Friday, May 31, 2013

Self Deadlock

Another problem is self-deadlock. Self-deadlock occurs when a single thread attempts to lock a mutex twice: the second attempt will block indefinitely. This can easily happen when the same resource is used at multiple levels within an algorithm.
In particular, consider a class that attempts to provide a threadsafe interface by synchronising all member function calls with a single internal mutex. The mutex is locked at the beginning of every method, and unlocked on method return. If that class now calls a member function from within a member function, there will be a self-deadlock.

To counter this problem there is the concept of recursive mutex. A recursive mutex will allow multiple locks from within a single thread to succeed, though that thread must unlock the mutex as many times as it has locked it. The disadvantage of a recursive mutex is a slight performance decrease.
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