Re: [PATCH 21/22] rust/hpet: Replace BqlRefCell<HPETRegisters> with Mutex<HPETRegisters>

[Zhao Liu <zhao1.liu@intel.com>]

> @@ -179,8 +180,8 @@ const fn deactivating_bit(old: u64, new: u64, shift: usize) -> bool {
>  fn timer_handler(timer_cell: &BqlRefCell<HPETTimer>) {
>      let mut t = timer_cell.borrow_mut();
>      // SFAETY: state field is valid after timer initialization.
> -    let regs = &mut unsafe { t.state.as_mut() }.regs.borrow_mut();
> -    t.callback(regs)
> +    let mut regs = unsafe { t.state.as_ref() }.regs.lock().unwrap();
> +    t.callback(&mut regs)
>  }

callback()
 -> arm_timer(): access timer N register
 -> update_irq(): modify global register (int_status or "isr" in C code)

So timer handler needs to lock Mutex. But this may cause deadlock:

timer_hanlder -> lock BQL   -> try to lock Mutex
MMIO access   -> lock Mutex -> try to lock BQL

C HPET doesn't have such deadlock issue since it doesn't lock Mutex in
timer handler.

I think it seems necessay to lock Mutex in timer handler since there's
no guarantee to avoid data race...

Thanks,
Zhao