Re: [PATCH] soundwire: bus_type: Avoid lockdep assert in sdw_drv_probe()

[Charles Keepax <ckeepax@opensource.cirrus.com>]

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@opensource.cirrus.com>
> ---

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

Thanks,
Charles