Editando: chan.go
// 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 ( "internal/abi" "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.PtrBytes == 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) lockInit(&c.lock, lockRankHchan) if debugChan { print("makechan: chan=", c, "; elemsize=", elem.Size_, "; 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)) } // full reports whether a send on c would block (that is, the channel is full). // It uses a single word-sized read of mutable state, so although // the answer is instantaneously true, the correct answer may have changed // by the time the calling function receives the return value. func full(c *hchan) bool { // c.dataqsiz is immutable (never written after the channel is created) // so it is safe to read at any time during channel operation. if c.dataqsiz == 0 { // Assumes that a pointer read is relaxed-atomic. return c.recvq.first == nil } // Assumes that a uint read is relaxed-atomic. return c.qcount == c.dataqsiz } // 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, traceBlockForever, 2) throw("unreachable") } if debugChan { print("chansend: chan=", c, "\n") } if raceenabled { racereadpc(c.raceaddr(), callerpc, abi.FuncPCABIInternal(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 full()). // 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. However, nothing here // guarantees forward progress. We rely on the side effects of lock release in // chanrecv() and closechan() to update this thread's view of c.closed and full(). if !block && c.closed == 0 && full(c) { 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 { racenotify(c, c.sendx, nil) } 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) // Signal to anyone trying to shrink our stack that we're about // to park on a channel. The window between when this G's status // changes and when we set gp.activeStackChans is not safe for // stack shrinking. gp.parkingOnChan.Store(true) gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanSend, traceBlockChanSend, 2) // 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 gp.activeStackChans = false closed := !mysg.success gp.param = nil if mysg.releasetime > 0 { blockevent(mysg.releasetime-t0, 2) } mysg.c = nil releaseSudog(mysg) if closed { if c.closed == 0 { throw("chansend: spurious wakeup") } panic(plainError("send on closed channel")) } 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. racenotify(c, c.recvx, nil) racenotify(c, c.recvx, sg) 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) sg.success = true 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, abi.FuncPCABIInternal(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 = unsafe.Pointer(sg) sg.success = false 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 = unsafe.Pointer(sg) sg.success = false 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) } } // empty reports whether a read from c would block (that is, the channel is // empty). It uses a single atomic read of mutable state. func empty(c *hchan) bool { // c.dataqsiz is immutable. if c.dataqsiz == 0 { return atomic.Loadp(unsafe.Pointer(&c.sendq.first)) == nil } return atomic.Loaduint(&c.qcount) == 0 } // 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, traceBlockForever, 2) throw("unreachable") } // Fast path: check for failed non-blocking operation without acquiring the lock. if !block && empty(c) { // After observing that the channel is not ready for receiving, we observe whether the // channel is closed. // // Reordering of these checks could lead to incorrect behavior when racing with a close. // For example, if the channel was open and not empty, was closed, and then drained, // reordered reads could incorrectly indicate "open and empty". To prevent reordering, // we use atomic loads for both checks, and rely on emptying and closing to happen in // separate critical sections under the same lock. This assumption fails when closing // an unbuffered channel with a blocked send, but that is an error condition anyway. if atomic.Load(&c.closed) == 0 { // 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. return } // The channel is irreversibly closed. Re-check whether the channel has any pending data // to receive, which could have arrived between the empty and closed checks above. // Sequential consistency is also required here, when racing with such a send. if empty(c) { // The channel is irreversibly closed and empty. if raceenabled { raceacquire(c.raceaddr()) } if ep != nil { typedmemclr(c.elemtype, ep) } return true, false } } var t0 int64 if blockprofilerate > 0 { t0 = cputicks() } lock(&c.lock) if c.closed != 0 { if c.qcount == 0 { if raceenabled { raceacquire(c.raceaddr()) } unlock(&c.lock) if ep != nil { typedmemclr(c.elemtype, ep) } return true, false } // The channel has been closed, but the channel's buffer have data. } else { // Just found waiting sender with not closed. 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 { racenotify(c, c.recvx, nil) } 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) // Signal to anyone trying to shrink our stack that we're about // to park on a channel. The window between when this G's status // changes and when we set gp.activeStackChans is not safe for // stack shrinking. gp.parkingOnChan.Store(true) gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanReceive, traceBlockChanRecv, 2) // someone woke us up if mysg != gp.waiting { throw("G waiting list is corrupted") } gp.waiting = nil gp.activeStackChans = false if mysg.releasetime > 0 { blockevent(mysg.releasetime-t0, 2) } success := mysg.success gp.param = nil mysg.c = nil releaseSudog(mysg) return true, success } // 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 { racenotify(c, c.recvx, nil) racenotify(c, c.recvx, sg) } // 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) sg.success = true if sg.releasetime != 0 { sg.releasetime = cputicks() } goready(gp, skip+1) } func chanparkcommit(gp *g, chanLock unsafe.Pointer) bool { // There are unlocked sudogs that point into gp's stack. Stack // copying must lock the channels of those sudogs. // Set activeStackChans here instead of before we try parking // because we could self-deadlock in stack growth on the // channel lock. gp.activeStackChans = true // Mark that it's safe for stack shrinking to occur now, // because any thread acquiring this G's stack for shrinking // is guaranteed to observe activeStackChans after this store. gp.parkingOnChan.Store(false) // Make sure we unlock after setting activeStackChans and // unsetting parkingOnChan. The moment we unlock chanLock // we risk gp getting readied by a channel operation and // so gp could continue running before everything before // the unlock is visible (even to gp itself). unlock((*mutex)(chanLock)) return true } // 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, ok = <-c: // ... foo // default: // ... bar // } // // as // // if selected, ok = selectnbrecv(&v, c); selected { // ... foo // } else { // ... bar // } func selectnbrecv(elem unsafe.Pointer, c *hchan) (selected, received bool) { return chanrecv(c, elem, false) } //go:linkname reflect_chansend reflect.chansend0 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 reflectlite_chanlen internal/reflectlite.chanlen func reflectlite_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 && !sgp.g.selectDone.CompareAndSwap(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)) } // Notify the race detector of a send or receive involving buffer entry idx // and a channel c or its communicating partner sg. // This function handles the special case of c.elemsize==0. func racenotify(c *hchan, idx uint, sg *sudog) { // We could have passed the unsafe.Pointer corresponding to entry idx // instead of idx itself. However, in a future version of this function, // we can use idx to better handle the case of elemsize==0. // A future improvement to the detector is to call TSan with c and idx: // this way, Go will continue to not allocating buffer entries for channels // of elemsize==0, yet the race detector can be made to handle multiple // sync objects underneath the hood (one sync object per idx) qp := chanbuf(c, idx) // When elemsize==0, we don't allocate a full buffer for the channel. // Instead of individual buffer entries, the race detector uses the // c.buf as the only buffer entry. This simplification prevents us from // following the memory model's happens-before rules (rules that are // implemented in racereleaseacquire). Instead, we accumulate happens-before // information in the synchronization object associated with c.buf. if c.elemsize == 0 { if sg == nil { raceacquire(qp) racerelease(qp) } else { raceacquireg(sg.g, qp) racereleaseg(sg.g, qp) } } else { if sg == nil { racereleaseacquire(qp) } else { racereleaseacquireg(sg.g, qp) } } }
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