From: Jerome Brunet <jbrunet@baylibre.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Kevin Hilman <khilman@baylibre.com>,
Brad Harper <bjharper@gmail.com>,
linux-amlogic@lists.infradead.org, linux-mmc@vger.kernel.org,
linux-rt-users@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mmc: host: meson-gx-mmc: fix possible deadlock condition for preempt_rt
Date: Fri, 25 Sep 2020 16:14:09 +0200 [thread overview]
Message-ID: <1jh7rmj64u.fsf@starbuckisacylon.baylibre.com> (raw)
In-Reply-To: <20200925134445.rk366jip5ne4x7em@linutronix.de>
On Fri 25 Sep 2020 at 15:44, Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:
> On 2020-09-25 11:11:42 [+0200], Jerome Brunet wrote:
>> I'm not sure about this.
>> As you have explained on IRC, I understand that IRQF_ONESHOT is causing
>> trouble with RT as the hard IRQ part of the thread will not be migrated
>> to a thread. That was certainly not the intent when putting this flag.
>
> That is my understanding as well.
>
>> This seems pretty unsafe to me. Maybe we could improve the driver so it
>> copes with this case gracefully. ATM, I don't think it would.
>
> Running the primary handler in hardirq context is bad, because it
> invokes meson_mmc_request_done() at the very end. And here:
> - mmc_complete_cmd() -> complete_all()
> There is a lockdep_assert_RT_in_threaded_ctx() which should trigger.
>
> - led_trigger_event() -> led_trigger_event()
> This should trigger a might_sleep() warning somewhere.
>
> So removing IRQF_ONESHOT is okay but it should additionally disable the
> IRQ source in meson_mmc_irq() and re-enable back in
> meson_mmc_irq_thread(). Otherwise the IRQ remains asserted and may fire
> multiple times before the thread has a chance to run.
Looks like we need to do manually what IRQF_ONESHOT was doing for us :(
This brings a few questions:
* The consideration you described is not mentioned near the description
of IRQF_ONESHOT. Maybe it should so other drivers with same intent
don't end up in the same pitfall ?
* Why doesn't RT move the IRQ with this flag ? Seems completly unrelated
to RT (maybe it is the same documentation problem)
* Can't we have flag doing the irq disable in the same way while still
allowing to RT to do its magic ? seems better than open coding it in
the driver ?
>
> Sebastian
next prev parent reply other threads:[~2020-09-25 14:14 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-23 13:04 [PATCH] mmc: host: meson-gx-mmc: fix possible deadlock condition for preempt_rt Brad Harper
2020-09-24 17:01 ` Kevin Hilman
2020-09-25 9:11 ` Jerome Brunet
2020-09-25 13:44 ` Sebastian Andrzej Siewior
2020-09-25 14:14 ` Jerome Brunet [this message]
2020-09-25 15:05 ` Sebastian Andrzej Siewior
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