Messages in this thread | | | Date | Tue, 19 Jun 2018 15:23:38 +0200 | From | Peter Zijlstra <> | 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.
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