Will improper use of Golang mutex cause 'fatal error: sync: unlock of unlocked mutex' error? How to avoid this problem?

Golang mutex: avoid "fatal error: sync: unlock of unlocked mutex"

In Go concurrent programming, mutex ( sync.Mutex ) is a key tool for protecting shared resources. However, incorrect use can result in a "fatal error: sync.Mutex: unlock of unlocked mutex" error. This error indicates an attempt to unlock an unlocked mutex, usually resulting from a false coordination of concurrent access and lock operations.

Let's analyze a code example that might cause this error:

 package main

import (
    "fmt"
    "sync"
)

type Sync struct {
    Name string
    age int
    Mu sync.Mutex
}

var (
    Cache *Sync
    CacheContainer Sync
)

func (s *Sync) GetTree() *Sync {
    s.Mu.Lock()
    defer s.Mu.Unlock()
    Cache = &Sync{Name: "abc", age: 18}
    CacheContainer = *Cache // Potential problem: copying data, causing lock protection to fail return &CacheContainer
}

func (s *Sync) GetTree2() *Sync {
    s.Mu.Lock()
    defer s.Mu.Unlock()
    Cache = &Sync{Name: "abc", age: 18}
    return Cache // Correct: return directly to protected variable}

In the GetTree function, CacheContainer is a local variable that copies the value of Cache . When GetTree function returns, the life cycle of CacheContainer ends, but Cache still exists and is accessed by other goroutines. If another goroutine tries to operate on CacheContainer and Cache is unlocked, it will cause unlock of unlocked mutex error.

The GetTree2 function avoids this problem, which directly returns the Cache pointer to ensure that all operations on the data are within the protection scope of the lock.

The root cause of the problem and solutions:

• Incorrect lock release timing: The lock release must correspond to the lock and can only be released by the goroutine holding the lock.
• Data replication: Avoid copying shared data within lock-protected code blocks, which will create a copy of the data and leave the lock's protection scope.
• Traps of global variables: In high concurrency environments, special attention should be paid to modifying global variables. If multiple goroutines operate global variables at the same time, problems may occur even if locks are used.

How to avoid the "unlock of unlocked mutex" error:

1. Make sure that each Lock() has a corresponding Unlock() : Using defer s.Mu.Unlock() is a best practice to ensure that the lock can be released correctly even if panic occurs.

2. Avoid creating copy of data within lock protection: directly manipulate shared data instead of creating copy.

3. Use global variables with caution: Try to avoid modifying global variables in high concurrency environments. If necessary, make sure all access is protected by locks.

4. Use more advanced concurrency primitives: For more complex concurrency scenarios, consider using more advanced concurrency primitives such as sync.RWMutex (read-write lock) or channel.

5. Carefully check the code logic: Carefully check the code logic to ensure that the locking and unlocking operations of the lock are correct and avoid deadlocks or other concurrency problems.

By following these suggestions, you can effectively avoid the "fatal error: sync: unlock of unlocked mutex" error and write more robust and safer concurrent Go code.

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