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sasha-s/go-deadlock

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Original source (github.com)
Tags: golang go debugging mutex
Clipped on: 2018-08-11
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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

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

Usage

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

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

And in another goroutine the order of locks is reversed:

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

Another common sources of deadlocs 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

bigtable/bttest: A race between GC and row mutations

Need a mutex that works with net.context?

I have one.

Grabbing an RLock twice from the same goroutine

This is, surprisingly, not a good idea!

From 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 or try it online with go-deadlock on wandbox:

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
() }
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