regulator: core: Avoid potential deadlock on regulator_unregister

Lockdep reports the following issue on my setup:

Possible unsafe locking scenario:

CPU0                    CPU1
----                    ----
lock((work_completion)(&(&rdev->disable_work)->work));
                        lock(regulator_list_mutex);
                        lock((work_completion)(&(&rdev->disable_work)->work));
lock(regulator_list_mutex);

The problem is that regulator_unregister takes the
regulator_list_mutex and then calls flush_work on disable_work. But
regulator_disable_work calls regulator_lock_dependent which will
also take the regulator_list_mutex. Resulting in a deadlock if the
flush_work call actually needs to flush the work.

Fix this issue by moving the flush_work outside of the
regulator_list_mutex. The list mutex is not used to guard the point at
which the delayed work is queued, so its use adds no additional safety.

Fixes: f8702f9e ("regulator: core: Use ww_mutex for regulators locking")
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/core.c

@@ -5061,10 +5061,11 @@ void regulator_unregister(struct regulator_dev *rdev)
 		regulator_put(rdev->supply);
 	}
 
+	flush_work(&rdev->disable_work.work);
+
 	mutex_lock(&regulator_list_mutex);
 
 	debugfs_remove_recursive(rdev->debugfs);
-	flush_work(&rdev->disable_work.work);
 	WARN_ON(rdev->open_count);
 	regulator_remove_coupling(rdev);
 	unset_regulator_supplies(rdev);