GHC 如何识别一个线程是 BlockedIndefinitelyOnMVar？
How does GHC recognize that a thread is BlockedIndefinitelyOnMVar?
如果线程等待 MVar that doesn't have a possible producer, GHC is usually smart enough to throw BlockedIndefinitelyOnMVar 并终止线程。这个检测是如何实现的？
它是否检查对 MVar 的所有引用都来自等待它的线程，还是类似的东西？
来自Control.Concurrent：
GHC attempts to detect when threads are deadlocked using the garbage
collector. A thread that is not reachable (cannot be found by
following pointers from live objects) must be deadlocked, and in this
case the thread is sent an exception. The exception is either
'BlockedIndefinitelyOnMVar', 'BlockedIndefinitelyOnSTM',
'NonTermination', or 'Deadlock', depending on the way in which the
thread is deadlocked.
Note that this feature is intended for debugging, and should not be
relied on for the correct operation of your program. There is no
guarantee that the garbage collector will be accurate enough to detect
your deadlock, and no guarantee that the garbage collector will run in
a timely enough manner. Basically, the same caveats as for finalizers
apply to deadlock detection.
There is a subtle interaction between deadlock detection and
finalizers (as created by 'Foreign.Concurrent.newForeignPtr' or the
functions in "System.Mem.Weak"): if a thread is blocked waiting for a
finalizer to run, then the thread will be considered deadlocked and
sent an exception. So preferably don't do this, but if you have no
alternative then it is possible to prevent the thread from being
considered deadlocked by making a 'StablePtr' pointing to it. Don't
forget to release the 'StablePtr' later with 'freeStablePtr'.
如果线程等待 MVar that doesn't have a possible producer, GHC is usually smart enough to throw BlockedIndefinitelyOnMVar 并终止线程。这个检测是如何实现的？
它是否检查对 MVar 的所有引用都来自等待它的线程，还是类似的东西？
来自Control.Concurrent：
GHC attempts to detect when threads are deadlocked using the garbage collector. A thread that is not reachable (cannot be found by following pointers from live objects) must be deadlocked, and in this case the thread is sent an exception. The exception is either 'BlockedIndefinitelyOnMVar', 'BlockedIndefinitelyOnSTM', 'NonTermination', or 'Deadlock', depending on the way in which the thread is deadlocked.
Note that this feature is intended for debugging, and should not be relied on for the correct operation of your program. There is no guarantee that the garbage collector will be accurate enough to detect your deadlock, and no guarantee that the garbage collector will run in a timely enough manner. Basically, the same caveats as for finalizers apply to deadlock detection.
There is a subtle interaction between deadlock detection and finalizers (as created by 'Foreign.Concurrent.newForeignPtr' or the functions in "System.Mem.Weak"): if a thread is blocked waiting for a finalizer to run, then the thread will be considered deadlocked and sent an exception. So preferably don't do this, but if you have no alternative then it is possible to prevent the thread from being considered deadlocked by making a 'StablePtr' pointing to it. Don't forget to release the 'StablePtr' later with 'freeStablePtr'.