Do condition variables still need a mutex if you're changing the checked value atomically?

Question

Here is the typical way to use a condition variable:

// The reader(s)
lock(some_mutex);
if(protected_by_mutex_var != desired_value)
    some_condition.wait(some_         


        

Answer 1

Imagine the following scenario:

| Thread 1                                            | Thread 2                                           |
| if(protected_by_mutex_var != desired_value) -> true |                                                    |
|                                                     | atomic_set(protected_by_mutex_var, desired_value); |
|                                                     | some_condition.notify_all();                       |
| some_condition.wait(some_mutex);                    |                                                    |

This situation sees Thread 1 waiting for a notify that may never come. Because the statements acting on the condition are not part of the variable read / atomic set, this presents a race condition.

Using the mutex effectively makes these actions inseparable (assuming all accesses to the variable behave properly and lock the mutex.)