Mutex (互斥体)

Abstract

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• Stands for Mutual Exclusion
• A Synchronisation (同步) mechanism that controls access to shared resources, such as variables, data structures, or critical sections of code, in a systems that use Concurrency (并发)
• Powered by Atomic Instruction

Avoid corrupted data

Prevent multiple Thread or Process (进程) from accessing the same resource simultaneously, which could lead to Race Condition (竞态条件).

Go Mutex

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• The program below simulates incrementing a counter. Each 1000 increments takes 100ms. We perform 10 iterations to get the counter from 0 to 10000. Uncomment the code blocks in the editor below to add in Mutex (互斥体) to prevent Race Condition (竞态条件).

Corrupted data!

Since the Goroutines are running in a Concurrency (并发) manner, both goroutines may read in the same counter value, one goroutine will overwrite the changes made by another goroutine - Race Condition (竞态条件). Uncomment the code blocks in the editor above to add in the Mutex (互斥体)!

Program runs perfectly without Mutex

Sometimes, without Mutex (互斥体), the program runs without any issues, this is because the Goroutines complete too fast, run a few times to see corrupted data caused by Race Condition (竞态条件).

Placement of m.Lock() & m.Unlock() matter!

Try place m.Lock() at the first line of the increment() function and mUnlock() at the last line of the increment(). Then re-run the program and observe the time take.

The time taken jump from 100ms to 1s! This is because Mutex forces the Goroutines to take turns to execute the code block between mLock() and m.Unlock(). This means instead of running 10 time.Sleep(100 * time.Millisecond) concurrently, which can complete in 100ms, we now need to run it one by one which accumulates to 1s. In this case, it is totally safe to run in a concurrent manner, the concurrency is destroyed due to improper place of m.Lock() & m.Unlock()!