Question

Ok.. here is some background on the issue. I have some ‘critical’ code that i’m trying to protect with a mutex. It goes something like this

Mutex.Lock()

// critical code
// some file IO

Mutex.Unlock().

Now the issue is that my program seems to be ‘stuck’ due to this. Let me explain with an example.

Thread_1 comes in; and go to Mutex.Lock() and starts executing the critical code. In the critical code; it needs to do some File IO. Now at this point; I believe a ‘context switch’ happens and Thread_2 comes in and blocks on the Mutex.Lock() (since Thread_1 has the lock). All seems fine but in my case; the program ‘hangs’ here.. The only thing I can think of is that somehow Thread_2 keeps blocking for ever and doesn’t switch back to Thread_1??

More info: using pthread_mutex_init and pthread_mutex_lock on linux.

Answer 1

As others have mentioned, you probably have a deadlock.

Sidenote:

You’ll want to make sure that there aren’t any uncaught exceptions thrown in the critical block of code. Otherwise the lock will never be released. You can use an RAII lock to overcome this issue:

class SingleLock {
public:
  SingleLock(Mutex &m) : m(m) { m.Lock(); }
  ~SingleLock() { m.Unlock(); }
private:
  Mutex m;
};

...
{ 
  SingleLock lock(mutex);
  // critical code // some file IO

}
...