with a close.
 	if !block && (c.dataqsiz == 0 && c.sendq.first == nil ||
 		c.dataqsiz > 0 && atomic.Loaduint(&c.qcount) == 0) &&
 		atomic.Load(&c.closed) == 0 {
 		return
 	}

 	var t0 int64
 	if blockprofilerate > 0 {
 		t0 = cputicks()
 	}

 	lock(&c.lock)

 	if c.closed != 0 && c.qcount == 0 {
 		if raceenabled {
 			raceacquire(c.raceaddr())
 		}
 		unlock(&c.lock)
 		if ep != nil {
 			typedmemclr(c.elemtype, ep)
 		}
 		return true, false
 	}

 	if sg := c.sendq.dequeue(); sg != nil {
 		// Found a waiting sender. If buffer is size 0, receive value
 		// directly from sender. Otherwise, receive from head of queue
 		// and add sender's value to the tail of the queue (both map to
 		// the same buffer slot because the queue is full).
 		recv(c, sg, ep, func() { unlock(&c.lock) }, 3)
 		return true, true
 	}

 	if c.qcount > 0 {
 		// Receive directly from queue
 		qp := chanbuf(c, c.recvx)
 		if raceenabled {
 			raceacquire(qp)
 			racerelease(qp)
 		}
 		if ep != nil {
 			typedmemmove(c.elemtype, ep, qp)
 		}
 		typedmemclr(c.elemtype, qp)
 		c.recvx++
 		if c.recvx == c.dataqsiz {
 			c.recvx = 0
 		}
 		c.qcount--
 		unlock(&c.lock)
 		return true, true
 	}

 	if !block {
 		unlock(&c.lock)
 		return false, false
 	}

 	// no sender available: block on this channel.
 	gp := getg()
 	mysg := acquireSudog()
 	mysg.releasetime = 0
 	if t0 != 0 {
 		mysg.releasetime = -1
 	}
 	// No stack splits between assigning elem and enqueuing mysg
 	// on gp.waiting where copystack can find it.
 	mysg.elem = ep
 	mysg.waitlink = nil
 	gp.waiting = mysg
 	mysg.g = gp
 	mysg.isSelect = false
 	mysg.c = c
 	gp.param = nil
 	c.recvq.enqueue(mysg)
 	goparkunlock(&c.lock, waitReasonChanReceive, traceEvGoBlockRecv, 3)

 	// someone woke us up
 	if mysg != gp.waiting {
 		throw("G waiting list is corrupted")
 	}
 	gp.waiting = nil
 	if mysg.releasetime > 0 {
 		blockevent(mysg.releasetime-t0, 2)
 	}
 	closed := gp.param == nil
 	gp.param = nil
 	mysg.c = nil
 	releaseSudog(mysg)
 	return true, !closed
 }

 // recv processes a receive operation on a full channel c.
 // There are 2 parts:
 // 1) The value sent by the sender sg is put into the channel
 //    and the sender is woken up to go on its merry way.
 // 2) The value received by the receiver (the current G) is
 //    written to ep.
 // For synchronous channels, both values are the same.
 // For asynchronous channels, the receiver gets its data from
 // the channel buffer and the sender's data is put in the
 // channel buffer.
 // Channel c must be full and locked. recv unlocks c with unlockf.
 // sg must already be dequeued from c.
 // A non-nil ep must point to the heap or the caller's stack.
 func recv(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func(), skip int) {
 	if c.dataqsiz == 0 {
 		if raceenabled {
 			racesync(c, sg)
 		}
 		if ep != nil {
 			// copy data from sender
 			recvDirect(c.elemtype, sg, ep)
 		}
 	} else {
 		// Queue is full. Take the item at the
 		// head of the queue. Make the sender enqueue
 		// its item at the tail of the queue. Since the
 		// queue is full, those are both the same slot.
 		qp := chanbuf(c, c.recvx)
 		if raceenabled {
 			raceacquire(qp)
 			racerelease(qp)
 			raceacquireg(sg.g, qp)
 			racereleaseg(sg.g, qp)
 		}
 		// copy data from queue to receiver
 		if ep != nil {
 			typedmemmove(c.elemtype, ep, qp)
 		}
 		// copy data from sender to queue
 		typedmemmove(c.elemtype, qp, sg.elem)
 		c.recvx++
 		if c.recvx == c.dataqsiz {
 			c.recvx = 0
 		}
 		c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz
 	}
 	sg.elem = nil
 	gp := sg.g
 	unlockf()
 	gp.param = unsafe.Pointer(sg)
 	if sg.releasetime != 0 {
 		sg.releasetime = cputicks()
 	}
 	goready(gp, skip+1)
 }

 // compiler implements
 //
 //	select {
 //	case c <- v:
 //		... foo
 //	default:
 //		... bar
 //	}
 //
 // as
 //
 //	if selectnbsend(c, v) {
 //		... foo
 //	} else {
 //		... bar
 //	}
 //
 func selectnbsend(c *hchan, elem unsafe.Pointer) (selected bool) {
 	return chansend(c, elem, false, getcallerpc())
 }

 // compiler implements
 //
 //	select {
 //	case v = <-c:
 //		... foo
 //	default:
 //		... bar
 //	}
 //
 // as
 //
 //	if selectnbrecv(&v, c) {
 //		... foo
 //	} else {
 //		... bar
 //	}
 //
 func selectnbrecv(elem unsafe.Pointer, c *hchan) (selected bool) {
 	selected, _ = chanrecv(c, elem, false)
 	return
 }

 // compiler implements
 //
 //	select {
 //	case v, ok = <-c:
 //		... foo
 //	default:
 //		... bar
 //	}
 //
 // as
 //
 //	if c != nil && selectnbrecv2(&v, &ok, c) {
 //		... foo
 //	} else {
 //		... bar
 //	}
 //
 func selectnbrecv2(elem unsafe.Pointer, received *bool, c *hchan) (selected bool) {
 	// TODO(khr): just return 2 values from this function, now that it is in Go.
 	selected, *received = chanrecv(c, elem, false)
 	return
 }

 //go:linkname reflect_chansend reflect.chansend
 func reflect_chansend(c *hchan, elem unsafe.Pointer, nb bool) (selected bool) {
 	return chansend(c, elem, !nb, getcallerpc())
 }

 //go:linkname reflect_chanrecv reflect.chanrecv
 func reflect_chanrecv(c *hchan, nb bool, elem unsafe.Pointer) (selected bool, received bool) {
 	return chanrecv(c, elem, !nb)
 }

 //go:linkname reflect_chanlen reflect.chanlen
 func reflect_chanlen(c *hchan) int {
 	if c == nil {
 		return 0
 	}
 	return int(c.qcount)
 }

 //go:linkname reflect_chancap reflect.chancap
 func reflect_chancap(c *hchan) int {
 	if c == nil {
 		return 0
 	}
 	return int(c.dataqsiz)
 }

 //go:linkname reflect_chanclose reflect.chanclose
 func reflect_chanclose(c *hchan) {
 	closechan(c)
 }

 func (q *waitq) enqueue(sgp *sudog) {
 	sgp.next = nil
 	x := q.last
 	if x == nil {
 		sgp.prev = nil
 		q.first = sgp
 		q.last = sgp
 		return
 	}
 	sgp.prev = x
 	x.next = sgp
 	q.last = sgp
 }

 func (q *waitq) dequeue() *sudog {
 	for {
 		sgp := q.first
 		if sgp == nil {
 			return nil
 		}
 		y := sgp.next
 		if y == nil {
 			q.first = nil
 			q.last = nil
 		} else {
 			y.prev = nil
 			q.first = y
 			sgp.next = nil // mark as removed (see dequeueSudog)
 		}

 		// if a goroutine was put on this queue because of a
 		// select, there is a small window between the goroutine
 		// being woken up by a different case and it grabbing the
 		// channel locks. Once it has the lock
 		// it removes itself from the queue, so we won't see it after that.
 		// We use a flag in the G struct to tell us when someone
 		// else has won the race to signal this goroutine but the goroutine
 		// hasn't removed itself from the queue yet.
 		if sgp.isSelect && !atomic.Cas(&sgp.g.selectDone, 0, 1) {
 			continue
 		}

 		return sgp
 	}
 }

 func (c *hchan) raceaddr() unsafe.Pointer {
 	// Treat read-like and write-like operations on the channel to
 	// happen at this address. Avoid using the address of qcount
 	// or dataqsiz, because the len() and cap() builtins read
 	// those addresses, and we don't want them racing with
 	// operations like close().
 	return unsafe.Pointer(&c.buf)
 }

 func racesync(c *hchan, sg *sudog) {
 	racerelease(chanbuf(c, 0))
 	raceacquireg(sg.g, chanbuf(c, 0))
 	racereleaseg(sg.g, chanbuf(c, 0))
 	raceacquire(chanbuf(c, 0))
 }
	// Copyright 2014 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package runtime

	// This file contains the implementation of Go channels.

	// Invariants:
	// At least one of c.sendq and c.recvq is empty,
	// except for the case of an unbuffered channel with a single goroutine
	// blocked on it for both sending and receiving using a select statement,
	// in which case the length of c.sendq and c.recvq is limited only by the
	// size of the select statement.
	//
	// For buffered channels, also:
	// c.qcount > 0 implies that c.recvq is empty.
	// c.qcount < c.dataqsiz implies that c.sendq is empty.

	import (
	"runtime/internal/atomic"
	"runtime/internal/math"
	"unsafe"
	)

	const (
	maxAlign = 8
	hchanSize = unsafe.Sizeof(hchan{}) + uintptr(-int(unsafe.Sizeof(hchan{}))&(maxAlign-1))
	debugChan = false
	)

	type hchan struct {
	qcount uint // total data in the queue
	dataqsiz uint // size of the circular queue
	buf unsafe.Pointer // points to an array of dataqsiz elements
	elemsize uint16
	closed uint32
	elemtype *_type // element type
	sendx uint // send index
	recvx uint // receive index
	recvq waitq // list of recv waiters
	sendq waitq // list of send waiters

	// lock protects all fields in hchan, as well as several
	// fields in sudogs blocked on this channel.
	//
	// Do not change another G's status while holding this lock
	// (in particular, do not ready a G), as this can deadlock
	// with stack shrinking.
	lock mutex
	}

	type waitq struct {
	first *sudog
	last *sudog
	}

	//go:linkname reflect_makechan reflect.makechan
	func reflect_makechan(t chantype, size int) hchan {
	return makechan(t, size)
	}

	func makechan64(t chantype, size int64) hchan {
	if int64(int(size)) != size {
	panic(plainError("makechan: size out of range"))
	}

	return makechan(t, int(size))
	}

	func makechan(t chantype, size int) hchan {
	elem := t.elem

	// compiler checks this but be safe.
	if elem.size >= 1<<16 {
	throw("makechan: invalid channel element type")
	}
	if hchanSize%maxAlign != 0 \|\| elem.align > maxAlign {
	throw("makechan: bad alignment")
	}

	mem, overflow := math.MulUintptr(elem.size, uintptr(size))
	if overflow \|\| mem > maxAlloc-hchanSize \|\| size < 0 {
	panic(plainError("makechan: size out of range"))
	}

	// Hchan does not contain pointers interesting for GC when elements stored in buf do not contain pointers.
	// buf points into the same allocation, elemtype is persistent.
	// SudoG's are referenced from their owning thread so they can't be collected.
	// TODO(dvyukov,rlh): Rethink when collector can move allocated objects.
	var c *hchan
	switch {
	case mem == 0:
	// Queue or element size is zero.
	c = (*hchan)(mallocgc(hchanSize, nil, true))
	// Race detector uses this location for synchronization.
	c.buf = c.raceaddr()
	case elem.kind&kindNoPointers != 0:
	// Elements do not contain pointers.
	// Allocate hchan and buf in one call.
	c = (*hchan)(mallocgc(hchanSize+mem, nil, true))
	c.buf = add(unsafe.Pointer(c), hchanSize)
	default:
	// Elements contain pointers.
	c = new(hchan)
	c.buf = mallocgc(mem, elem, true)
	}

	c.elemsize = uint16(elem.size)
	c.elemtype = elem
	c.dataqsiz = uint(size)

	if debugChan {
	print("makechan: chan=", c, "; elemsize=", elem.size, "; elemalg=", elem.alg, "; dataqsiz=", size, "\n")
	}
	return c
	}

	// chanbuf(c, i) is pointer to the i'th slot in the buffer.
	func chanbuf(c *hchan, i uint) unsafe.Pointer {
	return add(c.buf, uintptr(i)*uintptr(c.elemsize))
	}

	// entry point for c <- x from compiled code
	//go:nosplit
	func chansend1(c *hchan, elem unsafe.Pointer) {
	chansend(c, elem, true, getcallerpc())
	}

	/*
	* generic single channel send/recv
	* If block is not nil,
	* then the protocol will not
	* sleep but return if it could
	* not complete.
	*
	* sleep can wake up with g.param == nil
	* when a channel involved in the sleep has
	* been closed. it is easiest to loop and re-run
	* the operation; we'll see that it's now closed.
	*/
	func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
	if c == nil {
	if !block {
	return false
	}
	gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2)
	throw("unreachable")
	}

	if debugChan {
	print("chansend: chan=", c, "\n")
	}

	if raceenabled {
	racereadpc(c.raceaddr(), callerpc, funcPC(chansend))
	}

	// Fast path: check for failed non-blocking operation without acquiring the lock.
	//
	// After observing that the channel is not closed, we observe that the channel is
	// not ready for sending. Each of these observations is a single word-sized read
	// (first c.closed and second c.recvq.first or c.qcount depending on kind of channel).
	// Because a closed channel cannot transition from 'ready for sending' to
	// 'not ready for sending', even if the channel is closed between the two observations,
	// they imply a moment between the two when the channel was both not yet closed
	// and not ready for sending. We behave as if we observed the channel at that moment,
	// and report that the send cannot proceed.
	//
	// It is okay if the reads are reordered here: if we observe that the channel is not
	// ready for sending and then observe that it is not closed, that implies that the
	// channel wasn't closed during the first observation.
	if !block && c.closed == 0 && ((c.dataqsiz == 0 && c.recvq.first == nil) \|\|
	(c.dataqsiz > 0 && c.qcount == c.dataqsiz)) {
	return false
	}

	var t0 int64
	if blockprofilerate > 0 {
	t0 = cputicks()
	}

	lock(&c.lock)

	if c.closed != 0 {
	unlock(&c.lock)
	panic(plainError("send on closed channel"))
	}

	if sg := c.recvq.dequeue(); sg != nil {
	// Found a waiting receiver. We pass the value we want to send
	// directly to the receiver, bypassing the channel buffer (if any).
	send(c, sg, ep, func() { unlock(&c.lock) }, 3)
	return true
	}

	if c.qcount < c.dataqsiz {
	// Space is available in the channel buffer. Enqueue the element to send.
	qp := chanbuf(c, c.sendx)
	if raceenabled {
	raceacquire(qp)
	racerelease(qp)
	}
	typedmemmove(c.elemtype, qp, ep)
	c.sendx++
	if c.sendx == c.dataqsiz {
	c.sendx = 0
	}
	c.qcount++
	unlock(&c.lock)
	return true
	}

	if !block {
	unlock(&c.lock)
	return false
	}

	// Block on the channel. Some receiver will complete our operation for us.
	gp := getg()
	mysg := acquireSudog()
	mysg.releasetime = 0
	if t0 != 0 {
	mysg.releasetime = -1
	}
	// No stack splits between assigning elem and enqueuing mysg
	// on gp.waiting where copystack can find it.
	mysg.elem = ep
	mysg.waitlink = nil
	mysg.g = gp
	mysg.isSelect = false
	mysg.c = c
	gp.waiting = mysg
	gp.param = nil
	c.sendq.enqueue(mysg)
	goparkunlock(&c.lock, waitReasonChanSend, traceEvGoBlockSend, 3)
	// Ensure the value being sent is kept alive until the
	// receiver copies it out. The sudog has a pointer to the
	// stack object, but sudogs aren't considered as roots of the
	// stack tracer.
	KeepAlive(ep)

	// someone woke us up.
	if mysg != gp.waiting {
	throw("G waiting list is corrupted")
	}
	gp.waiting = nil
	if gp.param == nil {
	if c.closed == 0 {
	throw("chansend: spurious wakeup")
	}
	panic(plainError("send on closed channel"))
	}
	gp.param = nil
	if mysg.releasetime > 0 {
	blockevent(mysg.releasetime-t0, 2)
	}
	mysg.c = nil
	releaseSudog(mysg)
	return true
	}

	// send processes a send operation on an empty channel c.
	// The value ep sent by the sender is copied to the receiver sg.
	// The receiver is then woken up to go on its merry way.
	// Channel c must be empty and locked. send unlocks c with unlockf.
	// sg must already be dequeued from c.
	// ep must be non-nil and point to the heap or the caller's stack.
	func send(c hchan, sg sudog, ep unsafe.Pointer, unlockf func(), skip int) {
	if raceenabled {
	if c.dataqsiz == 0 {
	racesync(c, sg)
	} else {
	// Pretend we go through the buffer, even though
	// we copy directly. Note that we need to increment
	// the head/tail locations only when raceenabled.
	qp := chanbuf(c, c.recvx)
	raceacquire(qp)
	racerelease(qp)
	raceacquireg(sg.g, qp)
	racereleaseg(sg.g, qp)
	c.recvx++
	if c.recvx == c.dataqsiz {
	c.recvx = 0
	}
	c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz
	}
	}
	if sg.elem != nil {
	sendDirect(c.elemtype, sg, ep)
	sg.elem = nil
	}
	gp := sg.g
	unlockf()
	gp.param = unsafe.Pointer(sg)
	if sg.releasetime != 0 {
	sg.releasetime = cputicks()
	}
	goready(gp, skip+1)
	}

	// Sends and receives on unbuffered or empty-buffered channels are the
	// only operations where one running goroutine writes to the stack of
	// another running goroutine. The GC assumes that stack writes only
	// happen when the goroutine is running and are only done by that
	// goroutine. Using a write barrier is sufficient to make up for
	// violating that assumption, but the write barrier has to work.
	// typedmemmove will call bulkBarrierPreWrite, but the target bytes
	// are not in the heap, so that will not help. We arrange to call
	// memmove and typeBitsBulkBarrier instead.

	func sendDirect(t _type, sg sudog, src unsafe.Pointer) {
	// src is on our stack, dst is a slot on another stack.

	// Once we read sg.elem out of sg, it will no longer
	// be updated if the destination's stack gets copied (shrunk).
	// So make sure that no preemption points can happen between read & use.
	dst := sg.elem
	typeBitsBulkBarrier(t, uintptr(dst), uintptr(src), t.size)
	// No need for cgo write barrier checks because dst is always
	// Go memory.
	memmove(dst, src, t.size)
	}

	func recvDirect(t _type, sg sudog, dst unsafe.Pointer) {
	// dst is on our stack or the heap, src is on another stack.
	// The channel is locked, so src will not move during this
	// operation.
	src := sg.elem
	typeBitsBulkBarrier(t, uintptr(dst), uintptr(src), t.size)
	memmove(dst, src, t.size)
	}

	func closechan(c *hchan) {
	if c == nil {
	panic(plainError("close of nil channel"))
	}

	lock(&c.lock)
	if c.closed != 0 {
	unlock(&c.lock)
	panic(plainError("close of closed channel"))
	}

	if raceenabled {
	callerpc := getcallerpc()
	racewritepc(c.raceaddr(), callerpc, funcPC(closechan))
	racerelease(c.raceaddr())
	}

	c.closed = 1

	var glist gList

	// release all readers
	for {
	sg := c.recvq.dequeue()
	if sg == nil {
	break
	}
	if sg.elem != nil {
	typedmemclr(c.elemtype, sg.elem)
	sg.elem = nil
	}
	if sg.releasetime != 0 {
	sg.releasetime = cputicks()
	}
	gp := sg.g
	gp.param = nil
	if raceenabled {
	raceacquireg(gp, c.raceaddr())
	}
	glist.push(gp)
	}

	// release all writers (they will panic)
	for {
	sg := c.sendq.dequeue()
	if sg == nil {
	break
	}
	sg.elem = nil
	if sg.releasetime != 0 {
	sg.releasetime = cputicks()
	}
	gp := sg.g
	gp.param = nil
	if raceenabled {
	raceacquireg(gp, c.raceaddr())
	}
	glist.push(gp)
	}
	unlock(&c.lock)

	// Ready all Gs now that we've dropped the channel lock.
	for !glist.empty() {
	gp := glist.pop()
	gp.schedlink = 0
	goready(gp, 3)
	}
	}

	// entry points for <- c from compiled code
	//go:nosplit
	func chanrecv1(c *hchan, elem unsafe.Pointer) {
	chanrecv(c, elem, true)
	}

	//go:nosplit
	func chanrecv2(c *hchan, elem unsafe.Pointer) (received bool) {
	_, received = chanrecv(c, elem, true)
	return
	}

	// chanrecv receives on channel c and writes the received data to ep.
	// ep may be nil, in which case received data is ignored.
	// If block == false and no elements are available, returns (false, false).
	// Otherwise, if c is closed, zeros *ep and returns (true, false).
	// Otherwise, fills in *ep with an element and returns (true, true).
	// A non-nil ep must point to the heap or the caller's stack.
	func chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) {
	// raceenabled: don't need to check ep, as it is always on the stack
	// or is new memory allocated by reflect.

	if debugChan {
	print("chanrecv: chan=", c, "\n")
	}

	if c == nil {
	if !block {
	return
	}
	gopark(nil, nil, waitReasonChanReceiveNilChan, traceEvGoStop, 2)
	throw("unreachable")
	}

	// Fast path: check for failed non-blocking operation without acquiring the lock.
	//
	// After observing that the channel is not ready for receiving, we observe that the
	// channel is not closed. Each of these observations is a single word-sized read
	// (first c.sendq.first or c.qcount, and second c.closed).
	// Because a channel cannot be reopened, the later observation of the channel
	// being not closed implies that it was also not closed at the moment of the
	// first observation. We behave as if we observed the channel at that moment
	// and report that the receive cannot proceed.
	//
	// The order of operations is important here: reversing the operations can lead to
	// incorrect behavior when racing with a close.
	if !block && (c.dataqsiz == 0 && c.sendq.first == nil \|\|
	c.dataqsiz > 0 && atomic.Loaduint(&c.qcount) == 0) &&
	atomic.Load(&c.closed) == 0 {
	return
	}

	var t0 int64
	if blockprofilerate > 0 {
	t0 = cputicks()
	}

	lock(&c.lock)

	if c.closed != 0 && c.qcount == 0 {
	if raceenabled {
	raceacquire(c.raceaddr())
	}
	unlock(&c.lock)
	if ep != nil {
	typedmemclr(c.elemtype, ep)
	}
	return true, false
	}

	if sg := c.sendq.dequeue(); sg != nil {
	// Found a waiting sender. If buffer is size 0, receive value
	// directly from sender. Otherwise, receive from head of queue
	// and add sender's value to the tail of the queue (both map to
	// the same buffer slot because the queue is full).
	recv(c, sg, ep, func() { unlock(&c.lock) }, 3)
	return true, true
	}

	if c.qcount > 0 {
	// Receive directly from queue
	qp := chanbuf(c, c.recvx)
	if raceenabled {
	raceacquire(qp)
	racerelease(qp)
	}
	if ep != nil {
	typedmemmove(c.elemtype, ep, qp)
	}
	typedmemclr(c.elemtype, qp)
	c.recvx++
	if c.recvx == c.dataqsiz {
	c.recvx = 0
	}
	c.qcount--
	unlock(&c.lock)
	return true, true
	}

	if !block {
	unlock(&c.lock)
	return false, false
	}

	// no sender available: block on this channel.
	gp := getg()
	mysg := acquireSudog()
	mysg.releasetime = 0
	if t0 != 0 {
	mysg.releasetime = -1
	}
	// No stack splits between assigning elem and enqueuing mysg
	// on gp.waiting where copystack can find it.
	mysg.elem = ep
	mysg.waitlink = nil
	gp.waiting = mysg
	mysg.g = gp
	mysg.isSelect = false
	mysg.c = c
	gp.param = nil
	c.recvq.enqueue(mysg)
	goparkunlock(&c.lock, waitReasonChanReceive, traceEvGoBlockRecv, 3)

	// someone woke us up
	if mysg != gp.waiting {
	throw("G waiting list is corrupted")
	}
	gp.waiting = nil
	if mysg.releasetime > 0 {
	blockevent(mysg.releasetime-t0, 2)
	}
	closed := gp.param == nil
	gp.param = nil
	mysg.c = nil
	releaseSudog(mysg)
	return true, !closed
	}

	// recv processes a receive operation on a full channel c.
	// There are 2 parts:
	// 1) The value sent by the sender sg is put into the channel
	// and the sender is woken up to go on its merry way.
	// 2) The value received by the receiver (the current G) is
	// written to ep.
	// For synchronous channels, both values are the same.
	// For asynchronous channels, the receiver gets its data from
	// the channel buffer and the sender's data is put in the
	// channel buffer.
	// Channel c must be full and locked. recv unlocks c with unlockf.
	// sg must already be dequeued from c.
	// A non-nil ep must point to the heap or the caller's stack.
	func recv(c hchan, sg sudog, ep unsafe.Pointer, unlockf func(), skip int) {
	if c.dataqsiz == 0 {
	if raceenabled {
	racesync(c, sg)
	}
	if ep != nil {
	// copy data from sender
	recvDirect(c.elemtype, sg, ep)
	}
	} else {
	// Queue is full. Take the item at the
	// head of the queue. Make the sender enqueue
	// its item at the tail of the queue. Since the
	// queue is full, those are both the same slot.
	qp := chanbuf(c, c.recvx)
	if raceenabled {
	raceacquire(qp)
	racerelease(qp)
	raceacquireg(sg.g, qp)
	racereleaseg(sg.g, qp)
	}
	// copy data from queue to receiver
	if ep != nil {
	typedmemmove(c.elemtype, ep, qp)
	}
	// copy data from sender to queue
	typedmemmove(c.elemtype, qp, sg.elem)
	c.recvx++
	if c.recvx == c.dataqsiz {
	c.recvx = 0
	}
	c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz
	}
	sg.elem = nil
	gp := sg.g
	unlockf()
	gp.param = unsafe.Pointer(sg)
	if sg.releasetime != 0 {
	sg.releasetime = cputicks()
	}
	goready(gp, skip+1)
	}

	// compiler implements
	//
	// select {
	// case c <- v:
	// ... foo
	// default:
	// ... bar
	// }
	//
	// as
	//
	// if selectnbsend(c, v) {
	// ... foo
	// } else {
	// ... bar
	// }
	//
	func selectnbsend(c *hchan, elem unsafe.Pointer) (selected bool) {
	return chansend(c, elem, false, getcallerpc())
	}

	// compiler implements
	//
	// select {
	// case v = <-c:
	// ... foo
	// default:
	// ... bar
	// }
	//
	// as
	//
	// if selectnbrecv(&v, c) {
	// ... foo
	// } else {
	// ... bar
	// }
	//
	func selectnbrecv(elem unsafe.Pointer, c *hchan) (selected bool) {
	selected, _ = chanrecv(c, elem, false)
	return
	}

	// compiler implements
	//
	// select {
	// case v, ok = <-c:
	// ... foo
	// default:
	// ... bar
	// }
	//
	// as
	//
	// if c != nil && selectnbrecv2(&v, &ok, c) {
	// ... foo
	// } else {
	// ... bar
	// }
	//
	func selectnbrecv2(elem unsafe.Pointer, received bool, c hchan) (selected bool) {
	// TODO(khr): just return 2 values from this function, now that it is in Go.
	selected, *received = chanrecv(c, elem, false)
	return
	}

	//go:linkname reflect_chansend reflect.chansend
	func reflect_chansend(c *hchan, elem unsafe.Pointer, nb bool) (selected bool) {
	return chansend(c, elem, !nb, getcallerpc())
	}

	//go:linkname reflect_chanrecv reflect.chanrecv
	func reflect_chanrecv(c *hchan, nb bool, elem unsafe.Pointer) (selected bool, received bool) {
	return chanrecv(c, elem, !nb)
	}

	//go:linkname reflect_chanlen reflect.chanlen
	func reflect_chanlen(c *hchan) int {
	if c == nil {
	return 0
	}
	return int(c.qcount)
	}

	//go:linkname reflect_chancap reflect.chancap
	func reflect_chancap(c *hchan) int {
	if c == nil {
	return 0
	}
	return int(c.dataqsiz)
	}

	//go:linkname reflect_chanclose reflect.chanclose
	func reflect_chanclose(c *hchan) {
	closechan(c)
	}

	func (q waitq) enqueue(sgp sudog) {
	sgp.next = nil
	x := q.last
	if x == nil {
	sgp.prev = nil
	q.first = sgp
	q.last = sgp
	return
	}
	sgp.prev = x
	x.next = sgp
	q.last = sgp
	}

	func (q waitq) dequeue() sudog {
	for {
	sgp := q.first
	if sgp == nil {
	return nil
	}
	y := sgp.next
	if y == nil {
	q.first = nil
	q.last = nil
	} else {
	y.prev = nil
	q.first = y
	sgp.next = nil // mark as removed (see dequeueSudog)
	}

	// if a goroutine was put on this queue because of a
	// select, there is a small window between the goroutine
	// being woken up by a different case and it grabbing the
	// channel locks. Once it has the lock
	// it removes itself from the queue, so we won't see it after that.
	// We use a flag in the G struct to tell us when someone
	// else has won the race to signal this goroutine but the goroutine
	// hasn't removed itself from the queue yet.
	if sgp.isSelect && !atomic.Cas(&sgp.g.selectDone, 0, 1) {
	continue
	}

	return sgp
	}
	}

	func (c *hchan) raceaddr() unsafe.Pointer {
	// Treat read-like and write-like operations on the channel to
	// happen at this address. Avoid using the address of qcount
	// or dataqsiz, because the len() and cap() builtins read
	// those addresses, and we don't want them racing with
	// operations like close().
	return unsafe.Pointer(&c.buf)
	}

	func racesync(c hchan, sg sudog) {
	racerelease(chanbuf(c, 0))
	raceacquireg(sg.g, chanbuf(c, 0))
	racereleaseg(sg.g, chanbuf(c, 0))
	raceacquire(chanbuf(c, 0))
	}