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tailscale/syncs/locked.go

60 lines
1.5 KiB
Go

// Copyright (c) 2020 Tailscale Inc & 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 go1.13 && !go1.18
// +build go1.13,!go1.18
// This file makes assumptions about the inner workings of sync.Mutex and sync.RWMutex.
// This includes not just their memory layout but their invariants and functionality.
// To prevent accidents, it is limited to a known good subset of Go versions.
package syncs
import (
"sync"
"sync/atomic"
"unsafe"
)
const (
mutexLocked = 1
// sync.Mutex field offsets
stateOffset = 0
// sync.RWMutext field offsets
mutexOffset = 0
readerCountOffset = 16
)
// add returns a pointer with value p + off.
func add(p unsafe.Pointer, off uintptr) unsafe.Pointer {
return unsafe.Pointer(uintptr(p) + off)
}
// AssertLocked panics if m is not locked.
func AssertLocked(m *sync.Mutex) {
p := add(unsafe.Pointer(m), stateOffset)
if atomic.LoadInt32((*int32)(p))&mutexLocked == 0 {
panic("mutex is not locked")
}
}
// AssertRLocked panics if rw is not locked for reading or writing.
func AssertRLocked(rw *sync.RWMutex) {
p := add(unsafe.Pointer(rw), readerCountOffset)
if atomic.LoadInt32((*int32)(p)) != 0 {
// There are readers present or writers pending, so someone has a read lock.
return
}
// No readers.
AssertWLocked(rw)
}
// AssertWLocked panics if rw is not locked for writing.
func AssertWLocked(rw *sync.RWMutex) {
m := (*sync.Mutex)(add(unsafe.Pointer(rw), mutexOffset))
AssertLocked(m)
}