src/runtime/lock_futex.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 dragonfly || freebsd || linux

package runtime

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

// We use the uintptr mutex.key and note.key as a uint32.
//
//go:nosplit
func key32(p *uintptr) *uint32 {
	return (*uint32)(unsafe.Pointer(p))
}

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

func notewakeup(n *note) {
	old := atomic.Xchg(key32(&n.key), 1)
	if old != 0 {
		print("notewakeup - double wakeup (", old, ")\n")
		throw("notewakeup - double wakeup")
	}
	futexwakeup(key32(&n.key), 1)
}

func notesleep(n *note) {
	gp := getg()
	if gp != gp.m.g0 {
		throw("notesleep not on g0")
	}
	ns := int64(-1)
	if *cgo_yield != nil {
		// Sleep for an arbitrary-but-moderate interval to poll libc interceptors.
		ns = 10e6
	}
	for atomic.Load(key32(&n.key)) == 0 {
		gp.m.blocked = true
		futexsleep(key32(&n.key), 0, ns)
		if *cgo_yield != nil {
			asmcgocall(*cgo_yield, nil)
		}
		gp.m.blocked = false
	}
}

// May run with m.p==nil if called from notetsleep, so write barriers
// are not allowed.
//
//go:nosplit
//go:nowritebarrier
func notetsleep_internal(n *note, ns int64) bool {
	gp := getg()

	if ns < 0 {
		if *cgo_yield != nil {
			// Sleep for an arbitrary-but-moderate interval to poll libc interceptors.
			ns = 10e6
		}
		for atomic.Load(key32(&n.key)) == 0 {
			gp.m.blocked = true
			futexsleep(key32(&n.key), 0, ns)
			if *cgo_yield != nil {
				asmcgocall(*cgo_yield, nil)
			}
			gp.m.blocked = false
		}
		return true
	}

	if atomic.Load(key32(&n.key)) != 0 {
		return true
	}

	deadline := nanotime() + ns
	for {
		if *cgo_yield != nil && ns > 10e6 {
			ns = 10e6
		}
		gp.m.blocked = true
		futexsleep(key32(&n.key), 0, ns)
		if *cgo_yield != nil {
			asmcgocall(*cgo_yield, nil)
		}
		gp.m.blocked = false
		if atomic.Load(key32(&n.key)) != 0 {
			break
		}
		now := nanotime()
		if now >= deadline {
			break
		}
		ns = deadline - now
	}
	return atomic.Load(key32(&n.key)) != 0
}

func notetsleep(n *note, ns int64) bool {
	gp := getg()
	if gp != gp.m.g0 && gp.m.preemptoff != "" {
		throw("notetsleep not on g0")
	}

	return notetsleep_internal(n, ns)
}

// 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")
	}

	entersyscallblock()
	ok := notetsleep_internal(n, ns)
	exitsyscall()
	return ok
}

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

func checkTimeouts() {}

//go:nosplit
func semacreate(mp *m) {}

//go:nosplit
func semasleep(ns int64) int32 {
	mp := getg().m

	for v := atomic.Xadd(&mp.waitsema, -1); ; v = atomic.Load(&mp.waitsema) {
		if int32(v) >= 0 {
			return 0
		}
		futexsleep(&mp.waitsema, v, ns)
		if ns >= 0 {
			if int32(v) >= 0 {
				return 0
			} else {
				return -1
			}
		}
	}
}

//go:nosplit
func semawakeup(mp *m) {
	v := atomic.Xadd(&mp.waitsema, 1)
	if v == 0 {
		futexwakeup(&mp.waitsema, 1)
	}
}