linux-kernel - Re: Confusing lockdep message

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Date:   Mon, 20 May 2019 17:50:29 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     "Koenig, Christian" <Christian.Koenig@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Confusing lockdep message

On Mon, May 20, 2019 at 1:38 PM Koenig, Christian
<Christian.Koenig@....com> wrote:
>
> Please ignore this mail,
>
> I've fixed the double unlock and lockdep is still complaining about the
> nested locking, so I'm actually facing multiple issues here.

The way we model the ww-mutex stuff is that the acquire-ctx is treated
as a lockdep lock, and then we require that one if you take two or
more ww-mutexes (the nested locking stuff lockdep complains about).

So you already hold a ww-mutex while trying to get a 2nd one, without
holding a ww-mutex acquire ctx ticket. Could be a ww-mutex you forgot
to unlock somewhere.
-Daniel
>
> Sorry to waste your time,
> Christian.
>
> Am 20.05.19 um 13:19 schrieb Christian König:
> > Hi guys,
> >
> > writing the usual suspects about locking/lockdep stuff and also Daniel
> > in CC because he might have stumbled over this as well.
> >
> > It took me a while to figuring out what the heck lockdep was
> > complaining about. The relevant dmesg was the following:
> >> [  145.623005] ==================================
> >> [  145.623094] WARNING: Nested lock was not taken
> >> [  145.623184] 5.0.0-rc1+ #144 Not tainted
> >> [  145.623261] ----------------------------------
> >> [  145.623351] amdgpu_test/1411 is trying to lock:
> >> [  145.623442] 0000000098a1c4d3 (reservation_ww_class_mutex){+.+.},
> >> at: ttm_eu_reserve_buffers+0x46e/0x910 [ttm]
> >> [  145.623651]
> >>                but this task is not holding:
> >> [  145.623758] reservation_ww_class_acquire
> >> [  145.623836]
> >>                stack backtrace:
> >> [  145.623924] CPU: 4 PID: 1411 Comm: amdgpu_test Not tainted
> >> 5.0.0-rc1+ #144
> >> [  145.624058] Hardware name: System manufacturer System Product
> >> Name/PRIME X399-A, BIOS 0808 10/12/2018
> >> [  145.624234] Call Trace:
> >> ...
> >
> > The problem is now that the message is very confusion because the
> > issue was *not* that I tried to acquire a lock, but rather that I
> > accidentally released a lock twice.
> >
> > Now releasing a lock twice is a rather common mistake and I'm really
> > surprised that I didn't get that pointed out by lockdep immediately.
> >
> > Additional to that I'm pretty sure that this used to work correctly
> > sometimes in the past, so I'm either hitting a rare corner case or
> > this broke just recently.
> >
> > Anyway can somebody take a look? I can try to provide a test case if
> > required.
> >
> > Thanks in advance,
> > Christian.
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch