Ensure that a thread doesn't lock a mutex twice? - c++

Question

Say I have a thread running a member method like runController in the example below:
class SomeClass {
public:
SomeClass() {
// Start controller thread
mControllerThread = std::thread(&SomeClass::runController, this)
}
~SomeClass() {
// Stop controller thread
mIsControllerThreadInterrupted = true;
// wait for thread to die.
std::unique_lock<std:::mutex> lk(mControllerThreadAlive);
}
// Both controller and external client threads might call this
void modifyObject() {
std::unique_lock<std::mutex> lock(mObjectMutex);
mObject.doSomeModification();
}
//...
private:
std::mutex mObjectMutex;
Object mObject;
std::thread mControllerThread;
std::atomic<bool> mIsControllerInterrupted;
std::mutex mControllerThreadAlive;
void runController() {
std::unique_lock<std::mutex> aliveLock(mControllerThreadAlive);
while(!mIsControllerInterruped) {
// Say I need to synchronize on mObject for all of these calls
std::unique_lock<std::mutex> lock(mObjectMutex);
someMethodA();
modifyObject(); // but calling modifyObject will then lock mutex twice
someMethodC();
}
}
//...
};
And some (or all) of the subroutines in runController need to modify data that is shared between threads and guarded by a mutex. Some (or all) of them, might also be called by other threads that need to modify this shared data.
With all the glory of C++11 at my disposal, how can I ensure that no thread ever locks a mutex twice?
Right now, I'm passing unique_lock references into the methods as parameters as below. But this seems clunky, difficult to maintain, potentially disastrous, etc...
void modifyObject(std::unique_lock<std::mutex>& objectLock) {
// We don't even know if this lock manages the right mutex...
// so let's waste some time checking that.
if(objectLock.mutex() != &mObjectMutex)
throw std::logic_error();
// Lock mutex if not locked by this thread
bool wasObjectLockOwned = objectLock.owns_lock();
if(!wasObjectLockOwned)
objectLock.lock();
mObject.doSomeModification();
// restore previous lock state
if(!wasObjectLockOwned)
objectLock.unlock();
}
Thanks!

Answer 1

There are several ways to avoid this kind of programming error. I recommend doing it on a class design level:
separate between public and private member functions,
only public member functions lock the mutex,
and public member functions are never called by other member functions.
If a function is needed both internally and externally, create two variants of the function, and delegate from one to the other:
public:
// intended to be used from the outside
int foobar(int x, int y)
{
std::unique_lock<std::mutex> lock(mControllerThreadAlive);
return _foobar(x, y);
}
private:
// intended to be used from other (public or private) member functions
int _foobar(int x, int y)
{
// ... code that requires locking
}