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From: Pavel Machek <pavel@ucw.cz>
To: Andrea Righi <andrea.righi@canonical.com>
Cc: Boqun Feng <boqun.feng@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
	linux-kernel@vger.kernel.org
Subject: Re: lockdep: possible irq lock inversion dependency detected (trig->leddev_list_lock)
Date: Sun, 1 Nov 2020 17:28:38 +0100	[thread overview]
Message-ID: <20201101162838.GA24370@duo.ucw.cz> (raw)
In-Reply-To: <20201101092614.GB3989@xps-13-7390>

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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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  parent reply	other threads:[~2020-11-01 16:28 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-01  9:26 Andrea Righi
2020-10-31 10:17 ` Boqun Feng
2020-11-02  7:33   ` Andrea Righi
2020-11-02  8:56     ` Pavel Machek
2020-11-02  9:09       ` Andrea Righi
2020-11-06  7:40         ` Andrea Righi
2020-11-01 16:28 ` Pavel Machek [this message]
2020-11-02  7:39   ` Andrea Righi

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