Acquire a lock on two mutexes and avoid deadlock

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.

Answer 2

How about this?

void foo::copy(const foo & rhs)
{
    scopedLock lock(rhs.pMutex); // release mutex in destructor
    foo tmp(rhs);
    swap(tmp); // no throw swap locked internally
}

This is exception safe, and pretty thread safe as well. To be 100% thread save you'll need to review all code path and than re-review again with another set of eyes, after that review it again...

Answer 3

As @Mellester mentioned you can use std::lock for locking multiple mutexes avoiding deadlock.

#include <mutex>

void foo::copy(const foo& rhs)
{
    std::lock(pMutex, rhs.pMutex);

    std::lock_guard<std::mutex> l1(pMutex, std::adopt_lock);
    std::lock_guard<std::mutex> l2(rhs.pMutex, std::adopt_lock);

    // do copy
}

But note to check that rhs is not a *this since in this case std::lock will lead to UB due to locking same mutex.