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// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
//go:build ts_mutex_debug
package syncs
import (
"bytes"
"fmt"
"log"
"runtime"
"sync"
"unsafe"
"go4.org/mem"
)
// MutexDebugging indicates whether the "ts_mutex_debug" build tag is set
// and mutex debugging is enabled.
const MutexDebugging = true
type Mutex struct {
sync.Mutex
}
type RWMutex struct {
sync.RWMutex
}
func RequiresMutex(mu *Mutex) {
// TODO: check
}
// TODO(bradfitz): actually track stuff when in debug mode.
var bufPool = &sync.Pool{
New: func() any {
b := make([]byte, 16<<10)
return &b
},
}
func (m *Mutex) Lock() {
defer m.Mutex.Lock()
gid := curGoroutineID()
up := uintptr((unsafe.Pointer)(m))
bufp := bufPool.Get().(*[]byte)
defer bufPool.Put(bufp)
stack := (*bufp)[:runtime.Stack(*bufp, false)]
trackMu.Lock()
defer trackMu.Unlock()
gid = walkToParent(gid)
name, ok := mutexName[up]
if !ok {
name = "unnamed"
log.Printf("XXX unregistered Mutex.Lock %p called from:\n%s", m, stack)
}
switch name {
case "ipnlocal.LocalBackend.mu", "wgengine.userspaceEngine.wgLock", "ipnlocal.nodeBackend.mu":
if bytes.Contains(stack, []byte("wireguard-go/device.(*Device).RoutineReceiveIncoming")) {
log.Printf("XXX mutex Lock from wireguard land: %s, %s", name, stack)
}
}
gi, ok := goroutines[gid]
if !ok {
gi = &goroutineInfo{}
goroutines[gid] = gi
}
gi.holding = append(gi.holding, &heldLock{
mutexAddr: up,
name: name,
})
if len(gi.holding) > 1 {
names := make([]string, 0, len(gi.holding))
for i, hl := range gi.holding {
names = append(names, hl.name)
if i == 0 {
continue
}
lo := lockOrder{
first: gi.holding[i-1].name,
second: hl.name,
}
if lockOrders[lo.reverse()] {
log.Printf("mutex: potential deadlock detected: lock order violation: %q then %q (saw reverse order before); goroutine %d stack:\n%s", lo.first, lo.second, gid, stack)
} else {
if _, ok := lockOrders[lo]; !ok {
log.Printf("XXX learned new lock order: %q then %q", lo.first, lo.second)
lockOrders[lo] = true
}
}
}
log.Printf("XXX goroutine %v holding %q", gid, names)
}
}
func (m *Mutex) Unlock() {
defer m.Mutex.Unlock()
up := uintptr((unsafe.Pointer)(m))
gid := curGoroutineID()
trackMu.Lock()
defer trackMu.Unlock()
gid = walkToParent(gid)
name, ok := mutexName[up]
if !ok {
name = "unnamed"
}
gi, ok := goroutines[gid]
if !ok || len(gi.holding) == 0 {
log.Printf("mutex: unlock of %p (%s) by goroutine %d with no held locks", m, name, gid)
return
}
last := gi.holding[len(gi.holding)-1]
if last.mutexAddr != up {
log.Printf("mutex: unlock of %p (%s) by goroutine %d, but last held lock is %p (%s)", m, name, gid, last.mutexAddr, last.name)
return
}
gi.holding[len(gi.holding)-1] = nil
gi.holding = gi.holding[:len(gi.holding)-1]
if len(gi.holding) == 0 {
delete(goroutines, gid)
}
}
var (
trackMu sync.Mutex
mutexName = make(map[uintptr]string)
goroutines = make(map[uint64]*goroutineInfo)
parentGID = make(map[uint64]uint64) // child goroutine ID -> parent (for ForkJoinGo)
lockOrders = make(map[lockOrder]bool) // observed lock orderings
)
type lockOrder struct {
first string
second string
}
func (lo lockOrder) reverse() lockOrder {
return lockOrder{first: lo.second, second: lo.first}
}
type goroutineInfo struct {
holding []*heldLock
}
type heldLock struct {
mutexAddr uintptr
name string
// TODO: stack? [16]uintptr?
}
// RegisterMutex registers the given mutex with the given name for
// debugging purposes.
func RegisterMutex(mu *Mutex, name string) {
trackMu.Lock()
defer trackMu.Unlock()
up := uintptr((unsafe.Pointer)(mu))
mutexName[up] = name
runtime.AddCleanup(mu, func(up uintptr) {
trackMu.Lock()
defer trackMu.Unlock()
delete(mutexName, up)
}, up)
}
var littleBuf = sync.Pool{
New: func() interface{} {
buf := make([]byte, 64)
return &buf
},
}
var goroutineSpace = []byte("goroutine ")
func curGoroutineID() uint64 {
bp := littleBuf.Get().(*[]byte)
defer littleBuf.Put(bp)
b := *bp
b = b[:runtime.Stack(b, false)]
// Parse the 4707 out of "goroutine 4707 ["
b = bytes.TrimPrefix(b, goroutineSpace)
i := bytes.IndexByte(b, ' ')
if i < 0 {
panic(fmt.Sprintf("No space found in %q", b))
}
b = b[:i]
n, err := mem.ParseUint(mem.B(b), 10, 64)
if err != nil {
panic(fmt.Sprintf("Failed to parse goroutine ID out of %q: %v", b, err))
}
return n
}
func trackForkJoinPair(parent, child uint64, add bool) {
trackMu.Lock()
defer trackMu.Unlock()
if add {
parentGID[child] = parent
} else {
delete(parentGID, child)
}
}
func walkToParent(gid uint64) uint64 {
for {
p, ok := parentGID[gid]
if !ok {
return gid
}
gid = p
}
}
// ForkJoinGo is like go fn() but indicates that the goroutine
// is part of a fork-join parallelism pattern.
//
// This compiles to just "go fn()" in non-debug builds.
func ForkJoinGo(fn func()) {
parentGID := curGoroutineID()
go func() {
childGID := curGoroutineID()
trackForkJoinPair(parentGID, childGID, true)
defer trackForkJoinPair(parentGID, childGID, false)
fn()
}()
}
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