Monday, April 8, 2013

calling unknown code while holding lock (or not holding)

[[eff c#]] warns against calling "unknown" code while holding a lock. This advice applies to java and c++ too.


Now, What defines "unknown"? Any delegate can have an inv list containing unknown callbacks. Raising an event involves delegate invocation. If you receives an Action instance, realize it's a delegate instance too. 

Another category of "unknown code" might be implementations of an interface. You get a IComparable instance, and you just call Compare() without knowing the implementation

Suppose within obj1.method1() we grab a lock and then call the unknown code. Such unknown code could circle back to call obj1.method2(). If method2() also grabs the same lock, then no issue, thanks to c# reentrancy. Note c# reentrancy is implemented by counting -- http://stackoverflow.com/questions/3687505/recursive-nested-locking-in-c-sharp-with-the-lock-statement

If method2 is on another thread, then we get a classic single-lock deadlock -- method2 would be blocked by the lock held by method1(), hopefully for a short time only. If method1() is unfortunately waiting for method2 to finish its work on the other thread, then the deadlock would persist.

Even if there's no locking involved, method2 may circle back to invoke method1 (on the same or a separate thread, on the same obj1 or another object), recursively. If this recursion is unplanned (like unplanned pregnancy:), then stack overflow may ensue. 

Dumb luck if unplanned recursion doesn't hit stack overflow.

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