linux-kernel - Re: [PATCH 2/2] ASoC: max98090: fix lockdep warning

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Message-ID: <CA+Px+wWiZ9MWwi-moXo9rJrbgLFVEbOqjQMhOZmm5mRL7EeMbQ@mail.gmail.com>
Date:   Fri, 10 Jan 2020 09:05:59 +0800
From:   Tzung-Bi Shih <tzungbi@...gle.com>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     ALSA development <alsa-devel@...a-project.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Samsung SOC <linux-samsung-soc@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        Sylwester Nawrocki <s.nawrocki@...sung.com>,
        Dylan Reid <dgreid@...gle.com>,
        Jimmy Cheng-Yi Chiang <cychiang@...gle.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH 2/2] ASoC: max98090: fix lockdep warning

On Thu, Jan 9, 2020 at 7:09 PM Marek Szyprowski
<m.szyprowski@...sung.com> wrote:
> On 09.01.2020 06:36, Tzung-Bi Shih wrote:
> > On Wed, Jan 8, 2020 at 7:50 PM Marek Szyprowski
> > <m.szyprowski@...sung.com> wrote:
> >> Fix this by introducing a separate mutex only for serializing the SHDN
> >> hardware register related operations.
> > This fix makes less sense to me.  We used dapm_mutex intentionally
> > because: both DAPM and userspace mixer control would change SHDN bit
> > at the same time.

We should not use a separate lock.  Either mixer control or DAPM would
change the SHDN bit.  The patch overlooks the calling path from DAPM.
As a result, DAPM can change the bit in the middle of mixer control.

> Nope. This is just a lockdep warning about possible hypothetical
> situation on the test system during the normal boot. It doesn't mean
> that the circular dependency actually happens (if so, it would end in
> deadlock). It also doesn't mean that such circular dependency can be
> really triggered, because some other dependencies that not known to
> lockdep engine might protect against it. However the easiest way to fix
> it is to use fine-grained locking instead of reusing some framework
> locks for other purposes. Such approach is also usually a good practice.

If the possible circular locking is a hypothetical situation, shall we
ignore it since we are very sure userspace cannot see the control
devices when building the sound card?