Should You Hold a Lock Before Calling condition_variable::notify_one()?

Is the Lock Required Before Calling condition_variable.notify_one()?

In C , condition_variables are used to efficiently handle multi-threaded synchronization scenarios. When using condition_variables, it's crucial to understand the role of locks in conjunction with their methods.

Locking Before notify_one()

While it's not mandatory to hold a lock before calling condition_variable::notify_one(), it's generally considered good practice not to. However, there are a few reasons why you might choose to do so:

• Avoiding Deadlocks: Holding the lock can prevent deadlocks if multiple threads attempt to notify and lock the same mutex concurrently.

Example Explained

In the example provided, the first call to condition_variable::notify_one() is made without holding the lock, while subsequent calls acquire the lock first. This approach is valid and serves to prevent deadlocks as described earlier.

Rationale

First notify_one() without Lock:

• When the first notify_one() is called, the waits() thread is not yet blocked on the condition variable.
• By notifying without acquiring the lock, the waits() thread can immediately acquire the lock associated with the condition variable and continue execution.

Subsequent notify_one() with Lock:

• Once the waits() thread is blocked on the condition variable, holding the lock prevents other threads from acquiring it.
• This ensures that the signals() thread can continue modifying the shared state (in this case, i) reliably.

Performance Considerations

While holding the lock before notify_one() can prevent deadlocks, it may also lead to performance degradation.

Holding the lock forces the scheduling of the waits() thread, which may already be ready to run. This can cause unnecessary context switches and affect performance.

Conclusion

Whether or not to hold a lock before calling condition_variable::notify_one() depends on the specific scenario and performance requirements. If deadlock avoidance is a concern, holding the lock is recommended. If performance is a priority, notifying without holding the lock may be preferable. However, it's essential to consider the overall thread safety implications and design a robust synchronization strategy accordingly.

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