linux-kernel - Re: regulator: deadlock vs memory reclaim

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Message-ID: <20200810153928.GD6438@sirena.org.uk>
Date:   Mon, 10 Aug 2020 16:39:28 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     Dmitry Osipenko <digetx@...il.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: regulator: deadlock vs memory reclaim

On Mon, Aug 10, 2020 at 12:25:37AM +0200, Michał Mirosław wrote:

> 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.

OK, that's very much a corner case, it only applies in the case of
coupled regulators.  The most obvious thing here would be to move the
allocations on registration out of the locked region, we really only
need this in the regulator_find_coupler() call I think.  If the
regulator isn't coupled we don't need to take the lock at all.

> 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).

Could you be more specific about what you're looking at here?  There's
nothing too obvious jumping out from the code here other than the bit
around the coupling allocation, otherwise it looks like we're locking
list walks.

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