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Mutex Locking Rules in Thread Synchronization 🧵🔒

Introduction 🌟

Mutexes are used for providing thread synchronization by granting access to critical sections to one thread at a time. This document elaborates on the key rules you should follow when using mutexes for thread synchronization.

Rules to Follow 📜

Rule 1: Ownership of Mutex Lock and Unlock 🔐

  • Rule: If a thread (say T1) locks a mutex (say M), then only that thread (T1) should unlock the mutex (M).
  • Implication: No other thread can unlock the mutex on behalf of the thread that locked it.

Rule 2: Don't Unlock an Unlocked Mutex 🛑

  • Rule: A thread should not attempt to unlock a mutex that is already in an unlocked state.
  • Implication: Doing so will lead to undefined behavior and indicates faulty code.

Rule 3: Blocked Threads on Locked Mutex ⛔

  • Rule: If a mutex is locked by a thread (say T1), other threads (say T2, T3) will be blocked if they try to lock the same mutex.
  • Implication: This establishes mutual exclusivity among threads for accessing critical sections.

Rule 4: OS Scheduling Policy for Blocked Threads ⚙️

  • Rule: If multiple threads are blocked because they tried to lock an already locked mutex, the OS scheduling policy determines which thread gets the lock when the mutex is unlocked.
  • Implication: The granting of the lock among blocked threads is OS-dependent.

Rule 5: Self Deadlock Through Double Locking 🔄

  • Rule: If a thread attempts to lock a mutex it has already locked, it will enter a self-deadlock state.
  • Implication: Care must be taken to avoid re-locking an already locked mutex by the same thread.

Rule 6: Unlock Mutexes in LIFO Order 🔄

  • Rule: Mutexes should be unlocked in the reverse order in which they were locked.
  • Implication: Helps in avoiding deadlocks and ensures proper flow control.

Interview Questions and Answers ❓🤔

Q1: What should you do if you own a mutex lock?

A1: If you own a mutex lock, you should be the one to unlock it. No other thread can unlock it for you.

Q2: What happens if you try to unlock an already unlocked mutex?

A2: Trying to unlock an already unlocked mutex leads to undefined behavior and indicates that your program has a bug.

Q3: What is the role of OS scheduling policy in mutex locking?

A3: The OS scheduling policy decides which thread among the blocked threads gets the lock when a mutex is unlocked by its current owner.

Q4: Can a thread lock a mutex it has already locked?

A4: No, if a thread tries to lock a mutex it has already locked, it will enter into a self-deadlock situation.

Q5: In what order should mutexes be unlocked if you have locked multiple mutexes?

A5: Mutexes should be unlocked in the reverse order in which they were locked to avoid potential deadlocks.

By keeping these rules in mind, you can effectively implement thread synchronization using mutexes.

📚 Happy Learning! 🌈