Re: regulator: deadlock vs memory reclaim

[Dmitry Osipenko]

10.08.2020 01:25, Michał Mirosław пишет:
> Hi guys,
> 
> Commit f8702f9e4aa7 ("regulator: core: Use ww_mutex for regulators locking")
> from Nov 2018 tried to fix possible deadlocks when handling coupled
> regulators. Unfortunately it introduced another possible deadlock,
> as discovered by lockdep (see below), instead.
> 
> regulator_lock_dependent() starts by taking regulator_list_mutex, The
> same mutex covers eg. regulator initialization, including memory allocations
> that happen there. This will deadlock when you have filesystem on eg. eMMC
> (which uses a regulator to control module voltages) and you register
> a new regulator (hotplug a device?) when under memory pressure.
> 
> There is also another problem with regulator_lock_dependent(): all the
> w/w rollback stuff is useless: because of the outer lock, there can only
> be one contendant doing multiple-lock-grabbing procedure. In this setup,
> the procedure cannot detect other processes waiting on
> regulator_lock_dependent() and it cannot signal (wound a transaction of)
> current holders of locks taken by regulator_lock().
> 
> Basically, we have a BKL for regulator_enable() and we're using ww_mutex
> as a recursive mutex with no deadlock prevention whatsoever. The locks
> also seem to cover way to much (eg. initialization even before making the
> regulator visible to the system).
> 
> To fix the regulator vs memory reclaim path I tried pushing all allocations
> out of protected sections. After doing a few patches, though, I'm not sure
> I'm going in the right direction. Your thoughts?

IIRC, taking the regulator_list_mutex within regulator_lock_dependent()
is needed in order to protect the case of decoupling regulators.

Perhaps moving out allocations or making them GFP_NOWAIT should be the
easiest solution.