Re: [RFC PATCH v3 03/10] PM: Introduce an Energy Model management framework

[Peter Zijlstra]

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.