Question

The following code contains a potential deadlock, but seems to be necessary: to safely copy data to one container from another, both containers must be locked to prevent changes from occurring in another thread.

void foo::copy(const foo & rhs)
{
    pMutex->lock();
    rhs.pMutex->lock();
    // do copy
}

Foo has an STL container and “do copy” essentially consists of using std::copy. How do I lock both mutexes without introducing deadlock?

Answer 1

Impose some kind of total order on instances of foo and always acquire their locks in either increasing or decreasing order, e.g., foo1->lock() and then foo2->lock().

Another approach is to use functional semantics and instead write a foo::clone method that creates a new instance rather than clobbering an existing one.

If your code is doing lots of locking, you may need a complex deadlock-avoidance algorithm such as the banker’s algorithm.