linux-kernel - Re: [PATCH] soundwire: bus_type: Avoid lockdep assert in sdw_drv

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Date:   Wed, 23 Nov 2022 09:35:47 +0000
From:   Charles Keepax <ckeepax@...nsource.cirrus.com>
To:     Richard Fitzgerald <rf@...nsource.cirrus.com>
CC:     <vkoul@...nel.org>, <yung-chuan.liao@...ux.intel.com>,
        <pierre-louis.bossart@...ux.intel.com>, <sanyog.r.kale@...el.com>,
        <patches@...nsource.cirrus.com>, <alsa-devel@...a-project.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] soundwire: bus_type: Avoid lockdep assert in
 sdw_drv_probe()

On Mon, Nov 21, 2022 at 04:24:52PM +0000, Richard Fitzgerald wrote:
> Don't hold sdw_dev_lock while calling the peripheral driver
> probe() and remove() callbacks.
> 
> Holding sdw_dev_lock around the probe() and remove() calls
> causes a theoretical mutex inversion which lockdep will
> assert on. The peripheral driver probe will probably register
> a soundcard, which will take ALSA and ASoC locks. During
> normal operation a runtime resume suspend can be triggered
> while these locks are held and will then take sdw_dev_lock.
> 
> It's not necessary to hold sdw_dev_lock when calling the
> probe() and remove(), it is only used to prevent the bus core
> calling the driver callbacks if there isn't a driver or the
> driver is removing.
> 
> If sdw_dev_lock is held while setting and clearing the
> 'probed' flag this is sufficient to guarantee the safety of
> callback functions.
> 
> The potential race of a bus event happening while probe() is
> executing is the same as the existing race of the bus event
> handler taking the mutex first and processing the event
> before probe() can run. In both cases the event has already
> happened before the driver is probed and ready to accept
> callbacks.
> 
> Signed-off-by: Richard Fitzgerald <rf@...nsource.cirrus.com>
> ---

Reviewed-by: Charles Keepax <ckeepax@...nsource.cirrus.com>

Thanks,
Charles