When Should a Lock Be Acquired Before Calling condition_variable.notify_one()?

In multithreaded programming, condition_variables are used to signal waiting threads that a specific condition has been met. While it is necessary to hold a lock before calling condition_variable.wait(), there is some uncertainty regarding whether a lock is also required before calling notify_one().

Can I Lock the Mutex Before notify_one() and Why?

Locking the mutex before notify_one() is not mandatory, but it can be advantageous in certain scenarios. When the notifying thread releases the lock before calling notify_one(), it allows waiting threads to immediately start executing. However, if the notifying thread holds the lock, waiting threads must wait for it to be released before proceeding.

Rationale for Locking Before Subsequent Calls to notify_one()

In the example provided, the lock is acquired for subsequent calls to notify_one() because the i variable is being modified within the critical section after the initial notify_one(). This ensures that the waiting thread observes the updated value of i when resuming execution.

Avoiding a Possible Race Condition

Some may argue that not holding the lock during the first notify_one() call could lead to a race condition where the consumer thread prematurely resumes execution and misses the notification. However, this is not a valid concern.

According to the C standard, condition_variable::wait() behaves like an internal loop that continuously checks a predicate and waits if necessary. In this case, the predicate is [] { return i == 1; }. If i is not equal to 1 when the consumer thread checks it, the internal loop will simply terminate, preventing the thread from resuming execution.

Therefore, even if the producer thread calls notify_one() before the consumer thread checks i, the consumer thread will not miss the notification.

Conclusion

While not strictly required, locking the mutex before calling condition_variable.notify_one() can be beneficial in specific scenarios. It allows waiting threads to resume execution immediately and ensures that the waiting thread observes the updated state of shared data (in this case, the value of i). Understanding the behavior of condition_variables and properly synchronizing threads using locks is crucial for effective multithreaded programming.