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Date:   Wed, 15 Apr 2020 20:48:27 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     John Stultz <john.stultz@linaro.org>
Cc:     "Paul E. McKenney" <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>
Subject: Re: On trace_*_rcuidle functions in modules
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On Wed, 15 Apr 2020 17:06:24 -0700
John Stultz <john.stultz@linaro.org> wrote:

> So you're saying the recent change to move to using trace_*_rcuidle()
> was unnecessary?
> 
> Or is there a different notifier then cpu_pm_register_notifier() that
> the driver should be using (that one seems to be using
> atomic_notifier_chain_register())?

From looking at the trace event in __tcs_buffer_write() in
drivers/soc/qcom/rpmh-rsc.c, the _rcuidle() was added by:

efde2659b0fe8 ("drivers: qcom: rpmh-rsc: Use rcuidle tracepoints for rpmh")

Which shows a backtrace dump of:

     Call trace:
      dump_backtrace+0x0/0x174
      show_stack+0x20/0x2c
      dump_stack+0xc8/0x124
      lockdep_rcu_suspicious+0xe4/0x104
      __tcs_buffer_write+0x230/0x2d0
      rpmh_rsc_write_ctrl_data+0x210/0x270
      rpmh_flush+0x84/0x24c
      rpmh_domain_power_off+0x78/0x98
      _genpd_power_off+0x40/0xc0
      genpd_power_off+0x168/0x208
      genpd_power_off+0x1e0/0x208
      genpd_power_off+0x1e0/0x208
      genpd_runtime_suspend+0x1ac/0x220
      __rpm_callback+0x70/0xfc
      rpm_callback+0x34/0x8c
      rpm_suspend+0x218/0x4a4
      __pm_runtime_suspend+0x88/0xac
      psci_enter_domain_idle_state+0x3c/0xb4
      cpuidle_enter_state+0xb8/0x284
      cpuidle_enter+0x38/0x4c
      call_cpuidle+0x3c/0x68
      do_idle+0x194/0x260
      cpu_startup_entry+0x24/0x28
      secondary_start_kernel+0x150/0x15c


There's no notifier that calls this. This is called by the rpm_callback
logic. Perhaps that callback will require a call to rcu_irq_enter()
before calling the callback.

In any case, I think it is wrong that these callbacks are called
without RCU watching. The _rcuidle() on that tracepoint should be
removed, and we fix the code that gets there to ensure that RCU is
enabled. I agree with Peter, that no module code should be executed
without RCU watching.

-- Steve