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Date:   Mon, 2 Nov 2020 08:39:40 +0100
From:   Andrea Righi <andrea.righi@canonical.com>
To:     Pavel Machek <pavel@ucw.cz>
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)
Message-ID: <20201102073940.GB9930@xps-13-7390>
References: <20201101092614.GB3989@xps-13-7390>
 <20201101162838.GA24370@duo.ucw.cz>
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On Sun, Nov 01, 2020 at 05:28:38PM +0100, Pavel Machek wrote:
> 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?

As pointed out by Boqun this is not a deadlock condition, because the
read_lock() called from soft-irq context is recursive (I was missing
that in_interrupt() returns true also from soft-irq context).

But the initial lockdep warning was correct, so there is still a
potential deadlock condition between trig->leddev_list_lock and
host->lock. And I think this can be prevented simply by scheduling the
led triggering part in a separate work from ata_hsm_qs_complete(), so
that led_trigger_event() won't be called with host->lock held. I'll send
a patch soon to do that.

-Andrea