linux-kernel - Re: [PATCH] mmc: host: meson-gx-mmc: fix possible deadlock condition for preempt

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Message-ID: <20200925134445.rk366jip5ne4x7em@linutronix.de>
Date:   Fri, 25 Sep 2020 15:44:45 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Jerome Brunet <jbrunet@...libre.com>
Cc:     Kevin Hilman <khilman@...libre.com>,
        Brad Harper <bjharper@...il.com>,
        linux-amlogic@...ts.infradead.org, linux-mmc@...r.kernel.org,
        linux-rt-users@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mmc: host: meson-gx-mmc: fix possible deadlock condition
 for preempt_rt

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.

Sebastian