Should You Acquire a Lock Before Calling `condition_variable.notify_one()`?

Unlocking the Mystery of Acquiring Locks Before Notifying Conditions

The usage of condition variables is a fundamental aspect of synchronization in multithreaded environments. One common question that arises is whether it is necessary to acquire a lock before calling condition_variable.notify_one().

Acquiring Locks Before notify_one()

Technically, acquiring a lock before calling condition_variable.notify_one() is not mandatory. However, it is considered a good practice in certain scenarios.

Benefits of Locking

Locking before notifying can prevent the race condition between notifying and unlocking. For example, if the notifying thread releases the lock immediately after calling notify_one(), the notified thread may acquire the lock and access shared resources before the notifying thread has a chance to update them. To avoid this race condition, it is recommended to hold the lock until after the shared resources have been updated.

When Not to Lock

Locking is unnecessary in cases where the notifying thread is not modifying shared resources. For instance, if the notify_one() call is purely intended to wake up waiting threads without modifying data, locking would be redundant.

Exception for Predicate Wait

An exception to the general rule of locking before notify_one() arises when using predicate-based wait calls such as cv.wait(lk, []{return i == 1;}). In such cases, acquiring the lock before notify_one() is redundant because the predicate itself ensures that the waited-upon condition is met before the waiting thread resumes execution.

Example Analysis

In the example provided, the first notify_one() call is not locked because the notifying thread is not updating shared resources. However, the subsequent notify_one() calls are locked because they are within a loop where shared resources are modified (i is set to 1).

Conclusion

Whether or not to acquire a lock before condition_variable.notify_one() depends on the specific synchronization requirements. In most cases, locking is beneficial to prevent race conditions during data modification. However, it is unnecessary when the notify_one() call is solely intended to wake up waiting threads without modifying shared resources or when using predicate-based wait calls.

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