Why does the 'fatal error: sync: unlock of unlocked mutex' error appear? How to avoid this error?

Go language mutex use traps and ways to avoid "fatal error: sync: unlock of unlocked mutex"

Mutex in Go is an indispensable tool in concurrent programming and is used to protect shared resources. However, incorrect mutex usage can cause a "fatal error: sync: unlock of unlocked mutex" error. This article will analyze the causes of this error and provide effective solutions.

Problem scenario

In high concurrency environments, such as frequent clicks by the user or page refresh, the program may throw a "fatal error: sync: unlock of unlocked mutex" error. The following code snippet demonstrates the code pattern that can cause this error:

 fmt.Println("1. Start locking:", key)
s.Lock()
fmt.Println("2. Locking is completed:", key)

defer fmt.Println("4. Start unlocking:", key)
defer s.Unlock()
defer fmt.Println("5. Unlocking is completed:", key)

Although the defer statement ensures unlocking of the lock, an error can still occur in high concurrency situations.

Analysis of the root cause of error

The problem is the Sync structure in the code and its usage method:

 package category

import (
    "sync"
)

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

var (
    Cache *Sync
    CacheContainer Sync
)

// GetTree query list (Error example)
func (s *Sync) GetTree() *Sync {
    s.Mu.Lock()
    defer s.Mu.Unlock()
    Cache = &Sync{
        Name: "abc",
        Age: 18,
    }
    // This is the error: unlock CacheContainer again for the unlocked mutex = *Cache
    return &CacheContainer
}

// GetTree2 query list (correct example)
func (s *Sync) GetTree2() *Sync {
    s.Mu.Lock()
    defer s.Mu.Unlock()
    Cache = &Sync{
        Name: "abc",
        Age: 18,
    }
    Return Cache
}

In the GetTree function, the line CacheContainer = *Cache is the source of the error. CacheContainer is a global variable, and *Cache is a dereference to Cache pointer. In high concurrency scenarios, multiple goroutines execute the GetTree function at the same time, which may cause the same mutex to be unlocked multiple times, thereby triggering the "unlock of unlocked mutex" error.

Solution

To avoid this error, follow the following suggestions:

1. Avoid global variables: Try to avoid using global variables, especially those related to mutex locks. Instantiating Sync structures into local variables can effectively avoid concurrency problems.

2. Use mutex correctly: Make sure that each Lock() call has a corresponding Unlock() call and is done in the same goroutine. The GetTree2 function shows the correct way to use mutexes.

3. Detailed error handling: Add more detailed logging to the code to make it easier to track and debug if problems arise.

Through the above analysis and solutions, the "fatal error: sync: unlock of unlocked mutex" error can be effectively avoided and ensure that the Go language program runs stably in a concurrent environment. Remember to use mutexes with caution and avoid unnecessary replication of shared resources in concurrent environments.

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