Re: [PATCH] regulator: core: Avoid potential deadlock on regulator_unregister

[Dmitry Osipenko]

04.04.2019 18:32, Charles Keepax ÐÐÑÐÑ:
> 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: f8702f9e4aa7 ("regulator: core: Use ww_mutex for regulators locking")
> Signed-off-by: Charles Keepax <ckeepax@xxxxxxxxxxxxxxxxxxxxx>
> ---
> 
> This patch follows on from my email the other day [1].
> 
> After looking at things in more detail I am fairly confident this is
> a good fix. I do still have a slight nagging doubt that something
> should be protecting this flush_work from additional works being
> queued, and I can't see what that is. But as that is definitely not
> the regulator_list_mutex the patch is not making this any more
> dangerous. In practice I suspect this is fine as nothing should
> really be using a regulator that is about to be unregistered,
> or really this delayed work is probably the least of the systems
> problems.

If anything tries to use regulator after invoking the regulator_unregister(), then indeed it's already in trouble. The patch looks good to me, thanks.

Reviewed-by: Dmitry Osipenko <digetx@xxxxxxxxx>