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Date:   Tue, 19 Jun 2018 15:23:38 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Quentin Perret <quentin.perret@arm.com>
Cc:     rjw@rjwysocki.net, gregkh@linuxfoundation.org,
        linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        mingo@redhat.com, dietmar.eggemann@arm.com,
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        patrick.bellasi@arm.com, valentin.schneider@arm.com,
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        viresh.kumar@linaro.org, tkjos@google.com, joelaf@google.com,
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        pkondeti@codeaurora.org, juri.lelli@redhat.com,
        edubezval@gmail.com, srinivas.pandruvada@linux.intel.com,
        currojerez@riseup.net, javi.merino@kernel.org
Subject: Re: [RFC PATCH v3 03/10] PM: Introduce an Energy Model management
 framework
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On Tue, Jun 19, 2018 at 01:58:58PM +0100, Quentin Perret wrote:
> On Tuesday 19 Jun 2018 at 13:34:08 (+0200), Peter Zijlstra wrote:
> > On Mon, May 21, 2018 at 03:24:58PM +0100, Quentin Perret wrote:
> > > +struct em_freq_domain *em_cpu_get(int cpu)
> > > +{
> > > +	struct em_freq_domain *fd;
> > > +	unsigned long flags;
> > > +
> > > +	read_lock_irqsave(&em_data_lock, flags);
> > > +	fd = per_cpu(em_data, cpu);
> > > +	read_unlock_irqrestore(&em_data_lock, flags);
> > 
> > Why can't this use RCU? This is the exact thing read_locks are terrible
> > at and RCU excells at.
> 
> So the idea was that clients (the scheduler for ex) can get a reference
> to a frequency domain object once, and they're guaranteed it always
> exists without asking for it again.
> 
> For example, my proposal was to have the scheduler (patch 05) build its
> own private list of frequency domains on which it can iterate efficiently
> in the wake-up path. If we protect this per_cpu variable with RCU, then
> this isn't possible any-more. The scheduler will have to re-ask
> em_cpu_get() at every wake-up, and that makes iterating over frequency
> domains a whole lot more complex.
> 
> Does that make any sense ?

None what so ever... The lock doesn't guarantee stability any more than
RCU does.

If you hand out the pointer and then drop the read-lock, the write-lock
can proceed and change the pointer right after you.

The very easiest solution is to never change the data, as I think was
suggested elsewhere in the thread. Construct the thing once and then
never mutate.