linux-kernel - Re: lockdep: possible irq lock inversion dependency detected (trig->leddev_list

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Date:   Sun, 1 Nov 2020 17:28:38 +0100
From:   Pavel Machek <pavel@....cz>
To:     Andrea Righi <andrea.righi@...onical.com>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: lockdep: possible irq lock inversion dependency detected
 (trig->leddev_list_lock)

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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