Unlocking the Mystery behind Conditions: Lock or Not to Lock for notify_one()

The Question:

To ensure efficient thread coordination, std::condition_variables play a pivotal role. However, uncertainty arose regarding the necessity of acquiring a lock before invoking notify_one(): Is it mandatory, or is it an optional practice?

Unraveling the Enigma:

The answer is clear: it is not mandatory to hold a lock before calling notify_one(). However, acquiring the lock is a sound practice in certain scenarios. Let us delve into the reasoning behind this.

Why Lock?

• Pessimistic Approach: While holding a lock might seem redundant, it can be considered a pessimistic strategy. By releasing the lock before notifying the waiting thread, the notified thread will immediately attempt to reacquire it. This may result in contention and potential performance degradation, as both threads compete for the same resource.
• Maintaining Consistency: Certain use cases demand strict adherence to condition variable usage guidelines. Holding the lock throughout the update and wait operations ensures consistency in the data being guarded by the lock. This practice minimizes the risk of race conditions or data corruption.

The Example: A Tale of Two Notifications

The provided example raises questions about the inconsistent locking behavior for subsequent notify_one() calls. The absence of a lock for the initial call is explained by the wait operation that follows: The wait function will automatically acquire and release the lock, ensuring that the notified thread can proceed. However, the subsequent notify_one() calls are guarded by a lock because they do not involve a wait operation.

In summary, holding a lock before calling notify_one() is not a universal requirement but is a recommended practice for certain scenarios. It can mitigate potential performance issues and ensure data integrity.