#7970: Thread GC frees roots before thread actually finishes ---------------------------------+------------------------------------------ Reporter: joeyadams | Owner: simonmar Type: bug | Status: new Priority: high | Milestone: 7.8.1 Component: Runtime System | Version: 7.6.3 Keywords: | Os: Unknown/Multiple Architecture: Unknown/Multiple | Failure: Runtime crash Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: 7170 | ---------------------------------+------------------------------------------ Comment(by simonmar):
It only makes sense to call a thread "unreachable" if it will never run again.
Which is the case now - at the point where the thread is unreachable, it will never run again. So we send it an exception to tell it (at which point it runs again of course, but if it were not for the exception it would not run). The specific problem in this ticket is the interaction between threads and weak pointers. Right now, it is the case that * a reference from the finalizer of a weak pointer to a thread makes the thread reachable, even if the key of the weak pointer is not reachable. This means that you can have a thread blocked waiting for a finalizer to run. There's an alternative design, which would make the program in this ticket work: * a reference from a thread to a weak pointer makes the weak pointer reachable, even if the thread itself is not reachable. You can choose one of these, or neither, but not both. The reason is that the GC must do these steps in some order: * decide which weak pointers are unreachable, and make their finalizers alive * decide which threads are unreachable, and then make them alive if we did both at the same time, you would get *neither* of the behaviours above. So I think what the example in this ticket argues for is switching the order. I'm fine with that (I buy the argument) but it does potentially break the use case of having a thread blocked on a finalizer. I'm not sure whether anyone actually does that or not. -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7970#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler