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From:   John Stultz <john.stultz@linaro.org>
Date:   Wed, 15 Apr 2020 15:07:42 -0700
Message-ID: <CALAqxLUMnjAYdnTBNHmzjw3L8qfeZb8UVVaenzxtwN3UxNcWJg@mail.gmail.com>
Subject: Re: On trace_*_rcuidle functions in modules
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     paulmck@kernel.org, Josh Triplett <josh@joshtriplett.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Saravana Kannan <saravanak@google.com>,
        Todd Kjos <tkjos@google.com>, Stephen Boyd <sboyd@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>
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On Wed, Apr 15, 2020 at 2:49 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Wed, 15 Apr 2020 14:02:04 -0700
> John Stultz <john.stultz@linaro.org> wrote:
>
> >
> > So in my case your concerns may not be a problem, but I guess
> > generally it might. Though I'd hope the callback would be unregistered
> > (and whatever waiting for the grace period to complete be done) before
> > the module removal is complete. But maybe I'm still missing your
> > point?
>
> Hmm, you may have just brought up a problem here...
>
> You're saying that cpu_pm_register_notifier() callers are called from non
> RCU watching context? If that's the case, we have this:

Wait? what? I don't think I'm saying that.  :) I'm just trying to fix
a build issue that prevents a driver from being a module since it has
a trace_*_rcuidle call in it.

Honestly, I really don't know much about how the cpu pm notifiers
interact with rcu.  It's just this recent change, which seems
necessary, now seemingly prevents the driver from being built as a
module:
  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=efde2659b0fe835732047357b2902cca14f054d9

Maybe there's a better solution than using trace_*_rcuidle?

thanks
-john