Minimize time spent in a critical section

Reason

The less time is spent with a mutex taken, the less chance that another thread has to wait, and thread suspension and resumption are expensive.

Example

void do_something() // bad
{
    unique_lock<mutex> lck(my_lock);
    do0();  // preparation: does not need lock
    do1();  // transaction: needs locking
    do2();  // cleanup: does not need locking
}

Here, we are holding the lock for longer than necessary: We should not have taken the lock before we needed it and should have released it again before starting the cleanup. We could rewrite this to

void do_something() // bad
{
    do0();  // preparation: does not need lock
    my_lock.lock();
    do1();  // transaction: needs locking
    my_lock.unlock();
    do2();  // cleanup: does not need locking
}

But that compromises safety and violates the use RAII rule. Instead, add a block for the critical section:

void do_something() // OK
{
    do0();  // preparation: does not need lock
    {
        unique_lock<mutex> lck(my_lock);
        do1();  // transaction: needs locking
    }
    do2();  // cleanup: does not need locking
}

Enforcement

Impossible in general. Flag "naked" lock() and unlock().