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From:   Ulf Hansson <ulf.hansson@linaro.org>
Date:   Wed, 2 Sep 2020 17:58:55 +0200
Message-ID: <CAPDyKFrGj+8hOXi7sWxWNv2QP0=mx9pFKLG0JM-L5VNKUPDgeA@mail.gmail.com>
Subject: Re: WARNING: suspicious RCU usage - sdhci-pltfm: SDHCI platform and
 OF driver helper
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "Paul E. McKenney" <paulmck@kernel.org>,
        Lina Iyer <ilina@codeaurora.org>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Saravana Kannan <saravanak@google.com>,
        open list <linux-kernel@vger.kernel.org>,
        linux-mmc <linux-mmc@vger.kernel.org>,
        lkft-triage@lists.linaro.org, rcu@vger.kernel.org,
        Linux PM <linux-pm@vger.kernel.org>,
        Anders Roxell <anders.roxell@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Rajendra Nayak <rnayak@codeaurora.org>,
        John Stultz <john.stultz@linaro.org>,
        Stephen Boyd <sboyd@kernel.org>,
        Lars Povlsen <lars.povlsen@microchip.com>,
        madhuparnabhowmik10@gmail.com,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>
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On Wed, 2 Sep 2020 at 14:14, <peterz@infradead.org> wrote:
>
> On Wed, Sep 02, 2020 at 09:03:37AM +0200, Ulf Hansson wrote:
> > Lots of cpuidle drivers are using CPU_PM notifiers (grep for
> > cpu_pm_enter and you will see) from their idlestates ->enter()
> > callbacks. And for those we are already calling
> > rcu_irq_enter_irqson|off() in cpu_pm_notify() when firing them.
>
> Yeah, that particular trainwreck is on my todo list already ... then
> again, that list is forever overflowing.
>
> I'm thinking cpu_pm_unregister_notifier() is not a common thing? The few
> I looked at seem to suggest 'never' is a good approximation.

The trend is that drivers are turning into regular modules that may
also need to manage "->remove()", which may mean unregistering the
notifier. Of course, I don't know for sure whether that becomes a
problem, but it seems quite limiting.

>
> It would be fairly trivial to replace the atomic_notifier usage with a
> raw_notifier a lock and either stop-machine or IPIs. Better still would
> be if we can get rid of it entirely, but I can't tell in a hurry if that
> is possible.

Okay, let's see.

In any case, I was thinking that the patch with CPU idle flag, for
letting CPU idle drivers deal with RCU, that you proposed, seems like
a good first step.

At least it should enable us to solve the problem for runtime PM in
psci_enter_domain_idle_state(). Let me update the patch and send it
out, then we can continue the discussion over there.

Kind regards
Uffe