Go源码: channel
发布于 2019-06-03 · 本文总共 7134 字 · 阅读大约需要
21 分钟
channel
/usr/local/go/src/runtime/chan.go
源码
type hchan struct {
qcount uint // len //total data in the queue
dataqsiz uint // cap //size of the circular queue
buf unsafe.Pointer // 指向buffer channel底层数组 //points to an array of dataqsiz elements
elemsize uint16 // channel里元素的大小
closed uint32 //是否close
elemtype *_type //通道类型 // element type
sendx uint //指向下一个写位置 // send index
recvx uint //指向下一个读位置 // receive index
recvq waitq //阻塞在读的goroutine,双向链表 // list of recv waiters
sendq waitq //阻塞在写的goroutine,双向链表 //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 // 控制并发
}
waitq
type waitq struct {
first *sudog
last *sudog
}
mutex
// Mutual exclusion locks. In the uncontended case,
// as fast as spin locks (just a few user-level instructions),
// but on the contention path they sleep in the kernel.
// A zeroed Mutex is unlocked (no need to initialize each lock).
type mutex struct {
// Futex-based impl treats it as uint32 key,
// while sema-based impl as M* waitm.
// Used to be a union, but unions break precise GC.
key uintptr
}
make
send
// 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
}
给nil的channel发送数据会报错:fatal error: all goroutines are asleep - deadlock!
if c == nil {
if !block {
return false
}
gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2)
throw("unreachable")
}
fatal error: all goroutines are asleep - deadlock!
// Check for deadlock situation.
// The check is based on number of running M's, if 0 -> deadlock.
// sched.lock must be held.
func checkdead() {
// For -buildmode=c-shared or -buildmode=c-archive it's OK if
// there are no running goroutines. The calling program is
// assumed to be running.
if islibrary || isarchive {
return
}
// If we are dying because of a signal caught on an already idle thread,
// freezetheworld will cause all running threads to block.
// And runtime will essentially enter into deadlock state,
// except that there is a thread that will call exit soon.
if panicking > 0 {
return
}
// If we are not running under cgo, but we have an extra M then account
// for it. (It is possible to have an extra M on Windows without cgo to
// accommodate callbacks created by syscall.NewCallback. See issue #6751
// for details.)
var run0 int32
if !iscgo && cgoHasExtraM {
mp := lockextra(true)
haveExtraM := extraMCount > 0
unlockextra(mp)
if haveExtraM {
run0 = 1
}
}
...
getg().m.throwing = -1 // do not dump full stacks
throw("all goroutines are asleep - deadlock!")
}
给已经closed的channel发送数据会panic
if c.closed != 0 {
unlock(&c.lock)
panic(plainError("send on closed channel"))
}
正常发送
1.如果有goroutine阻塞在channel,取出等待的goroutine,唤醒
2.如果缓冲区还有空间,将数据发到buffer
3.如果buffer已满,阻塞当前goroutine
receive
给nil的channel接收数据会报错(同发送一样)
如果channel已经closed
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
}
使用value,ok := <- a_chan的方式读
正常接收
1.如果有发送的goroutine阻塞在channel,取出等待的goroutine,唤醒
2.如果缓冲区还有可用数据,接收
3.如果buffer为空,阻塞当前goroutine
close
c.closed = 1
// release all readers
// release all writers (they will panic)
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)
}
}
select channel
go版本
go version go1.13.6 darwin/amd64