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