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vlovich123 • today at 4:52 PM • 1 reply • view on HN

There’s no global lock. There’s a linear MutexKey<N> that a lock of Level >= N has to be acquired with. Aquiring it consumes MutexKey<N> and hands you back MutexKey<Level+1> where Level is the N of the level you’re locking.

There’s no priority inversion possible because locks can only ever be held in decreasing orders of priority - you can’t acquire a low priority lock and then a high priority lock since your remaining MutexKey won’t have the right level.

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jcalvinowens • today at 5:19 PM

In the example it seems pretty clear to me that:

    Mutex::new(AppConfig::default());
...is meant to be acquiring a mutex protecting some global config object, yes? That's what I'm calling a "global lock".

> There’s no priority inversion possible because locks can only ever be held in decreasing orders of priority

    T1               T2
    --               --
    small_lock();
                     big_lock();
                     small_lock(); <--- Spins waiting for T1
...and now any other thread that needs big_lock() spins waiting for T2 to release it, but T2 is spinning waiting for T1 to release the (presumably less critical) small lock.

If small_lock is never ever acquired without acquiring big_lock first, small_lock serves no purpose and should be deleted from the program.

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