src/runtime/lock_sema.go

// Copyright 2011 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.

//go:build aix || darwin || netbsd || openbsd || plan9 || solaris || windows

package runtime

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

const (
	locked uintptr = 1
)

// One-time notifications.
func noteclear(n *note) {
	n.key = 0
}

func notewakeup(n *note) {
	var v uintptr
	for {
		v = atomic.Loaduintptr(&n.key)
		if atomic.Casuintptr(&n.key, v, locked) {
			break
		}
	}

	// Successfully set waitm to locked.
	// What was it before?
	switch {
	case v == 0:
		// Nothing was waiting. Done.
	case v == locked:
		// Two notewakeups! Not allowed.
		throw("notewakeup - double wakeup")
	default:
		// Must be the waiting m. Wake it up.
		semawakeup((*m)(unsafe.Pointer(v)))
	}
}

func notesleep(n *note) {
	gp := getg()
	if gp != gp.m.g0 {
		throw("notesleep not on g0")
	}
	semacreate(gp.m)
	if !atomic.Casuintptr(&n.key, 0, uintptr(unsafe.Pointer(gp.m))) {
		// Must be locked (got wakeup).
		if n.key != locked {
			throw("notesleep - waitm out of sync")
		}
		return
	}
	// Queued. Sleep.
	gp.m.blocked = true
	if *cgo_yield == nil {
		semasleep(-1)
	} else {
		// Sleep for an arbitrary-but-moderate interval to poll libc interceptors.
		const ns = 10e6
		for atomic.Loaduintptr(&n.key) == 0 {
			semasleep(ns)
			asmcgocall(*cgo_yield, nil)
		}
	}
	gp.m.blocked = false
}

//go:nosplit
func notetsleep_internal(n *note, ns int64, gp *g, deadline int64) bool {
	// gp and deadline are logically local variables, but they are written
	// as parameters so that the stack space they require is charged
	// to the caller.
	// This reduces the nosplit footprint of notetsleep_internal.
	gp = getg()

	// Register for wakeup on n->waitm.
	if !atomic.Casuintptr(&n.key, 0, uintptr(unsafe.Pointer(gp.m))) {
		// Must be locked (got wakeup).
		if n.key != locked {
			throw("notetsleep - waitm out of sync")
		}
		return true
	}
	if ns < 0 {
		// Queued. Sleep.
		gp.m.blocked = true
		if *cgo_yield == nil {
			semasleep(-1)
		} else {
			// Sleep in arbitrary-but-moderate intervals to poll libc interceptors.
			const ns = 10e6
			for semasleep(ns) < 0 {
				asmcgocall(*cgo_yield, nil)
			}
		}
		gp.m.blocked = false
		return true
	}

	deadline = nanotime() + ns
	for {
		// Registered. Sleep.
		gp.m.blocked = true
		if *cgo_yield != nil && ns > 10e6 {
			ns = 10e6
		}
		if semasleep(ns) >= 0 {
			gp.m.blocked = false
			// Acquired semaphore, semawakeup unregistered us.
			// Done.
			return true
		}
		if *cgo_yield != nil {
			asmcgocall(*cgo_yield, nil)
		}
		gp.m.blocked = false
		// Interrupted or timed out. Still registered. Semaphore not acquired.
		ns = deadline - nanotime()
		if ns <= 0 {
			break
		}
		// Deadline hasn't arrived. Keep sleeping.
	}

	// Deadline arrived. Still registered. Semaphore not acquired.
	// Want to give up and return, but have to unregister first,
	// so that any notewakeup racing with the return does not
	// try to grant us the semaphore when we don't expect it.
	for {
		v := atomic.Loaduintptr(&n.key)
		switch v {
		case uintptr(unsafe.Pointer(gp.m)):
			// No wakeup yet; unregister if possible.
			if atomic.Casuintptr(&n.key, v, 0) {
				return false
			}
		case locked:
			// Wakeup happened so semaphore is available.
			// Grab it to avoid getting out of sync.
			gp.m.blocked = true
			if semasleep(-1) < 0 {
				throw("runtime: unable to acquire - semaphore out of sync")
			}
			gp.m.blocked = false
			return true
		default:
			throw("runtime: unexpected waitm - semaphore out of sync")
		}
	}
}

func notetsleep(n *note, ns int64) bool {
	gp := getg()
	if gp != gp.m.g0 {
		throw("notetsleep not on g0")
	}
	semacreate(gp.m)
	return notetsleep_internal(n, ns, nil, 0)
}

// same as runtime·notetsleep, but called on user g (not g0)
// calls only nosplit functions between entersyscallblock/exitsyscall.
func notetsleepg(n *note, ns int64) bool {
	gp := getg()
	if gp == gp.m.g0 {
		throw("notetsleepg on g0")
	}
	semacreate(gp.m)
	entersyscallblock()
	ok := notetsleep_internal(n, ns, nil, 0)
	exitsyscall()
	return ok
}

func beforeIdle(int64, int64) (*g, bool) {
	return nil, false
}

func checkTimeouts() {}