Golang atomic.Cas函数代码示例

// Called to receive the next queued signal.
// Must only be called from a single goroutine at a time.
//go:linkname signal_recv os/signal.signal_recv
func signal_recv() uint32 {
	for {
		// Serve any signals from local copy.
		for i := uint32(0); i < _NSIG; i++ {
			if sig.recv[i/32]&(1<<(i&31)) != 0 {
				sig.recv[i/32] &^= 1 << (i & 31)
				return i
			}
		}

		// Wait for updates to be available from signal sender.
	Receive:
		for {
			switch atomic.Load(&sig.state) {
			default:
				throw("signal_recv: inconsistent state")
			case sigIdle:
				if atomic.Cas(&sig.state, sigIdle, sigReceiving) {
					notetsleepg(&sig.note, -1)
					noteclear(&sig.note)
					break Receive
				}
			case sigSending:
				if atomic.Cas(&sig.state, sigSending, sigIdle) {
					break Receive
				}
			}
		}

		// Incorporate updates from sender into local copy.
		for i := range sig.mask {
			sig.recv[i] = atomic.Xchg(&sig.mask[i], 0)
		}
	}
}

func lock(l *mutex) {
	gp := getg()

	if gp.m.locks < 0 {
		throw("runtime·lock: lock count")
	}
	gp.m.locks++

	// Speculative grab for lock.
	v := atomic.Xchg(key32(&l.key), mutex_locked)
	if v == mutex_unlocked {
		return
	}

	// wait is either MUTEX_LOCKED or MUTEX_SLEEPING
	// depending on whether there is a thread sleeping
	// on this mutex. If we ever change l->key from
	// MUTEX_SLEEPING to some other value, we must be
	// careful to change it back to MUTEX_SLEEPING before
	// returning, to ensure that the sleeping thread gets
	// its wakeup call.
	wait := v

	// On uniprocessors, no point spinning.
	// On multiprocessors, spin for ACTIVE_SPIN attempts.
	spin := 0
	if ncpu > 1 {
		spin = active_spin
	}
	for {
		// Try for lock, spinning.
		for i := 0; i < spin; i++ {
			for l.key == mutex_unlocked {
				if atomic.Cas(key32(&l.key), mutex_unlocked, wait) {
					return
				}
			}
			procyield(active_spin_cnt)
		}

		// Try for lock, rescheduling.
		for i := 0; i < passive_spin; i++ {
			for l.key == mutex_unlocked {
				if atomic.Cas(key32(&l.key), mutex_unlocked, wait) {
					return
				}
			}
			osyield()
		}

		// Sleep.
		v = atomic.Xchg(key32(&l.key), mutex_sleeping)
		if v == mutex_unlocked {
			return
		}
		wait = mutex_sleeping
		futexsleep(key32(&l.key), mutex_sleeping, -1)
	}
}

// sweeps one span
// returns number of pages returned to heap, or ^uintptr(0) if there is nothing to sweep
//go:nowritebarrier
func sweepone() uintptr {
	_g_ := getg()

	// increment locks to ensure that the goroutine is not preempted
	// in the middle of sweep thus leaving the span in an inconsistent state for next GC
	_g_.m.locks++
	sg := mheap_.sweepgen
	for {
		idx := atomic.Xadd(&sweep.spanidx, 1) - 1
		if idx >= uint32(len(work.spans)) {
			mheap_.sweepdone = 1
			_g_.m.locks--
			return ^uintptr(0)
		}
		s := work.spans[idx]
		if s.state != mSpanInUse {
			s.sweepgen = sg
			continue
		}
		if s.sweepgen != sg-2 || !atomic.Cas(&s.sweepgen, sg-2, sg-1) {
			continue
		}
		npages := s.npages
		if !s.sweep(false) {
			npages = 0
		}
		_g_.m.locks--
		return npages
	}
}

// Sweeps spans in list until reclaims at least npages into heap.
// Returns the actual number of pages reclaimed.
func (h *mheap) reclaimList(list *mSpanList, npages uintptr) uintptr {
	n := uintptr(0)
	sg := mheap_.sweepgen
retry:
	for s := list.first; s != nil; s = s.next {
		if s.sweepgen == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) {
			list.remove(s)
			// swept spans are at the end of the list
			list.insertBack(s)
			unlock(&h.lock)
			snpages := s.npages
			if s.sweep(false) {
				n += snpages
			}
			lock(&h.lock)
			if n >= npages {
				return n
			}
			// the span could have been moved elsewhere
			goto retry
		}
		if s.sweepgen == sg-1 {
			// the span is being sweept by background sweeper, skip
			continue
		}
		// already swept empty span,
		// all subsequent ones must also be either swept or in process of sweeping
		break
	}
	return n
}

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 sgp participates in a select and is already signaled, ignore it
		if sgp.selectdone != nil {
			// claim the right to signal
			if *sgp.selectdone != 0 || !atomic.Cas(sgp.selectdone, 0, 1) {
				continue
			}
		}

		return sgp
	}
}

//go:nosplit
func semasleep(ns int64) int32 {
	_g_ := getg()

	// Compute sleep deadline.
	var tsp *timespec
	if ns >= 0 {
		var ts timespec
		var nsec int32
		ns += nanotime()
		ts.set_sec(timediv(ns, 1000000000, &nsec))
		ts.set_nsec(nsec)
		tsp = &ts
	}

	for {
		v := atomic.Load(&_g_.m.waitsemacount)
		if v > 0 {
			if atomic.Cas(&_g_.m.waitsemacount, v, v-1) {
				return 0 // semaphore acquired
			}
			continue
		}

		// Sleep until unparked by semawakeup or timeout.
		ret := lwp_park(tsp, 0, unsafe.Pointer(&_g_.m.waitsemacount), nil)
		if ret == _ETIMEDOUT {
			return -1
		}
	}
}

// Dequeue the `sudog` from the waiters linked list
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 sgp.selectdone != nil {
			if *sgp.selectdone != 0 || !atomic.Cas(sgp.selectdone, 0, 1) {
				continue
			}
		}

		return sgp
	}
}

// sweeps one span
// returns number of pages returned to heap, or ^uintptr(0) if there is nothing to sweep
//go:nowritebarrier
func sweepone() uintptr {
	_g_ := getg()

	// increment locks to ensure that the goroutine is not preempted
	// in the middle of sweep thus leaving the span in an inconsistent state for next GC
	_g_.m.locks++
	sg := mheap_.sweepgen
	for {
		idx := atomic.Xadd(&sweep.spanidx, 1) - 1
		if idx >= uint32(len(work.spans)) {
			mheap_.sweepdone = 1
			_g_.m.locks--
			if debug.gcpacertrace > 0 && idx == uint32(len(work.spans)) {
				print("pacer: sweep done at heap size ", memstats.heap_live>>20, "MB; allocated ", mheap_.spanBytesAlloc>>20, "MB of spans; swept ", mheap_.pagesSwept, " pages at ", mheap_.sweepPagesPerByte, " pages/byte\n")
			}
			return ^uintptr(0)
		}
		s := work.spans[idx]
		if s.state != mSpanInUse {
			s.sweepgen = sg
			continue
		}
		if s.sweepgen != sg-2 || !atomic.Cas(&s.sweepgen, sg-2, sg-1) {
			continue
		}
		npages := s.npages
		if !s.sweep(false) {
			npages = 0
		}
		_g_.m.locks--
		return npages
	}
}

// gcLockStackBarriers synchronizes with tracebacks of gp's stack
// during sigprof for installation or removal of stack barriers. It
// blocks until any current sigprof is done tracebacking gp's stack
// and then disallows profiling tracebacks of gp's stack.
//
// This is necessary because a sigprof during barrier installation or
// removal could observe inconsistencies between the stkbar array and
// the stack itself and crash.
//
//go:nosplit
func gcLockStackBarriers(gp *g) {
	// Disable preemption so scanstack cannot run while the caller
	// is manipulating the stack barriers.
	acquirem()
	for !atomic.Cas(&gp.stackLock, 0, 1) {
		osyield()
	}
}

//go:nosplit
func gcTryLockStackBarriers(gp *g) bool {
	mp := acquirem()
	result := atomic.Cas(&gp.stackLock, 0, 1)
	if !result {
		releasem(mp)
	}
	return result
}

func cansemacquire(addr *uint32) bool {
	for {
		v := atomic.Load(addr)
		if v == 0 {
			return false
		}
		if atomic.Cas(addr, v, v-1) {
			return true
		}
	}
}

// If asked to move to or from a Gscanstatus this will throw. Use the castogscanstatus
// and casfrom_Gscanstatus instead.
// casgstatus will loop if the g->atomicstatus is in a Gscan status until the routine that
// put it in the Gscan state is finished.
//go:nosplit
func casgstatus(gp *g, oldval, newval uint32) {
	if (oldval&_Gscan != 0) || (newval&_Gscan != 0) || oldval == newval {
		systemstack(func() {
			print("runtime: casgstatus: oldval=", hex(oldval), " newval=", hex(newval), "\n")
			throw("casgstatus: bad incoming values")
		})
	}

	if oldval == _Grunning && gp.gcscanvalid {
		// If oldvall == _Grunning, then the actual status must be
		// _Grunning or _Grunning|_Gscan; either way,
		// we own gp.gcscanvalid, so it's safe to read.
		// gp.gcscanvalid must not be true when we are running.
		print("runtime: casgstatus ", hex(oldval), "->", hex(newval), " gp.status=", hex(gp.atomicstatus), " gp.gcscanvalid=true\n")
		throw("casgstatus")
	}

	// See http://golang.org/cl/21503 for justification of the yield delay.
	const yieldDelay = 5 * 1000
	var nextYield int64

	// loop if gp->atomicstatus is in a scan state giving
	// GC time to finish and change the state to oldval.
	for i := 0; !atomic.Cas(&gp.atomicstatus, oldval, newval); i++ {
		if oldval == _Gwaiting && gp.atomicstatus == _Grunnable {
			systemstack(func() {
				throw("casgstatus: waiting for Gwaiting but is Grunnable")
			})
		}
		// Help GC if needed.
		// if gp.preemptscan && !gp.gcworkdone && (oldval == _Grunning || oldval == _Gsyscall) {
		// 	gp.preemptscan = false
		// 	systemstack(func() {
		// 		gcphasework(gp)
		// 	})
		// }
		// But meanwhile just yield.
		if i == 0 {
			nextYield = nanotime() + yieldDelay
		}
		if nanotime() < nextYield {
			for x := 0; x < 10 && gp.atomicstatus != oldval; x++ {
				procyield(1)
			}
		} else {
			osyield()
			nextYield = nanotime() + yieldDelay/2
		}
	}
	if newval == _Grunning && gp.gcscanvalid {
		// Run queueRescan on the system stack so it has more space.
		systemstack(func() { queueRescan(gp) })
	}
}

// sweeps one span
// returns number of pages returned to heap, or ^uintptr(0) if there is nothing to sweep
//go:nowritebarrier
func sweepone() uintptr {
	_g_ := getg()

	// increment locks to ensure that the goroutine is not preempted
	// in the middle of sweep thus leaving the span in an inconsistent state for next GC
	_g_.m.locks++
	sg := mheap_.sweepgen
	for {
		s := mheap_.sweepSpans[1-sg/2%2].pop()
		if s == nil {
			mheap_.sweepdone = 1
			_g_.m.locks--
			if debug.gcpacertrace > 0 && atomic.Cas(&sweep.pacertracegen, sg-2, sg) {
				print("pacer: sweep done at heap size ", memstats.heap_live>>20, "MB; allocated ", mheap_.spanBytesAlloc>>20, "MB of spans; swept ", mheap_.pagesSwept, " pages at ", mheap_.sweepPagesPerByte, " pages/byte\n")
			}
			return ^uintptr(0)
		}
		if s.state != mSpanInUse {
			// This can happen if direct sweeping already
			// swept this span, but in that case the sweep
			// generation should always be up-to-date.
			if s.sweepgen != sg {
				print("runtime: bad span s.state=", s.state, " s.sweepgen=", s.sweepgen, " sweepgen=", sg, "\n")
				throw("non in-use span in unswept list")
			}
			continue
		}
		if s.sweepgen != sg-2 || !atomic.Cas(&s.sweepgen, sg-2, sg-1) {
			continue
		}
		npages := s.npages
		if !s.sweep(false) {
			// Span is still in-use, so this returned no
			// pages to the heap and the span needs to
			// move to the swept in-use list.
			npages = 0
		}
		_g_.m.locks--
		return npages
	}
}

// Called from sighandler to send a signal back out of the signal handling thread.
// Reports whether the signal was sent. If not, the caller typically crashes the program.
func sigsend(s uint32) bool {
	bit := uint32(1) << uint(s&31)
	if !sig.inuse || s >= uint32(32*len(sig.wanted)) || sig.wanted[s/32]&bit == 0 {
		return false
	}

	// Add signal to outgoing queue.
	for {
		mask := sig.mask[s/32]
		if mask&bit != 0 {
			return true // signal already in queue
		}
		if atomic.Cas(&sig.mask[s/32], mask, mask|bit) {
			break
		}
	}

	// Notify receiver that queue has new bit.
Send:
	for {
		switch atomic.Load(&sig.state) {
		default:
			throw("sigsend: inconsistent state")
		case sigIdle:
			if atomic.Cas(&sig.state, sigIdle, sigSending) {
				break Send
			}
		case sigSending:
			// notification already pending
			break Send
		case sigReceiving:
			if atomic.Cas(&sig.state, sigReceiving, sigIdle) {
				notewakeup(&sig.note)
				break Send
			}
		}
	}

	return true
}

func goexitsall(status *byte) {
	var buf [_ERRMAX]byte
	if !atomic.Cas(&exiting, 0, 1) {
		return
	}
	getg().m.locks++
	n := copy(buf[:], goexits)
	n = copy(buf[n:], gostringnocopy(status))
	pid := getpid()
	for mp := (*m)(atomic.Loadp(unsafe.Pointer(&allm))); mp != nil; mp = mp.alllink {
		if mp.procid != 0 && mp.procid != pid {
			postnote(mp.procid, buf[:])
		}
	}
	getg().m.locks--
}