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Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1g7Lfy-0006J7-Ao; Tue, 02 Oct 2018 14:29:26 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id DD852202B9CBE; Tue, 2 Oct 2018 16:29:24 +0200 (CEST) Date: Tue, 2 Oct 2018 16:29:24 +0200 From: Peter Zijlstra To: Quentin Perret Cc: rjw@rjwysocki.net, linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, gregkh@linuxfoundation.org, mingo@redhat.com, dietmar.eggemann@arm.com, morten.rasmussen@arm.com, chris.redpath@arm.com, patrick.bellasi@arm.com, valentin.schneider@arm.com, vincent.guittot@linaro.org, thara.gopinath@linaro.org, viresh.kumar@linaro.org, tkjos@google.com, joel@joelfernandes.org, smuckle@google.com, adharmap@codeaurora.org, skannan@codeaurora.org, pkondeti@codeaurora.org, juri.lelli@redhat.com, edubezval@gmail.com, srinivas.pandruvada@linux.intel.com, currojerez@riseup.net, javi.merino@kernel.org Subject: Re: [PATCH v7 03/14] PM: Introduce an Energy Model management framework Message-ID: <20181002142924.GI26858@hirez.programming.kicks-ass.net> References: <20180912091309.7551-1-quentin.perret@arm.com> <20180912091309.7551-4-quentin.perret@arm.com> <20181002123031.GZ3439@hirez.programming.kicks-ass.net> <20181002125115.245r3ocusvyiexno@queper01-lin> <20181002134857.GE26858@hirez.programming.kicks-ass.net> <20181002140430.fpeiqzblbcaewg6n@queper01-lin> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181002140430.fpeiqzblbcaewg6n@queper01-lin> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 02, 2018 at 03:05:23PM +0100, Quentin Perret wrote: > On Tuesday 02 Oct 2018 at 15:48:57 (+0200), Peter Zijlstra wrote: > > +/** > > + * em_cpu_get() - Return the performance domain for a CPU > > + * @cpu : CPU to find the performance domain for > > + * > > + * Return: the performance domain to which 'cpu' belongs, or NULL if it doesn't > > + * exist. > > + */ > > +struct em_perf_domain *em_cpu_get(int cpu) > > +{ > > + return READ_ONCE(per_cpu(em_data, cpu)); > > +} > > +EXPORT_SYMBOL_GPL(em_cpu_get); > > > > But your read side doesn't take, not is required to take em_pd_mutex. > > > > At that point, the mutex_unlock() doesn't guarantee anything. > > > > A CPU observing the em_data store, doesn't need to observe the store > > that filled the data structure it points to. > > Right but even if I add the smp_store_release(), I can still have a > CPU observing em_data while another is in the process of updating it. > So, if smp_store_release() doesn't guarantee that readers will see a > complete update, do I actually get something interesting from it ? > (That's not a rhetorical question, I'm actually wondering :-) I thought the update would fail if em_data was already set. That is, you can only set this thing up _once_ and then you'll have to forever live with it. Or did I read that wrong? If you want to allow updates, you'll have to do the whole RCU thing, at which point you'll need rcu_assign_pointer(), which again is exactly smp_store_release() :-)