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https://github.com/sasha-s/go-deadlock

Online deadlock detection in go (golang)
https://github.com/sasha-s/go-deadlock

deadlock golang mutex online-deadlock-detection

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Online deadlock detection in go (golang)

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# Online deadlock detection in go (golang). [![Try it online](https://img.shields.io/badge/try%20it-online-blue.svg)](https://wandbox.org/permlink/hJc6QCZowxbNm9WW) [![Docs](https://godoc.org/github.com/sasha-s/go-deadlock?status.svg)](https://godoc.org/github.com/sasha-s/go-deadlock) [![Build Status](https://travis-ci.com/sasha-s/go-deadlock.svg?branch=master)](https://travis-ci.com/sasha-s/go-deadlock) [![codecov](https://codecov.io/gh/sasha-s/go-deadlock/branch/master/graph/badge.svg)](https://codecov.io/gh/sasha-s/go-deadlock) [![version](https://badge.fury.io/gh/sasha-s%2Fgo-deadlock.svg)](https://github.com/sasha-s/go-deadlock/releases)  [![Go Report Card](https://goreportcard.com/badge/github.com/sasha-s/go-deadlock)](https://goreportcard.com/report/github.com/sasha-s/go-deadlock) [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0) 



## Why

Deadlocks happen and are painful to debug.



## What

go-deadlock provides (RW)Mutex drop-in replacements for sync.(RW)Mutex.

It would not work if you create a spaghetti of channels.

Mutexes only.



## Installation

```sh

go get github.com/sasha-s/go-deadlock/...

```



## Usage

```go

import "github.com/sasha-s/go-deadlock"

var mu deadlock.Mutex

// Use normally, it works exactly like sync.Mutex does.

mu.Lock()



defer mu.Unlock()

// Or

var rw deadlock.RWMutex

rw.RLock()

defer rw.RUnlock()

```



### Deadlocks

One of the most common sources of deadlocks is inconsistent lock ordering:

say, you have two mutexes A and B, and in some goroutines you have

```go

A.Lock() // defer A.Unlock() or similar.

...

B.Lock() // defer B.Unlock() or similar.

```

And in another goroutine the order of locks is reversed:

```go

B.Lock() // defer B.Unlock() or similar.

...

A.Lock() // defer A.Unlock() or similar.

```



Another common sources of deadlocks is duplicate take a lock in a goroutine:

```

A.Rlock() or lock()



A.lock() or A.RLock()

```



This does not guarantee a deadlock (maybe the goroutines above can never be running at the same time), but it usually a design flaw at least.



go-deadlock can detect such cases (unless you cross goroutine boundary - say lock A, then spawn a goroutine, block until it is singals, and lock B inside of the goroutine), even if the deadlock itself happens very infrequently and is painful to reproduce!



Each time go-deadlock sees a lock attempt for lock B, it records the order A before B, for each lock that is currently being held in the same goroutine, and it prints (and exits the program by default) when it sees the locking order being violated.



In addition, if it sees that we are waiting on a lock for a long time (opts.DeadlockTimeout, 30 seconds by default), it reports a potential deadlock, also printing the stacktrace for a goroutine that is currently holding the lock we are desperately trying to grab.



## Sample output

#### Inconsistent lock ordering:

```

POTENTIAL DEADLOCK: Inconsistent locking. saw this ordering in one goroutine:

happened before

inmem.go:623 bttest.(*server).ReadModifyWriteRow { r.mu.Lock() } <<<<<

inmem_test.go:118 bttest.TestConcurrentMutationsReadModifyAndGC.func4 { _, _ = s.ReadModifyWriteRow(ctx, rmw()) }



happened after

inmem.go:629 bttest.(*server).ReadModifyWriteRow { tbl.mu.RLock() } <<<<<

inmem_test.go:118 bttest.TestConcurrentMutationsReadModifyAndGC.func4 { _, _ = s.ReadModifyWriteRow(ctx, rmw()) }



in another goroutine: happened before

inmem.go:799 bttest.(*table).gc { t.mu.RLock() } <<<<<

inmem_test.go:125 bttest.TestConcurrentMutationsReadModifyAndGC.func5 { tbl.gc() }



happend after

inmem.go:814 bttest.(*table).gc { r.mu.Lock() } <<<<<

inmem_test.go:125 bttest.TestConcurrentMutationsReadModifyAndGC.func5 { tbl.gc() }

```



#### Waiting for a lock for a long time:



```

POTENTIAL DEADLOCK:

Previous place where the lock was grabbed

goroutine 240 lock 0xc820160440

inmem.go:799 bttest.(*table).gc { t.mu.RLock() } <<<<<

inmem_test.go:125 bttest.TestConcurrentMutationsReadModifyAndGC.func5 { tbl.gc() }



Have been trying to lock it again for more than 40ms

goroutine 68 lock 0xc820160440

inmem.go:785 bttest.(*table).mutableRow { t.mu.Lock() } <<<<<

inmem.go:428 bttest.(*server).MutateRow { r := tbl.mutableRow(string(req.RowKey)) }

inmem_test.go:111 bttest.TestConcurrentMutationsReadModifyAndGC.func3 { s.MutateRow(ctx, req) }



Here is what goroutine 240 doing now

goroutine 240 [select]:

github.com/sasha-s/go-deadlock.lock(0xc82028ca10, 0x5189e0, 0xc82013a9b0)

        /Users/sasha/go/src/github.com/sasha-s/go-deadlock/deadlock.go:163 +0x1640

github.com/sasha-s/go-deadlock.(*Mutex).Lock(0xc82013a9b0)

        /Users/sasha/go/src/github.com/sasha-s/go-deadlock/deadlock.go:54 +0x86

google.golang.org/cloud/bigtable/bttest.(*table).gc(0xc820160440)

        /Users/sasha/go/src/google.golang.org/cloud/bigtable/bttest/inmem.go:814 +0x28d

google.golang.org/cloud/bigtable/bttest.TestConcurrentMutationsReadModifyAndGC.func5(0xc82015c760, 0xc820160440)      /Users/sasha/go/src/google.golang.org/cloud/bigtable/bttest/inmem_test.go:125 +0x48

created by google.golang.org/cloud/bigtable/bttest.TestConcurrentMutationsReadModifyAndGC

        /Users/sasha/go/src/google.golang.org/cloud/bigtable/bttest/inmem_test.go:126 +0xb6f

```



## Used in

[cockroachdb: Potential deadlock between Gossip.SetStorage and Node.gossipStores](https://github.com/cockroachdb/cockroach/issues/7972)



[bigtable/bttest: A race between GC and row mutations](https://code-review.googlesource.com#/c/5301/)



## Need a mutex that works with net.context?

I have [one](https://github.com/sasha-s/go-csync).



## Grabbing an RLock twice from the same goroutine

This is, surprisingly, not a good idea!



From [RWMutex](https://golang.org/pkg/sync/#RWMutex) docs:



>If a goroutine holds a RWMutex for reading and another goroutine might call Lock, no goroutine should expect to be able to acquire a read lock until the initial read lock is released. In particular, this prohibits recursive read locking. This is to ensure that the lock eventually becomes available; a blocked Lock call excludes new readers from acquiring the lock.



The following code will deadlock — [run the example on playground](https://play.golang.org/p/AkL-W63nq5f) or [try it online with go-deadlock on wandbox](https://wandbox.org/permlink/JwnL0GMySBju4SII):

```go

package main



import (

	"fmt"

	"sync"

)



func main() {

	var mu sync.RWMutex



	chrlockTwice := make(chan struct{}) // Used to control rlockTwice

	rlockTwice := func() {

		mu.RLock()

		fmt.Println("first Rlock succeeded")

		<-chrlockTwice

		<-chrlockTwice

		fmt.Println("trying to Rlock again")

		mu.RLock()

		fmt.Println("second Rlock succeeded")

		mu.RUnlock()

		mu.RUnlock()

	}



	chLock := make(chan struct{}) // Used to contol lock

	lock := func() {

		<-chLock

		fmt.Println("about to Lock")

		mu.Lock()

		fmt.Println("Lock succeeded")

		mu.Unlock()

		<-chLock

	}



	control := func() {

		chrlockTwice <- struct{}{}

		chLock <- struct{}{}



		close(chrlockTwice)

		close(chLock)

	}



	go control()

	go lock()

	rlockTwice()

}

```

## Configuring go-deadlock



Have a look at [Opts](https://pkg.go.dev/github.com/sasha-s/go-deadlock#pkg-variables).



* `Opts.Disable`: disables deadlock detection altogether

* `Opts.DisableLockOrderDetection`: disables lock order based deadlock detection.

* `Opts.DeadlockTimeout`: blocking on mutex for longer than DeadlockTimeout is considered a deadlock. ignored if negative

* `Opts.OnPotentialDeadlock`: callback for then deadlock is detected

* `Opts.MaxMapSize`: size of happens before // happens after table

* `Opts.PrintAllCurrentGoroutines`:  dump stacktraces of all goroutines when inconsistent locking is detected, verbose

* `Opts.LogBuf`: where to write deadlock info/stacktraces