linux-kernel - Re: [RFC][PATCH] cpu

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Message-ID: <20200903153555.GN29330@paulmck-ThinkPad-P72>
Date:   Thu, 3 Sep 2020 08:35:55 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     peterz@...radead.org
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        Lina Iyer <ilina@...eaurora.org>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Saravana Kannan <saravanak@...gle.com>,
        open list <linux-kernel@...r.kernel.org>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        lkft-triage@...ts.linaro.org, rcu@...r.kernel.org,
        Linux PM <linux-pm@...r.kernel.org>,
        Anders Roxell <anders.roxell@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Lars Povlsen <lars.povlsen@...rochip.com>,
        madhuparnabhowmik10@...il.com,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC][PATCH] cpu_pm: Remove RCU abuse

On Thu, Sep 03, 2020 at 05:08:19PM +0200, peterz@...radead.org wrote:
> On Thu, Sep 03, 2020 at 04:36:35PM +0200, Ulf Hansson wrote:
> > On Thu, 3 Sep 2020 at 15:53, <peterz@...radead.org> wrote:
> > >  static int cpu_pm_notify(enum cpu_pm_event event)
> > >  {
> > >         int ret;
> > >
> > > +       lockdep_assert_irqs_disabled();
> > 
> > Nitpick, maybe the lockdep should be moved to a separate patch.
> 
> Well, the unregister relies on IRQs being disabled here, so I figured
> asserting this was a good thing ;-)
> 
> Starting the audit below, this might not in fact be true, which then
> invalidates the unregister implementation. In particular the notifier in
> arch/arm/kernel/hw_breakpoint.c seems to unconditionally enable IRQs.
> 
> > > +       ret = raw_notifier_call_chain(&cpu_pm_notifier_chain, event, NULL);
> > 
> > Converting to raw_notifiers seems reasonable - if we need to avoid the
> > RCU usage.
> > 
> > My point is, I wonder about if the notifier callbacks themselves are
> > safe from RCU usage. For example, I would not be surprised if tracing
> > is happening behind them.
> 
> A bunch of them seem to call into the clk domain stuff, and I think
> there's tracepoints in that.
> 
> > Moreover, I am not sure that we really need to prevent and limit
> > tracing from happening. Instead we could push rcu_idle_enter|exit()
> > further down to the arch specific code in the cpuidle drivers, as you
> > kind of all proposed earlier.
> 
> Well, at some point the CPU is in a really dodgy state, ISTR there being
> ARM platforms where you have to manually leave the cache coherency
> fabric and all sorts of insanity. There should be a definite cut-off on
> tracing before that.
> 
> Also, what is the point of all this clock and power domain callbacks, if
> not to put the CPU into an extremely low power state, surely you want to
> limit the amount of code that's ran when the CPU is in such a state.
> 
> > In this way, we can step by step, move to a new "version" of
> > cpu_pm_enter() that doesn't have to deal with rcu_irq_enter_irqson(),
> > because RCU hasn't been pushed to idle yet.
> 
> That should be easy enough to audit. The thing is that mainline is now
> generating (debug) splats, and some people are upset with this.
> 
> If you're ok with ARM not being lockdep clean while this is being
> reworked I'm perfectly fine with that.
> 
> (There used to be a separate CONFIG for RCU-lockdep, but that seems to
> have been removed)

CONFIG_PROVE_RCU still gates RCU_LOCKDEP_WARN(), but it is now a
def_bool that follows CONFIG_PROVE_LOCKING.

It would not be hard to make CONFIG_PROVE_RCU separately settable only
for arm, if that would help.

							Thanx, Paul