Re: lockdep: possible irq lock inversion dependency detected (trig->leddev_list_lock)

Pavel Machek
2020-11-01T16:28:41Z

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Hi!

> I'm getting the following lockdep splat (see below).
>
> Apparently this warning starts to be reported after applying:
>
> e918188611f0 ("locking: More accurate annotations for read_lock()")
>
> It looks like a false positive to me, but it made me think a bit and
> IIUC there can be still a potential deadlock, even if the deadlock
> scenario is a bit different than what lockdep is showing.
>
> In the assumption that read-locks are recursive only in_interrupt()
> context (as stated in e918188611f0), the following scenario can still
> happen:
>
> CPU0 CPU1
> ---- ----
> read_lock(&trig->leddev_list_lock);
> write_lock(&trig->leddev_list_lock);
> <soft-irq>
> kbd_bh()
> -> read_lock(&trig->leddev_list_lock);
>
> *** DEADLOCK ***
>
> The write-lock is waiting on CPU1 and the second read_lock() on CPU0
> would be blocked by the write-lock *waiter* on CPU1 => deadlock.
>
> In that case we could prevent this deadlock condition using a workqueue
> to call kbd_propagate_led_state() instead of calling it directly from
> kbd_bh() (even if lockdep would still report the false positive).

console.c is already using bh to delay work from
interrupt. But... that should not be neccessary. led_trigger_event
should already be callable from interrupt context, AFAICT.

Could this be resolved by doing the operations directly from keyboard
interrupt?

Best regards,
Pavel
--
HTTP://www.livejournal.com/~pavelmachek

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